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Lab Terms A–Z

405 entries.

A B C D E F G H I K L M N O P R S T U V W X Z

A

Abdominal aorta ultrasound: Imaging study that uses sound waves to view the abdominal aorta from the diaphragm down to where it divides into the iliac arteries, mainly to detect an aneurysm. It is an imaging test, not a laboratory test.
Abdominal obstruction series: Set of plain X-rays of the abdomen taken lying and upright to look for bowel obstruction or free air.
Acetest: A tablet test that detects ketones (acetoacetate and acetone) in urine, serum or plasma using the nitroprusside reaction. It is used to confirm ketonuria, for example in diabetic ketoacidosis or starvation.
Acetylcholinesterase (AChE): Enzyme that breaks down the neurotransmitter acetylcholine to end a nerve signal. The lab measures red-cell AChE to detect organophosphate pesticide poisoning, and amniotic-fluid AChE to support a diagnosis of open neural tube defects.
Acid phosphatase (ACP): Enzyme that works best at acidic pH and is found in the prostate, bone (osteoclasts), red cells, platelets and liver. It was once a prostate cancer marker and is still used forensically to detect semen.
Acid-fast bacilli (AFB): Bacteria, chiefly Mycobacterium species, that keep carbol fuchsin or auramine stain after an acid-alcohol wash. AFB smears (Ziehl-Neelsen or fluorescent stain) and culture of sputum and other specimens are key to diagnosing tuberculosis.
Acquired coagulation factor deficiencies: Shortage of one or more clotting factors that develops during life rather than being inherited, for example from liver disease, vitamin K deficiency, DIC, anticoagulant drugs or factor inhibitors. Usually several factors are affected.
Activated partial thromboplastin time (aPTT): Clotting test on citrated plasma that measures how long the intrinsic and common pathways take to form a clot after adding an activator, phospholipid and calcium. It is used to monitor unfractionated heparin and screen for factor deficiencies.
Agar: Gel-forming polysaccharide extracted from red seaweed that solidifies culture media. Most bacteria cannot digest it, and it stays solid at incubation temperatures, melting near 85 °C and setting at about 32 to 40 °C.
Agar diffusion (disk diffusion) test: Susceptibility method, such as Kirby-Bauer, in which antibiotic-impregnated disks are placed on an agar plate (usually Mueller-Hinton) evenly seeded with the test organism. The size of each inhibition zone shows susceptibility.
Agglutination test: Test in which antibodies cross-link particulate antigens, such as red cells, bacteria or coated latex beads, producing visible clumps. It is widely used in blood grouping and rapid serology.
Airway flow rate: Speed at which air moves in or out of the lungs through the airways, measured during pulmonary function testing. Reduced flow rates suggest obstructive disease such as asthma or COPD.
Alanine aminotransferase (ALT): Enzyme found mainly in liver cells that transfers an amino group from alanine; formerly called SGPT. Because it is fairly liver-specific, a raised serum ALT is a sensitive marker of hepatocellular injury.
Alkaline phosphatase (ALP): Group of isoenzymes active at alkaline pH, found mainly in liver (bile duct lining), bone (osteoblasts), placenta and intestine. Raised serum ALP suggests cholestasis or increased bone turnover; it is normally higher in growing children.
Alpha-fetoprotein (AFP): Glycoprotein made by the fetal yolk sac and liver. Maternal serum AFP is used in screening for neural tube defects, and in adults it is a tumor marker for hepatocellular carcinoma and some germ cell tumors.
Amniocentesis: Procedure in which amniotic fluid is withdrawn by a needle passed through the abdominal and uterine walls into the amniotic sac. The fluid is used for fetal chromosome and genetic testing, infection studies or lung maturity tests.
Amnioscopy: Viewing of the amniotic fluid through the intact membranes with an endoscope inserted through the cervix, to judge its color, for example meconium staining. It is rarely performed today.
Angiography: Imaging of blood vessels after injection of a contrast agent, using X-rays, CT or MRI. It shows narrowing, blockage, aneurysm or bleeding.
Anti-DNase B (ADB): Antibody against deoxyribonuclease B, an enzyme released by group A Streptococcus. A raised titer indicates recent streptococcal infection and complements the ASO test, especially after skin infections.
Antidiuretic hormone (ADH): Hormone, also called vasopressin, made in the hypothalamus and released from the posterior pituitary. It makes the kidney collecting ducts reabsorb water, concentrating the urine.
Antinuclear antibody (ANA) test: Test, usually indirect immunofluorescence on HEp-2 cells, that detects autoantibodies against nuclear components. It is a sensitive screen for systemic lupus erythematosus and related connective tissue diseases, but a positive result is not specific.
Antistreptolysin O (ASO) titer: Measures antibodies to streptolysin O, a toxin made by group A Streptococcus. A raised or rising titer supports recent infection and helps diagnose post-streptococcal complications such as rheumatic fever and glomerulonephritis.
Aorta ultrasonography: Ultrasound imaging of the aorta, most often the abdominal part, used to detect and measure an aneurysm. Same examination as abdominal aorta ultrasound.
Arterial Doppler studies: Ultrasound studies that use the Doppler effect to assess the speed and direction of blood flow in arteries. They detect narrowing or blockage, for example in peripheral arterial disease.
Arteriography of the lower extremities: X-ray imaging of the femoral artery and its branches in the legs after injection of contrast, to locate narrowing or blockage.
Arthrocentesis: Needle aspiration of fluid from a joint space. The synovial fluid is sent to the lab for cell count, crystal examination, Gram stain and culture.
Arthrography: X-ray, CT or MRI imaging of a joint after contrast is injected into the joint capsule, showing cartilage, ligaments and other soft tissue; also called joint radiography.
Arthroscopy: Examination of the inside of a joint through a small fiberoptic instrument called an arthroscope. It allows diagnosis, biopsy and minor surgical repair.
Aspartate aminotransferase (AST): Enzyme found in liver, heart, skeletal muscle, kidney and red cells; formerly called SGOT. Raised serum AST indicates tissue damage, often liver; hemolyzed samples give falsely high results.
Auditory brainstem response (ABR, BAER): Test that records the brain's electrical responses to repeated clicks or tones through scalp electrodes. It assesses the hearing pathway up to the brainstem and is used for newborn hearing screening.

B

Bands (band neutrophils): Slightly immature neutrophils whose nucleus is curved like a band and not yet divided into lobes. An increase (left shift) usually signals acute bacterial infection or inflammation; also called stabs.
Barium enema (BE): X-ray examination of the colon and rectum after barium contrast is introduced through the rectum. It has largely been replaced by colonoscopy and CT colonography.
Barium swallow: X-ray study of the pharynx and esophagus while the patient swallows barium contrast, showing swallowing problems, strictures, reflux and masses; also called esophagography.
Basophilia: A raised absolute basophil count in the blood. A persistent basophilia strongly suggests a myeloproliferative neoplasm, especially chronic myeloid leukemia.
Bence Jones protein: Free immunoglobulin light chains (kappa or lambda) that pass into urine. They point to a plasma cell neoplasm such as multiple myeloma and are detected by urine protein electrophoresis and immunofixation, not routine dipsticks.
Bicarbonate (HCO3-): Main buffer anion in blood, controlled by the kidneys. It is usually reported as total CO2 in electrolyte panels and is low in metabolic acidosis and high in metabolic alkalosis.
Biliary system scan: Nuclear medicine study (HIDA scan) that follows a radioactive tracer through the liver, gallbladder and bile ducts. It detects acute cholecystitis, bile duct blockage and bile leaks.
Bilirubin: Yellow pigment produced when heme from broken-down red cells is degraded. The liver conjugates it for excretion in bile; raised levels cause jaundice. Protect samples from light, which breaks it down.
Bladder ultrasonography: Ultrasound imaging of the urinary bladder, used to judge its wall and contents and to measure urine left after voiding.
Bleeding tendencies: Disorders in which bleeding is excessive or prolonged because of defects in blood vessels, platelets, coagulation factors or overactive fibrinolysis. Screening includes platelet count, PT, aPTT and fibrinogen.
Bleeding time: Largely obsolete test timing how long a small standardized skin cut takes to stop bleeding, reflecting platelet plug formation. It is poorly reproducible and has been replaced by platelet function analyzers and aggregation studies.
Blood buffer system: Chemical systems, chiefly bicarbonate-carbonic acid plus proteins, hemoglobin and phosphate, that resist pH change. They keep arterial blood pH within about 7.35 to 7.45.
Blood urea nitrogen (BUN): Measure of the nitrogen present as urea in serum or plasma. It reflects kidney function but is also raised by dehydration, high protein intake or gastrointestinal bleeding, so it is interpreted alongside creatinine.
Bone scan: Nuclear medicine imaging of the skeleton after injection of a technetium-labeled tracer. Areas of increased bone turnover, such as metastases, fractures or infection, show as hot spots.
Brain scan: Nuclear medicine study of the brain using an injected radioactive tracer, now mostly performed as SPECT or PET to assess blood flow or metabolism.
Breast ultrasonography: Ultrasound imaging of the breast, used to tell solid lumps from fluid-filled cysts and to guide needle biopsy.
Broad casts: Wide urinary casts formed in dilated, damaged distal tubules or collecting ducts where urine flow is slow. They indicate advanced chronic kidney disease and are sometimes called renal failure casts.
Bronchoscopy: Endoscopic examination of the larynx, trachea and bronchi. It allows collection of bronchial washings, bronchoalveolar lavage and biopsies for cytology, histology and microbiology.

C

C-peptide: Connecting peptide cut from proinsulin and released in equal amounts to insulin. Because it is not affected by injected insulin, it measures the patient's own insulin production, e.g. helping to distinguish type 1 from type 2 diabetes.
C-reactive protein (CRP): Acute-phase protein made by the liver that rises within hours of inflammation, infection or tissue injury. It is not an antibody; high-sensitivity CRP is also used for cardiovascular risk.
Calcitonin: Hormone secreted by the parafollicular C cells of the thyroid that lowers blood calcium. In the lab it is mainly a tumor marker for medullary thyroid carcinoma.
Calcium (Ca): Essential positive ion needed for bone, muscle contraction, nerve function and blood clotting. About half is free (ionized) and physiologically active; EDTA and citrate bind calcium, so these samples are unsuitable.
Casts: Cylindrical structures formed in kidney tubules from Tamm-Horsfall protein (uromodulin), which may trap cells or debris. Their type in urine sediment helps localize and characterize kidney disease.
Catheterized specimen: Urine collected through a sterile catheter, either a straight catheter passed for the collection or from the sampling port of an indwelling catheter, never from the drainage bag. It reduces contamination.
Cellophane tape test: Clear adhesive tape pressed on the perianal skin first thing in the morning, before washing, then placed on a slide to look for Enterobius vermicularis (pinworm) eggs.
Cellular casts: Urinary casts containing trapped cells. Red cell casts point to glomerulonephritis, white cell casts to pyelonephritis or interstitial nephritis, and renal tubular epithelial cell casts to tubular injury.
Central nervous system (CNS): The brain and spinal cord. In the lab, CNS infections and bleeding are investigated mainly through cerebrospinal fluid analysis.
Cerebrospinal fluid (CSF): Clear, colorless fluid made mainly by the choroid plexus that circulates through the brain ventricles and around the brain and spinal cord. CSF analysis diagnoses meningitis, bleeding and some neurological diseases.
Cerebrospinal fluid flow scan: Nuclear medicine study in which a tracer injected into the CSF is followed to check the size and patency of CSF pathways and its reabsorption, also called cisternography.
Chem 12: Older name for an automated panel measuring 12 blood chemistry analytes at once. Such panels are now usually ordered as a basic or comprehensive metabolic panel (BMP or CMP).
Chem 24: Older name for an automated panel measuring 24 blood chemistry analytes at once, now largely replaced by standard metabolic and organ panels.
Chest ultrasonography: Ultrasound imaging of the chest, mainly to detect and guide drainage of pleural fluid.
Chest X-ray: Plain radiograph of the chest showing the lungs, heart, great vessels, pleura, mediastinum and bony thorax.
Chloride (Cl-): Main negative ion of extracellular fluid, which moves with sodium to help regulate water balance and acid-base status. It is measured in electrolyte panels, sweat tests and anion gap calculations.
Chocolate agar: Enriched medium made with blood that has been heated so the cells lyse, releasing hemin (X factor) and NAD (V factor). It grows fastidious organisms such as Haemophilus and Neisseria.
Cholecystography: X-ray study of the gallbladder after an oral contrast agent, also called a gallbladder (GB) series. It has been replaced by ultrasound.
Cholesterol: Sterol lipid used to build cell membranes, bile acids, steroid hormones and vitamin D. Serum total, HDL and LDL cholesterol are measured to assess cardiovascular risk.
Chyle: Milky lymph containing chylomicrons (absorbed dietary fat) carried from the intestine through the lymphatics. Finding it in pleural or peritoneal fluid indicates thoracic duct or lymphatic damage.
Cisternal puncture: Collection of CSF by inserting a needle below the occipital bone, between it and the first cervical vertebra, into the cisterna magna. It is rarely used, only when lumbar puncture is impossible.
Cisternography: Nuclear medicine study of CSF flow after tracer is injected into the CSF, showing the patency of CSF pathways and its reabsorption; same as a CSF flow scan.
Clean-catch midstream specimen: Urine collected after cleansing the genital area, passing the first part of the stream and catching the middle portion in a sterile container. It is the standard specimen for urine culture.
Clinitest: Copper-reduction tablet test that detects reducing substances in urine, such as glucose, galactose and lactose, not glucose alone. It is used to screen infants for galactosemia.
Clot retraction test: Old test observing how well a clot shrinks in a tube over time, which depends on platelet number and function. Poor retraction occurs in thrombocytopenia and Glanzmann thrombasthenia.
Coagulation factors: Plasma proteins, most made in the liver, that act in a chain of reactions to produce a fibrin clot. They are numbered by Roman numerals, and several (II, VII, IX, X) need vitamin K.
Cold spots: Areas on a nuclear medicine scan that take up less tracer than surrounding tissue, as seen with some tumors, cysts or poor blood supply.
Colonoscopy: Endoscopic examination of the entire colon from the rectum to the cecum, often including the end of the ileum. It allows removal of polyps and biopsies.
Color-flow Doppler imaging: Ultrasound mode that overlays color on the image to show the direction and speed of blood flow in the heart or vessels.
Colposcope: Low-power binocular magnifying instrument with a light, used to examine the cervix, vagina and vulva.
Colposcopy: Magnified examination of the cervix and vagina with a colposcope, usually after an abnormal Pap smear or HPV test, often with biopsy of suspicious areas.
Common pathway: Final section of the classic coagulation cascade in which activated factor X, with factor V, converts prothrombin to thrombin, which then turns fibrinogen into fibrin. It is assessed by both PT and aPTT.
Complement fixation test: Serological test that detects antibodies (or antigens) by showing that complement has been used up in an antigen-antibody reaction, so indicator sheep red cells are not lysed. Largely replaced by EIA.
Complete blood count (CBC): Automated panel on EDTA blood reporting red cell, white cell and platelet counts, hemoglobin, hematocrit and red cell indices, often with a white cell differential. It is the most requested hematology test.
Computed (axial) tomography (CT, CAT): Imaging technique in which a rotating X-ray source and detectors feed data to a computer that builds detailed cross-sectional images of the body.
Computed tomography (CT) of the body: CT imaging of the chest, abdomen, pelvis, spine or limbs that produces detailed cross-sectional pictures, often with contrast.
Computed tomography (CT) of the brain: CT imaging of the skull and brain, used urgently to detect bleeding, stroke, tumors or injury; images can be reconstructed in three dimensions.
Confirmatory tests: Tests using a different method or principle to check a screening result, for example Acetest, Ictotest or sulfosalicylic acid after a positive urine dipstick. They reduce false positives.
Conn syndrome: Primary hyperaldosteronism, usually from an adrenal adenoma or hyperplasia, causing hypertension and often low potassium. Screened by a raised aldosterone-to-renin ratio.
Continuous-wave Doppler: Doppler ultrasound mode in which one crystal sends and another receives continuously, so very high blood-flow velocities can be measured, although the exact depth cannot be located.
Coombs test (antiglobulin test): Test using anti-human globulin to show antibody or complement on red cells. The direct test (DAT) detects cells coated in the body; the indirect test (IAT) detects red-cell antibodies in serum, e.g. crossmatching.
Coronary angiography: X-ray imaging of the coronary arteries and heart chambers after contrast is injected through a catheter, to locate narrowing or blockage.
Cortisol: Main glucocorticoid hormone from the adrenal cortex, regulating metabolism and stress responses. Levels peak in the early morning, so sample time must be recorded.
Creatine kinase (CK): Enzyme in skeletal muscle, heart and brain, also called creatine phosphokinase (CPK). Raised in muscle injury; its CK-MB isoenzyme was once the main heart attack marker but troponin has replaced it.
Creatinine: Waste product formed at a steady rate from creatine and creatine phosphate in muscle and cleared by the kidneys. Serum creatinine and eGFR are the standard tests of kidney function.
Culdoscopy: Endoscopic view of the pelvic organs through the pouch of Douglas (rectouterine pouch) via the vagina; largely replaced by laparoscopy.
Culture: Growing microorganisms from a specimen on or in suitable media under controlled conditions, so they can be isolated, identified and tested for antimicrobial susceptibility.
Culture and sensitivity (C&S): Combined request to grow and identify a pathogen from a specimen and then test which antimicrobials inhibit it, guiding treatment.
Culture medium: Nutrient preparation, liquid or solidified with agar, that supports microbial growth. Media may be general, enriched, selective or differential.
Cystine: Amino acid formed by two cysteine molecules joined by a disulfide bond. In acid urine it forms colorless hexagonal crystals, which always indicate cystinuria.
Cystinuria: Inherited defect in kidney and gut transport of cystine, ornithine, lysine and arginine. Poorly soluble cystine forms urinary crystals and kidney stones.
Cystography: X-ray imaging of the urinary bladder after it is filled with contrast through a catheter.
Cystometrogram (CMG): Recording of bladder pressure against volume as the bladder is filled, to assess bladder muscle and nerve function.
Cystoscopy: Endoscopic examination of the urethra and bladder, including the ureteral openings and, in men, the prostatic urethra.
Cystourethrogram: X-ray imaging of the bladder and urethra with contrast, often taken during voiding to detect reflux.

D

Differential white cell count (diff): Part of the CBC giving the percentage and absolute number of each white cell type (neutrophils, lymphocytes, monocytes, eosinophils and basophils), by analyzer or stained blood smear.
Diffusing capacity of the lung (DLCO): Lung function test measuring how readily gas passes from the alveoli into the blood, usually using a small amount of carbon monoxide.
Diffusion (pulmonary): Passive movement of oxygen from alveoli into capillary blood and of carbon dioxide in the reverse direction, down their pressure gradients.
Digital subtraction angiography (DSA): Angiography in which a pre-contrast image is digitally subtracted from post-contrast images so only the contrast-filled vessels remain visible.
Dipyridamole-thallium stress test: Nuclear heart perfusion scan in which dipyridamole, a coronary vasodilator, is used instead of exercise to reveal areas of poor blood flow.
Direct (conjugated) bilirubin: Bilirubin joined to glucuronic acid in the liver, making it water-soluble for excretion in bile. It rises in cholestasis and hepatocellular disease and can appear in urine.
Direct fluorescent antibody (DFA) test: Test in which a fluorescent-labeled antibody is applied directly to a specimen; glowing under a fluorescence microscope shows the target antigen is present.
Dobutamine-thallium stress test: Nuclear heart perfusion scan in which dobutamine, a drug that raises heart rate and contractility, replaces exercise for patients unable to exercise.
Doppler blood vessel studies: Ultrasound studies of arteries or veins that measure flow direction and speed to detect narrowing, blockage or clots.
Doppler ultrasound: Ultrasound method that detects the frequency shift of echoes from moving red cells, allowing measurement of blood flow speed and direction.
Dressler syndrome: Pericarditis, often with fever and pleural effusion, developing weeks after a heart attack or heart surgery, thought to be autoimmune. It is now uncommon.
Duke method: Obsolete bleeding time technique using a small puncture of the earlobe and timing until bleeding stops by blotting with filter paper. It is poorly standardized.
Duodenoscopy: Endoscopic examination of the duodenum, usually as part of upper endoscopy or ERCP.
Duplex scanning: Ultrasound that combines real-time gray-scale images of a vessel or organ with Doppler measurement of the blood flowing through it.

E

Echocardiography: Noninvasive ultrasound examination of the heart's chambers, valves, walls and blood flow.
Effusion: Abnormal build-up of fluid in a body cavity such as the pleural, pericardial or peritoneal space. Lab tests classify it as a transudate or exudate.
Electrocardiography (ECG, EKG): Recording from skin electrodes of the heart's electrical activity, used to detect rhythm problems, ischemia and heart attack.
Electrodiagnostic studies: Tests that record the body's electrical activity, such as ECG, EEG, EMG and nerve conduction, to assess heart, brain, nerve and muscle function.
Electroencephalography (EEG): Recording from scalp electrodes of the electrical activity of the brain's cortex, used mainly in epilepsy and sleep disorders.
Electromyography (EMG): Recording of the electrical activity of skeletal muscles, at rest and during contraction, to detect muscle or nerve disease.
Electromyoneurography: Combined study of muscle electrical activity (EMG) and nerve conduction.
Electroneurography (nerve conduction study): Test measuring the speed and strength of electrical signals travelling along peripheral nerves.
Electrophysiology study (EPS): Invasive cardiac test in which catheter electrodes record and stimulate the heart from inside, to map arrhythmias and conduction defects.
Endoscopic retrograde cholangiopancreatography (ERCP): Endoscopic procedure in which contrast is injected into the bile and pancreatic ducts through the duodenal papilla for X-ray imaging; stones can be removed and brushings taken for cytology.
Enzyme immunoassay (EIA): Immunoassay that uses an enzyme label; the enzyme converts a substrate to a colored, fluorescent or light-emitting product showing that antigen-antibody binding has occurred.
Enzyme-linked immunosorbent assay (ELISA): Enzyme immunoassay in which antigen or antibody is fixed to a solid surface, usually a microplate well. An enzyme-labeled reagent produces a color change that is proportional to the analyte in sandwich and indirect formats (inversely proportional in competitive formats).
Eosinopenia: Lower than normal eosinophil count, commonly caused by acute stress, corticosteroids or Cushing syndrome.
Eosinophilia: Raised eosinophil count, typically due to allergy, asthma, parasitic (especially helminth) infection, drug reactions or some neoplasms.
Erythrocyte sedimentation rate (ESR): Distance in millimeters that red cells fall in one hour in a vertical tube of anticoagulated blood (Westergren method). It is a nonspecific marker of inflammation, raised by fibrinogen and globulins.
Esophagogastroduodenoscopy (EGD): Upper endoscopy examining the esophagus, stomach and duodenum, often with biopsies, for example for Helicobacter pylori testing.
Esophagoscopy: Endoscopic examination of the esophagus.
Esterase: Any enzyme that splits esters into an acid and an alcohol. The urine dipstick detects leukocyte esterase released by neutrophils, indicating white cells in urine.
Event marker: Button on a Holter or event monitor that the patient presses when symptoms occur, so the recording can be matched to symptoms.
Evoked response studies: Tests recording the brain's electrical responses to specific visual, sound or touch stimuli, to assess sensory pathways.
Exercise stress test: Test recording ECG, heart rate and blood pressure while the patient exercises on a treadmill or bicycle, to reveal exercise-induced ischemia.
Extrinsic pathway: Coagulation pathway started when tissue factor from damaged tissue binds factor VII, activating factor X. It is assessed by the prothrombin time (PT).
Exudate: Effusion with high protein, LDH and often many cells, formed when inflammation, infection or cancer makes vessels leaky. Light's criteria separate it from a transudate.

F

Factor assay: Test that measures the activity of a single coagulation factor, usually by mixing patient plasma with plasma lacking that factor. It confirms specific deficiencies such as hemophilia A or B.
Fasting blood glucose (FBG, FBS): Plasma glucose after at least 8 hours without calories. A result of 7.0 mmol/L (126 mg/dL) or more, confirmed on repeat, is diagnostic of diabetes; use fluoride tubes if testing is delayed.
Fasting urine specimen: Second urine passed after an overnight fast, discarding the first morning void. It is used mainly for glucose monitoring.
Fatty cast: Urinary cast containing lipid droplets or oval fat bodies, which show Maltese crosses under polarized light. It is typical of nephrotic syndrome.
Ferritin: Main iron-storage protein; serum ferritin reflects body iron stores. A low level confirms iron deficiency, but it rises in inflammation, liver disease and malignancy because it is an acute-phase protein.
Fetal ultrasound: Another name for obstetric ultrasound, which images the fetus, placenta and amniotic fluid.
Fetoscopy: Direct viewing of the fetus inside the uterus with a fine endoscope, now used mainly for fetal surgery.
Fibrin: Insoluble protein strands formed from fibrinogen by thrombin; cross-linked by factor XIII, it forms the mesh that holds a blood clot together.
Fibrinogen (factor I): Soluble plasma protein made by the liver that thrombin converts to fibrin. It is also an acute-phase protein, and low levels occur in DIC and liver failure.
Fibrinogen assay: Test measuring functional fibrinogen in citrated plasma, usually by the Clauss method, in which excess thrombin is added and clotting time is inversely related to fibrinogen level.
Fibrinolysis: Breakdown of fibrin clots by plasmin, releasing fibrin degradation products such as D-dimer.
First morning specimen: First urine passed after waking, ideally after about 8 hours in bed. It is the most concentrated sample, preferred for pregnancy tests and detecting protein and nitrite.
Flocculation: Form of precipitation in which soluble antigen and antibody form fine visible clumps (floccules). VDRL and RPR are flocculation tests.
Fluorescent treponemal antibody absorption (FTA-ABS) test: Indirect immunofluorescence test detecting antibodies specific to Treponema pallidum, after absorbing out cross-reacting antibodies. It confirms syphilis at all stages; because it usually stays positive for life, it cannot distinguish past from current infection or monitor treatment.
Foam stability test: Bedside test of fetal lung maturity in which amniotic fluid is shaken with ethanol; a lasting ring of bubbles shows enough surfactant; also called the shake test. It is now largely obsolete.
Forward (direct) ABO grouping: Blood grouping by mixing the patient's red cells with anti-A and anti-B reagents to detect A and B antigens on the cells.

G

Gallbladder and biliary system scan: Nuclear medicine (HIDA) scan tracing a radioactive tracer through the liver, gallbladder and bile ducts; same as a biliary system scan.
Gallbladder ultrasonography: Ultrasound imaging of the gallbladder and bile ducts; the first-choice test for gallstones.
Gallium-67 scan: Nuclear medicine scan using gallium-67, which collects in areas of inflammation, infection and some tumors such as lymphoma.
Gas exchange tests: Tests measuring how effectively oxygen and carbon dioxide move between the alveoli and the blood, e.g. arterial blood gases and diffusing capacity.
Gastric emptying scan: Nuclear medicine study in which the patient eats a meal labeled with a tracer and imaging measures how quickly the stomach empties.
Gastric secretion analysis: Measurement of the volume and acid output of gastric juice collected by nasogastric tube, sometimes after stimulation. It is now rarely performed except in suspected gastrinoma.
Gastrin: Hormone released by G cells in the stomach antrum that stimulates gastric acid secretion. A very high fasting serum gastrin suggests Zollinger-Ellison syndrome.
Gastroesophageal reflux scan: Nuclear medicine study that detects stomach contents flowing back into the esophagus after a tracer-labeled drink.
Gastrointestinal bleeding scan: Nuclear medicine study using the patient's own red cells labeled with technetium to locate active bleeding in the gut.
Gastrointestinal reflux scan: Nuclear medicine study showing backward flow of labeled gastric contents into the esophagus; same as a gastroesophageal reflux scan.
Gastroscopy: Endoscopic examination of the stomach, usually as part of an upper GI endoscopy (EGD).
Gestational trophoblastic disease: Group of pregnancy-related placental tumors from benign hydatidiform mole to malignant choriocarcinoma. Serum hCG is used for diagnosis and follow-up.
Glucagon: Hormone from pancreatic alpha cells that raises blood glucose by stimulating glycogen breakdown and glucose production in the liver, opposing insulin.
Granular casts: Urinary casts containing fine or coarse granules from broken-down cells or protein aggregates. Few may follow exercise; many, especially muddy brown casts, suggest acute tubular injury.
Gray-scale (B-mode) ultrasound imaging: Ultrasound display in which returning echoes are shown as dots of varying brightness, building a cross-sectional picture of tissue.
Growth hormone (GH): Hormone, also called somatotropin, from the anterior pituitary that promotes growth and influences metabolism, largely by stimulating IGF-1. Its release is pulsatile, so stimulation or suppression tests are used.
Gynecologic laparoscopy: Endoscopic viewing of the uterus, fallopian tubes and ovaries through a small abdominal incision, for diagnosis or surgery.

H

Head and brain MRI: Magnetic resonance imaging of the brain, face and head, including its blood vessels, giving detailed soft-tissue images without radiation.
Hemagglutination inhibition (HAI) test: Test in which patient antibodies prevent a virus from clumping red cells; the highest dilution that still blocks clumping gives the antibody titer. Used for influenza and formerly rubella.
Hematocrit (Hct): Proportion of blood volume occupied by red cells, expressed as a percentage or fraction; also called packed cell volume. It is measured by centrifugation or calculated by analyzers (RBC count × MCV).
Heme: Iron-containing porphyrin ring (protoporphyrin IX with ferrous iron) that binds oxygen in hemoglobin; each hemoglobin molecule has four heme groups.
Hemoglobin (Hb): Iron-containing protein in red cells, made of four globin chains each with a heme group, that carries oxygen from lungs to tissues. Low levels define anemia.
Hemostasis: The body's process for stopping bleeding from a damaged vessel through vessel constriction, a platelet plug, fibrin clot formation and later clot removal.
Hepatobiliary ultrasonography: Ultrasound imaging of the liver, gallbladder and bile ducts.
High-density lipoprotein (HDL): Smallest, densest lipoprotein, which carries cholesterol from tissues back to the liver. Higher HDL cholesterol is linked to lower cardiovascular risk, hence the name 'good cholesterol'.
Holter monitoring: Continuous ECG recording over 24 to 48 hours or longer with a portable device, to catch intermittent arrhythmias during daily life.
Hot spots: Areas on a nuclear medicine scan that take up more tracer than surrounding tissue, as in bone metastases, fractures or infection.
Human chorionic gonadotropin (hCG): Hormone made by the placenta that maintains early pregnancy. It is detectable in serum about 8 to 11 days after conception and is the basis of pregnancy tests; also a tumor marker.
Hyaline cast: Clear, colorless urinary cast made only of Tamm-Horsfall protein (uromodulin). A few are normal, especially after exercise or dehydration; best seen with reduced light or phase contrast.
Hypercoagulable state (thrombophilia): Increased tendency to form blood clots, either inherited (e.g. factor V Leiden) or acquired (e.g. antiphospholipid syndrome, cancer, pregnancy).
Hyperventilation EEG: EEG recorded while the patient breathes deeply and rapidly for a few minutes; the resulting low CO2 can provoke abnormal activity, especially absence seizures.
Hypochromic anemia: Anemia in which red cells have less hemoglobin than normal and look pale, with low MCH and MCHC. It is typical of iron deficiency and thalassemia.
Hysterosalpingography: X-ray imaging of the uterine cavity and fallopian tubes after contrast is injected through the cervix, often in infertility investigation.

I

Ictotest: Tablet test using a diazo reaction to confirm bilirubin in urine. It is more sensitive and less affected by interference than the dipstick.
Inclusion bodies: Abnormal structures inside cells visible on stained smears, such as Döhle bodies in neutrophils or Howell-Jolly bodies in red cells; Heinz bodies need a supravital stain such as crystal violet to be seen. They give clues to infection, toxicity or specific disorders.
Indirect (unconjugated) bilirubin: Water-insoluble bilirubin from heme breakdown, carried on albumin to the liver for conjugation. It rises in hemolysis, Gilbert syndrome and newborn jaundice and does not appear in urine.
Indirect fluorescent antibody (IFA) test: Two-step test: patient antibody binds a fixed antigen on a slide, then fluorescent-labeled anti-human globulin reveals it. Used for ANA and many infectious serologies.
Inherited coagulation factor deficiencies: Congenital lack or dysfunction of a clotting factor, such as factor VIII (hemophilia A), factor IX (hemophilia B) or von Willebrand factor.
Insulin: Hormone from pancreatic beta cells that lowers blood glucose by moving it into cells and promoting its storage as glycogen and fat.
Intermediate (I): Susceptibility category meaning an antimicrobial may work if higher doses are used or it concentrates at the site of infection. EUCAST now calls this 'susceptible, increased exposure'.
Intravascular ultrasound (IVUS): Imaging from inside a blood vessel, usually a coronary artery, using a tiny ultrasound probe on a catheter to show the vessel wall and plaque.
Intravenous urography (IVU): X-ray series taken after intravenous contrast to show the kidneys, ureters and bladder; also called intravenous pyelography. It has largely been replaced by CT urography.
Intrinsic pathway: Coagulation pathway started by contact activation of factor XII and involving factors XI, IX and VIII, all present in blood. It is assessed by the aPTT.
Iodine-131 scan: Nuclear medicine study of thyroid tissue using radioactive iodine-131, mainly to find residual or metastatic thyroid cancer.
Isoenzymes: Different molecular forms of the same enzyme that catalyze the same reaction, e.g. CK-MM, CK-MB and CK-BB or LD1 to LD5. Their pattern can indicate which tissue is damaged.
Ivy method: Bleeding time technique in which a blood pressure cuff is inflated to 40 mmHg and standard cuts are made on the forearm, timing until bleeding stops. Now largely obsolete.

K

Karyotype: Organized display of a cell's chromosomes, arranged by size and banding pattern, prepared from dividing cells. It detects changes in chromosome number and large structural abnormalities.
Kidney scan: Nuclear medicine study of kidney size, shape, position and function.
Kidney ultrasonography: Ultrasound imaging of the kidneys to assess size, cysts, stones, masses and obstruction.
Kidneys, ureters and bladder (KUB) X-ray: Plain X-ray of the abdomen covering the kidneys, ureters and bladder, mainly to look for stones.

L

Lactate dehydrogenase (LD, LDH): Enzyme found in almost all tissues, highest in heart, liver, muscle, kidney and red cells. It rises with tissue damage, hemolysis and some cancers; hemolyzed samples give falsely high results.
Laparoscopy: Examination of the abdominal and pelvic organs through a small incision using an endoscope called a laparoscope.
Latency period: Time between a stimulus and the recorded response, for example in nerve conduction or evoked potential studies.
Latex agglutination: Test in which latex beads are coated with antigen or antibody; a matching analyte in the sample causes visible clumping. Used for rapid antigen tests and rheumatoid factor.
Lecithin: Phosphatidylcholine, the main phospholipid in lung surfactant. Its rise in amniotic fluid late in pregnancy indicates fetal lung maturity.
Lecithin/sphingomyelin (L/S) ratio: Amniotic fluid measure of fetal lung maturity; a ratio of about 2.0 or higher suggests mature lungs. It has largely been replaced by the lamellar body count.
Leukocyte esterase test: Urine dipstick test detecting esterase from neutrophils, including lysed ones, indicating pyuria and supporting urinary tract infection.
Leukocytosis: White blood cell count above the reference range (roughly above 11 × 10^9/L in adults), usually due to infection, inflammation, stress, steroids or leukemia.
Leukopenia: White blood cell count below the reference range, usually due to low neutrophils or lymphocytes.
Limulus amebocyte lysate (LAL) assay: Test using horseshoe crab blood cell extract that clots in the presence of bacterial endotoxin from gram-negative bacteria. It is mainly used to test pharmaceuticals and devices for endotoxin.
Lipase: Pancreatic enzyme that digests triglycerides. Serum lipase above 3 times the upper limit supports acute pancreatitis and is more specific than amylase.
Lipoprotein: Particle of lipids and apolipoproteins that carries cholesterol and triglycerides in blood; main classes are chylomicrons, VLDL, LDL and HDL.
Liver scan: Nuclear medicine study of liver size, shape and function.
Liver ultrasonography: Ultrasound imaging of the liver and bile ducts.
Long-bone radiography: X-ray images of the long bones of the arms and legs.
Low-density lipoprotein (LDL): Lipoprotein formed from VLDL breakdown that carries most blood cholesterol to tissues. High LDL cholesterol is a key cardiovascular risk factor, the 'bad cholesterol'.
Lumbar puncture: Insertion of a needle into the subarachnoid space, usually between L3/L4 or L4/L5, to measure pressure and collect CSF, normally into numbered sterile tubes.
Lung volumes and capacities: Measurements of air in the lungs at different stages of breathing, such as tidal volume, vital capacity and total lung capacity.
Lymphangiography: X-ray imaging of lymph vessels and nodes after contrast injection; rarely used now.
Lymphocytopenia: Lymphocyte count below the reference range, seen e.g. in HIV infection, steroid therapy and severe illness.
Lymphocytosis: Lymphocyte count above the reference range, typical of viral infections, pertussis and chronic lymphocytic leukemia.
Lytes (electrolytes): Informal term for the electrolyte panel: sodium, potassium, chloride and bicarbonate (total CO2).

M

M-mode echocardiography: Echocardiography mode showing movement of heart structures along a single line over time, used to measure wall thickness and chamber size.
Magnesium (Mg): Mainly intracellular ion that is a cofactor for many enzymes and is needed for nerve and muscle function. Hemolysis falsely raises serum magnesium.
Magnetic resonance imaging (MRI): Imaging using a strong magnetic field and radio waves to produce detailed soft-tissue images without ionizing radiation.
Mammography: Low-dose X-ray imaging of the breast for screening and diagnosis of breast cancer.
Maple syrup urine disease (MSUD): Inherited deficiency of the enzyme that breaks down the branched-chain amino acids leucine, isoleucine and valine. They build up, giving urine a sweet smell; detected by newborn screening.
Mean corpuscular hemoglobin (MCH): Average mass of hemoglobin in one red cell, in picograms, calculated from hemoglobin and RBC count.
Mean corpuscular hemoglobin concentration (MCHC): Average hemoglobin concentration in red cells, calculated as hemoglobin divided by hematocrit (g/dL). Low in iron deficiency; high in spherocytosis or with sample errors such as lipemia.
Mean corpuscular volume (MCV): Average red cell size in femtoliters. It classifies anemia as microcytic, normocytic or macrocytic.
Mean platelet volume (MPV): Average platelet size measured by the analyzer. Larger platelets suggest increased production, as in immune thrombocytopenia.
Meconium: Dark green first stool of a newborn, formed in the fetal intestine. Meconium in amniotic fluid may indicate fetal distress; it is also tested for drug exposure.
Mediastinoscopy: Endoscopic examination of the mediastinum through a small neck incision, mainly to biopsy lymph nodes for lung cancer staging.
Microhemagglutination (MHA): Hemagglutination test performed in small volumes, typically in microtiter plates, where red cell clumping is read by eye or with a magnifier.
Microhemagglutination assay for Treponema pallidum (MHA-TP): Treponemal test in which sheep red cells coated with T. pallidum antigen clump in the presence of patient antibodies. It has been replaced by the TP-PA particle agglutination test and immunoassays.
Monocytosis: Monocyte count above the reference range, seen in chronic infections such as tuberculosis, recovery from infection, and chronic myelomonocytic leukemia.
Mucoprotein: Protein carrying a large carbohydrate component, found in mucus and connective tissue. Tamm-Horsfall protein, the matrix of urinary casts, is an example.
Multigated acquisition (MUGA) scan: Nuclear medicine study using labeled red cells to measure how well the heart pumps (ejection fraction) and how its walls move.
Musculoskeletal MRI: MRI of bones, joints, muscles, tendons and ligaments.
Myelography: X-ray or CT imaging of the spinal canal after contrast is injected into the subarachnoid space.

N

Neonatal head ultrasonography: Ultrasound of a newborn's brain through the open fontanelle to detect bleeding or hydrocephalus; also called echoencephalography.
Neurosyphilis: Syphilis infection of the central nervous system, which can occur at any stage. A reactive CSF VDRL is highly specific, supported by raised CSF cells and protein.
Neutropenia: Absolute neutrophil count below the reference range (usually below 1.5 × 10^9/L), increasing the risk of bacterial and fungal infection.
Neutrophilia: Absolute neutrophil count above the reference range, most often caused by bacterial infection, inflammation, stress or corticosteroids.
Nitrite urine test: Dipstick test detecting nitrite made when bacteria, mostly Enterobacterales, reduce dietary nitrate in urine. A positive result suggests UTI; a negative one does not exclude it.

O

Obstetric ultrasonography: Ultrasound imaging of the fetus, placenta and amniotic fluid during pregnancy.
Ocular ultrasonography: Ultrasound imaging of the eye and orbit.
Opalescent: Describes CSF or other fluid that is slightly cloudy and milky, usually from increased white cells or protein, as in meningitis.
Oral glucose tolerance test (OGTT): Test in which plasma glucose is measured fasting and 2 hours after a standard 75 g oral glucose load. It diagnoses diabetes and gestational diabetes; urine glucose is no longer part of it.
Oximetry: Measurement of the oxygen saturation of hemoglobin, either with a CO-oximeter on a blood sample or noninvasively with a pulse oximeter.

P

Packed cell volume (PCV): Percentage of whole blood made up of red cells after centrifugation in a capillary tube; same as hematocrit.
Pancreas ultrasonography: Ultrasound imaging of the pancreas.
Paracentesis: Needle aspiration of fluid from the peritoneal cavity. The ascitic fluid is analyzed for cells, protein, albumin, culture and cytology.
Paramyxoviridae: Family of enveloped, negative-sense single-stranded RNA viruses that includes mumps, measles and parainfluenza viruses. RSV and human metapneumovirus, once grouped here, are now in the separate family Pneumoviridae.
Parathyroid scan: Nuclear medicine scan, usually with sestamibi, to locate overactive parathyroid glands.
Partial thromboplastin time (PTT): Clotting test of the intrinsic and common pathways. The activated form (aPTT), which adds a contact activator, is the version used today.
Pelvic ultrasonography: Ultrasound imaging of the pelvic organs, in women the uterus and ovaries.
Pelviscopy: Endoscopic examination of the pelvic cavity; another term for gynecologic laparoscopy.
Percutaneous transhepatic cholangiography (PTC): X-ray imaging of the bile ducts after contrast is injected through a needle passed through the skin into the liver.
Percutaneous ultrasonic lithotripsy (PUL): Breaking up kidney stones with ultrasonic energy delivered through a probe passed into the kidney via the skin.
Pericardiocentesis: Needle aspiration of fluid from the pericardial sac, for relief of pressure and laboratory analysis.
Peritoneoscopy: Endoscopic examination of the peritoneal cavity; another name for laparoscopy.
Persantine-thallium stress test: Heart perfusion scan using Persantine (dipyridamole) as the stress agent; same as the dipyridamole-thallium stress test.
Phenylketonuria (PKU): Inherited deficiency of phenylalanine hydroxylase, so phenylalanine builds up and damages the brain. Detected by newborn screening and treated with a low-phenylalanine diet.
Phosphate (phosphorus): Mineral mostly stored in bone with calcium, also vital for energy (ATP) and buffering. The lab measures serum inorganic phosphate; delayed separation or hemolysis falsely raises it.
Photic stimulation EEG: EEG recorded while a strobe light flashes at different frequencies, which may provoke seizure activity in photosensitive patients.
Plasmin: Enzyme formed from plasminogen that digests fibrin and fibrinogen to dissolve clots; also called fibrinolysin.
Platelet aggregation: Platelets sticking to one another to form a plug; in the lab, aggregometry measures this response to agents such as ADP, collagen and ristocetin.
Platelet count: Number of platelets per volume of blood, normally about 150 to 400 × 10^9/L. Small clots in the sample or EDTA-induced platelet clumping (pseudothrombocytopenia) give falsely low counts.
Polymerase chain reaction (PCR): Technique that copies a target DNA sequence millions of times through repeated heating and cooling cycles with a heat-stable DNA polymerase. It detects pathogens, mutations and genetic markers in almost any specimen.
Positron emission tomography (PET): Nuclear imaging that detects positron-emitting tracers, such as FDG, to map metabolic activity, e.g. in cancer staging.
Postprandial blood glucose (PPBG): Plasma glucose measured, usually 2 hours after a meal, to monitor diabetes control. It is not a standard diagnostic test; diagnosis uses fasting glucose, OGTT or HbA1c.
Precipitation: Reaction in which soluble antigen and antibody combine in optimal proportions to form a visible insoluble complex, as in immunodiffusion.
Primary (thyroidal) hypothyroidism: Underactive thyroid due to disease of the gland itself, such as Hashimoto thyroiditis. Results show high TSH and low free T4.
Proctoscopy: Endoscopic examination of the anal canal and rectum.
Prostate ultrasonography: Ultrasound imaging of the prostate, usually through the rectum, often to guide biopsy.
Prostate-specific antigen (PSA): Serine protease made by prostate epithelium that liquefies semen. Serum PSA is a marker for prostate cancer but also rises in benign enlargement and prostatitis.
Prostatic acid phosphatase (PAP): Prostate form of acid phosphatase once measured in blood for prostate cancer, now largely replaced by PSA. It is still used in forensic semen detection.
Prothrombin (factor II): Vitamin K-dependent plasma protein made by the liver that is converted to thrombin during coagulation.
Prothrombin time (PT): Clotting test of the extrinsic and common pathways (factors VII, X, V, II and fibrinogen) in citrated plasma. Reported as an INR to monitor warfarin.
Pulmonary diffusion: Movement of oxygen and carbon dioxide across the alveolar-capillary membrane.
Pulmonary perfusion: Blood flow through the lung capillaries, which must match ventilation for efficient gas exchange.
Pulmonary perfusion scan: Nuclear medicine study using labeled albumin particles to map blood flow in the lungs, typically to detect pulmonary embolism.
Pulmonary ventilation: Movement of air between the atmosphere and the alveoli.
Pulmonary ventilation scan: Nuclear medicine study in which a radioactive gas or aerosol is inhaled to show air distribution in the lungs, often combined with a perfusion scan.
Pulse oximetry (SpO2): Noninvasive estimate of arterial oxygen saturation using light absorbance through a finger or earlobe. It can be falsely normal in carbon monoxide poisoning.
Pulsed-wave Doppler: Doppler mode in which one crystal sends short pulses and listens between them, measuring blood flow speed at a chosen depth, but with a limit on the highest measurable velocity.

R

RA cells (ragocytes): Neutrophils in synovial fluid with dark cytoplasmic inclusions of immune complexes, such as rheumatoid factor. Seen in rheumatoid arthritis and other inflammatory arthritis.
Radioactive iodine uptake (RAIU): Measurement of how much of an oral radioactive iodine dose the thyroid takes up over time, used to assess hyperthyroidism.
Radioimmunoassay (RIA): Immunoassay using a radioactive label to measure hormones, drugs and other analytes. Largely replaced by enzyme and chemiluminescent methods.
Radionuclides: Unstable isotopes that emit radiation as they decay; in medicine they are used as tracers for imaging and for therapy.
Random urine specimen: Urine collected at any time without special preparation; convenient for routine screening but can be diluted.
Rapid plasma reagin (RPR) test: Nontreponemal syphilis screen in which charcoal-labeled cardiolipin antigen flocculates with reagin antibodies. Titers monitor treatment; positives need treponemal confirmation.
Rapid slide test (Monospot): Rapid agglutination test for heterophile antibodies that supports a diagnosis of infectious mononucleosis caused by Epstein-Barr virus.
Rapid surfactant test: Quick test estimating the amount of surfactant in amniotic fluid to assess fetal lung maturity; also known as the shake or foam stability test.
Reagent strips (dipsticks): Plastic strips with chemically treated pads that change color to indicate analytes in urine, such as glucose, protein, blood, ketones and nitrite. Timing and storage affect results.
Reagin: Antibodies against cardiolipin-lecithin-cholesterol antigen produced in syphilis; also found in some other conditions (biological false positives). Detected by RPR and VDRL tests.
Red blood cell (RBC) count: Number of red cells per volume of blood, reported as × 10^12/L.
Red blood cell indices: Calculated measures of red cell size and hemoglobin content (MCV, MCH, MCHC) plus RDW, used to classify anemia.
Red blood cell tests: The red cell parameters of the CBC: RBC count, hemoglobin, hematocrit, the indices and RDW.
Resistant (R): Susceptibility category meaning the organism is unlikely to respond to an antimicrobial at normal doses because the result is beyond the clinical breakpoint.
Retrograde pyelography (RP): X-ray imaging of the ureters and renal pelvis after contrast is injected upward through catheters placed during cystoscopy.
Reverse (indirect) ABO grouping: Blood grouping by testing the patient's serum or plasma with known A1 and B red cells to detect anti-A and anti-B antibodies. It must agree with forward grouping.
Rh (D) typing: Test showing whether red cells carry the RhD antigen. It is key for transfusion and preventing hemolytic disease of the fetus and newborn.
Rice bodies: Small white rice-grain-like particles of fibrin and debris in synovial fluid or joint tissue, seen in rheumatoid arthritis and tuberculous arthritis.

S

Scrotal scan: Nuclear medicine study of blood flow to the testes, historically used to differentiate torsion from infection.
Scrotal ultrasonography: Ultrasound imaging of the scrotum and testes, often with Doppler to assess blood flow.
Segmented neutrophils (segs): Mature neutrophils whose nucleus is divided into two to five lobes connected by thin strands; the main circulating white cell.
Sensitivity testing (antimicrobial susceptibility testing, AST): Laboratory testing of a bacterial or fungal isolate against antimicrobials, by disk diffusion, MIC or automated methods, to guide treatment.
Sensory evoked potentials: Electrical responses of the brain recorded after stimulation of a sensory pathway, such as vision, hearing or touch.
Septicemia: Older term for serious infection with organisms multiplying in the bloodstream. Current practice uses 'bacteremia' and 'sepsis'; blood cultures are the key lab test.
Shake test: Rapid test in which amniotic fluid shaken with ethanol shows a stable foam ring if enough surfactant is present, indicating fetal lung maturity.
Sigmoidoscopy: Endoscopic examination of the rectum and sigmoid colon.
Signal-averaged electrocardiography (SAECG): ECG technique that averages many beats to detect tiny late potentials, identifying patients at risk of ventricular arrhythmias, for example after a heart attack.
Single-photon emission computed tomography (SPECT): Nuclear imaging in which a rotating gamma camera builds three-dimensional images of tracer distribution, e.g. heart perfusion.
Skull radiography: Plain X-rays of the skull and facial bones, including the sinuses.
Sleep EEG: EEG recorded during sleep, which can reveal seizure activity not seen while awake and help assess sleep disorders.
SMAC 12: Sequential Multiple Analyzer with Computer, an early Technicon automated analyzer; SMAC 12 was a 12-analyte chemistry profile. The term is historical.
Small-bowel follow-through (SBFT): Series of X-rays following barium from the duodenum through the small intestine to the ileocecal valve.
Sodium (Na+): Main positive ion of extracellular fluid, controlling water distribution, osmolality and blood pressure. Abnormal levels (hyponatremia or hypernatremia) often reflect water balance.
Somatosensory evoked response (SER): Recording of brain and spinal responses to electrical stimulation of peripheral nerves, testing the touch and position-sense pathways.
Special-purpose specimens: Urine specimens collected in a specific way for particular tests, such as fasting, postprandial, timed or 24-hour collections.
Specific gravity (SG): Urine density compared with water, reflecting concentration of dissolved solutes and the kidney's concentrating ability. Usually about 1.005 to 1.030.
Sperm morphology: Assessment of sperm shape (head, midpiece and tail) on a stained smear, reported as percentage of normal forms by WHO strict criteria.
Sperm motility: Proportion of sperm that move and how well, graded as rapidly progressive, slowly progressive, non-progressive or immotile (WHO 2021). Should be assessed within an hour of collection, ideally within 30 minutes.
Spherocytosis: Presence of spherocytes, small round red cells without central pallor, seen in hereditary spherocytosis and warm autoimmune hemolytic anemia. MCHC is often raised.
Sphingomyelin: Phospholipid in amniotic fluid that stays fairly constant during pregnancy, so it is used as the reference in the L/S ratio.
Spinal radiography: X-rays of all or part of the spine.
Spirometry: Lung function test measuring volumes and flow rates of air breathed in and out, such as FEV1 and FVC.
Spleen scan: Nuclear medicine study of spleen size, shape and function.
Spleen ultrasonography: Ultrasound imaging of the spleen.
Sputum: Mucus and other material coughed up from the lower airways, not saliva. A good sample has many pus cells and few squamous cells.
Stabs: Another name for band neutrophils, immature neutrophils with an unsegmented curved nucleus.
Steatorrhea: Excess fat in feces, giving pale, bulky, greasy stools; typical of malabsorption or pancreatic insufficiency.
Stool culture: Culture of feces on selective media to detect enteric pathogens such as Salmonella, Shigella and Campylobacter.
Straight catheter method: Urine collection by passing a sterile catheter through the urethra into the bladder and removing it after collection.
Sulfonamides: Synthetic antibacterial drugs that block folate synthesis. Some form needle-shaped crystals in acid urine.
Supernatant: Clear liquid above the sediment after a sample, such as urine or blood, has been centrifuged.
Suprapubic aspiration: Collection of urine by needle through the abdominal wall above the pubis into the bladder; it gives an uncontaminated specimen for culture.
Surfactant: Mixture of phospholipids, mainly lecithin, and proteins that lowers surface tension in the alveoli, preventing collapse. Premature babies lacking it develop respiratory distress syndrome.
Susceptible (S): Susceptibility category (formerly 'sensitive') meaning an infection is likely to respond to normal doses of the antimicrobial, based on clinical breakpoints.
Sweat test: Test that collects sweat after pilocarpine iontophoresis and measures its chloride. A sweat chloride of 60 mmol/L or more supports cystic fibrosis.
Syndrome of inappropriate ADH secretion (SIADH): Excess antidiuretic hormone secretion causing water retention and dilutional hyponatremia, with inappropriately concentrated urine.

T

T-tube cholangiography: X-ray of the bile ducts using contrast injected through a T-shaped drainage tube left after gallbladder or bile duct surgery.
Testicular scan: Nuclear medicine study of the scrotal contents; same as a scrotal scan.
Testosterone: Main male sex hormone, mostly from the testes, needed for sperm production and male characteristics. Levels are highest in the morning.
Thallium stress test: Nuclear medicine test in which thallium-201 shows heart muscle blood flow at rest and during exercise or drug stress.
Thoracentesis: Needle aspiration of fluid from the pleural space for relief or laboratory analysis.
Thoracic ultrasonography: Ultrasound imaging of the chest wall, pleura and lungs, especially to find pleural fluid.
Thoracoscopy: Endoscopic examination of the pleural cavity, allowing pleural biopsy.
Thrombin (factor IIa): Enzyme formed from prothrombin that converts fibrinogen to fibrin and activates platelets and other factors.
Thrombin time (TT): Time for citrated plasma to clot after adding thrombin. Prolonged by low or abnormal fibrinogen and by heparin or direct thrombin inhibitors.
Thrombocythemia: Persistently high platelet count due to a bone marrow disorder, e.g. essential thrombocythemia; distinguished from reactive thrombocytosis.
Thyroid ultrasonography: Ultrasound imaging of the thyroid gland, especially to assess nodules.
Thyroid-stimulating hormone (TSH): Hormone from the anterior pituitary that stimulates the thyroid to make T4 and T3.
Thyroxine (T4): Main hormone made by thyroid follicular cells; converted in tissues to more active T3. Free T4 is usually measured.
Timed urine specimen: Collection of all urine over a set period, such as 24 hours: discard the first void, collect all later urine, and include the final void.
TORCH panel: Serology for congenital infections: Toxoplasma, Other (e.g. syphilis, parvovirus B19, VZV), Rubella, Cytomegalovirus and Herpes simplex. Targeted testing is now preferred.
Total bilirubin: Sum of direct (conjugated) and indirect (unconjugated) bilirubin in serum.
Total iron-binding capacity (TIBC): Maximum amount of iron that serum proteins, chiefly transferrin, can bind; an indirect measure of transferrin. High in iron deficiency.
Tracer: Radioactive substance given in small amounts so its distribution can be followed; another name for radionuclide tracer.
Transabdominal ultrasound: Ultrasound performed by moving the transducer over the abdomen, e.g. in pregnancy.
Transcranial Doppler studies: Doppler ultrasound through the skull to measure blood flow in the brain's main arteries.
Transesophageal echocardiography (TEE): Echocardiography using a probe in the esophagus behind the heart for clearer images of valves and chambers.
Transferrin: Main plasma protein, made by the liver, that carries iron. Levels rise in iron deficiency and fall in inflammation.
Transferrin saturation: Percentage of transferrin binding sites occupied by iron, calculated as serum iron ÷ TIBC × 100. Low in iron deficiency; high in iron overload.
Transtracheal aspiration: Collection of lower respiratory secretions by needle and catheter through the cricothyroid membrane, bypassing mouth flora. Rarely performed now.
Transudate: Effusion with low protein and few cells caused by altered pressure, as in heart failure or cirrhosis, not local inflammation.
Transvaginal ultrasound: Ultrasound using a probe, covered with a sterile sheath, inserted into the vagina for close views of the uterus, ovaries or early pregnancy.
Triglycerides: Main storage form of fat, three fatty acids on glycerol. They rise sharply after meals, so a 9 to 12 hour fast was traditionally required; many guidelines now accept non-fasting samples for routine screening and reserve fasting samples for high results.
Triiodothyronine (T3): Most active thyroid hormone, mostly produced from T4 in tissues. Measured mainly when hyperthyroidism is suspected.
Triple renal scan: Nuclear medicine study assessing kidney perfusion, function and excretion.
TSH test: Immunoassay measuring serum TSH; it is the first-line thyroid function test. High in primary hypothyroidism, low in hyperthyroidism.
Two-dimensional (2-D) echocardiography: Echocardiography mode showing real-time cross-sectional images of the heart.
Type 1 diabetes mellitus (formerly IDDM): Diabetes caused mainly by autoimmune destruction of pancreatic beta cells, leading to absolute insulin deficiency. Autoantibodies (e.g. GAD) and low C-peptide support the diagnosis.
Type 2 diabetes mellitus (formerly NIDDM): Diabetes caused by insulin resistance combined with gradually failing insulin secretion, leading to raised blood glucose. Diagnosed by fasting glucose, OGTT or HbA1c.
Tyrosine: Amino acid made from phenylalanine; precursor of thyroid hormones, catecholamines and melanin.

U

Upper gastrointestinal (UGI) series: X-ray study of the esophagus, stomach and duodenum after barium is swallowed.
Uptake: Accumulation of a radionuclide in a tissue, for example the thyroid.
Urea: Main nitrogen waste product of protein breakdown, made in the liver from ammonia and excreted by the kidneys.
Urethral pressure profile: Measurement of pressure along the urethra, as a catheter is withdrawn, to assess urinary continence.
Uric acid: End product of purine breakdown. High levels cause gout and kidney stones; urate crystals are seen in joint fluid.
Urine sediment: Material that settles after centrifuging urine, examined microscopically for cells, casts, crystals and organisms.
Urobilinogen: Colorless compound formed from bilirubin by gut bacteria; some is reabsorbed and excreted in urine. Low or absent in bile duct blockage.
Urodynamic studies: Tests measuring bladder and urethral pressures and urine flow to assess bladder function.
Uroflowmetry: Measurement of urine flow rate over time during urination; low flow suggests obstruction.

V

Venereal Disease Research Laboratory (VDRL) test: Nontreponemal flocculation test for reagin antibodies used to screen for syphilis. It is the preferred test for neurosyphilis on CSF.
Venography of the lower extremities: X-ray imaging of the leg veins after contrast is injected, e.g. for clots; largely replaced by ultrasound.
Venous Doppler studies: Ultrasound of the veins to assess blood flow and detect clots, e.g. deep vein thrombosis.
Ventricular puncture: Collection of CSF directly from a lateral ventricle of the brain, usually through a surgical drain or shunt.
Very-low-density lipoprotein (VLDL): Triglyceride-rich lipoprotein made by the liver, which is broken down to LDL. Raised in hypertriglyceridemia.
Visual evoked response (VER): Recording of the visual cortex responses to flashes or a changing checkerboard, testing the optic pathways, e.g. in multiple sclerosis.

W

Waxy cast: Brittle, highly refractive urinary cast with cracked edges, formed by degeneration of other casts in slow-flowing tubules. It indicates advanced kidney disease.
Western blot: Method that separates proteins by gel electrophoresis, transfers them to a membrane and probes them with antibodies. Once used to confirm HIV, it has been replaced by newer assays.
White blood cell (WBC) count: Number of white cells per volume of blood, part of the CBC; normally about 4 to 11 × 10^9/L in adults.
Wound culture: Culture of a wound swab or, preferably, tissue or pus, for aerobic and anaerobic pathogens.

X

Xanthochromia: Yellow, orange or pink tint of CSF supernatant, usually from red cell breakdown after subarachnoid hemorrhage; also high protein or jaundice.

Z

Zollinger-Ellison syndrome: Condition caused by a gastrin-secreting tumor (gastrinoma), leading to high gastric acid and severe peptic ulcers. Fasting serum gastrin is markedly raised.
Zone of inhibition: Clear area around an antibiotic disk where bacteria did not grow. Its diameter is compared with breakpoints to classify the isolate as S, I or R.