MLT GlobeQCHP exam preparation (Qatar) – page 35
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
A pregnant woman's anti-D titer was 8 at 20 weeks. The frozen earlier sample is tested in parallel with a new sample at 24 weeks. Which new titer indicates a significant rise?
Titration has an error of about one dilution, so only a rise of two or more dilutions (fourfold, e.g. 8 to 32) is significant. A change from 8 to 16 may be technical variation.
A cold-reacting anti-M shows stronger reactions after the plasma is:
Many examples of anti-M are pH-dependent and react better when the test is acidified. DTT would destroy IgM anti-M rather than enhance it.
Red cells lacking all Rh antigens (Rhnull) typically show:
Rh proteins, with RHAG, maintain membrane integrity. Rhnull cells are stomatocytic and spherocytic with shortened survival, causing mild hemolytic anemia.
Why is a sample from a patient with extreme leukocytosis at risk of 'leukocyte larceny'?
Very high white cell or platelet counts consume oxygen rapidly after collection, causing spurious hypoxemia. Immediate analysis (or chilling if a delay is unavoidable) and comparison with pulse oximetry help identify it.
Most enzymatic lactate methods and sensors measure:
Lactate oxidase and lactate dehydrogenase methods are specific for L-lactate. D-lactic acidosis (e.g. short bowel syndrome) needs a D-lactate-specific assay.
Using eAG (mg/dL) = 28.7 × HbA1c − 46.7, what is the estimated average glucose for an HbA1c of 7.0%?
28.7 × 7.0 = 200.9; 200.9 − 46.7 = 154 mg/dL (about 8.6 mmol/L). 183 mg/dL corresponds to an HbA1c of about 8%.
PTH samples are best collected into EDTA because:
Intact PTH is degraded by proteases, and it is more stable in EDTA plasma, especially if separation is delayed. Samples should be processed promptly.
Diadenosine pentaphosphate and AMP are added to CK reagents to:
Adenylate kinase from red cells or platelets also forms ATP and would falsely raise CK; these inhibitors block it. Sulfhydryl protection is the role of N-acetylcysteine.
A markedly icteric sample (total bilirubin 20 mg/dL, 342 µmol/L) is tested for cholesterol with a peroxidase (Trinder) end-point method. The expected bias is:
Bilirubin reacts with hydrogen peroxide and competes in the peroxidase step, causing negative interference in Trinder-type assays such as cholesterol and enzymatic creatinine.
Immunofixation shows a sharp band in the lambda lane only; the IgG, IgA and IgM lanes show no matching band. The best next step is:
A light-chain band with no matching G, A or M heavy chain is either free lambda or an uncommon IgD or IgE monoclonal protein. Testing with anti-IgD and anti-IgE separates these.
The uricase method for uric acid measures a decrease in absorbance at 293 nm because:
Uricase converts uric acid, which absorbs UV at about 293 nm, into allantoin, which does not, so absorbance falls. Many analysers instead couple the H2O2 formed to a peroxidase colour reaction.
A patient with plasma glucose above 600 mg/dL (33 mmol/L) has a falsely high MCV on an impedance analyzer. The reason is that:
Cells adapted to hyperosmolar plasma take up water in the diluent before sizing, raising MCV and Hct and lowering MCHC. Glucose does not interfere with the hemoglobin absorbance reading.
Using a Miller disc, 30 reticulocytes are counted in the large squares and 200 red cells in the small squares (area one-ninth of the large square). What is the reticulocyte percentage?
Retic % = reticulocytes in large squares ÷ (RBCs in small squares × 9) × 100 = 30 ÷ 1800 × 100 = 1.7%. 15% comes from forgetting to multiply the small-square count by 9.
A man has an isolated raised hemoglobin, normal white cells and platelets, and a family history of the same. The oxygen P50 is low. The most likely cause is:
High-affinity variants hold oxygen tightly (low P50), causing tissue hypoxia and a compensatory rise in erythropoietin and red cells. HbM variants cause cyanosis from methemoglobin, not erythrocytosis with low P50.
An elderly woman with sudden large bruises has a prolonged aPTT that corrects in an immediate 1:1 mix but becomes prolonged again after 2 hours at 37 °C. This suggests:
Factor VIII autoantibodies are time- and temperature-dependent, so prolongation appears after incubation. Lupus anticoagulant usually acts immediately and causes thrombosis, not bruising.
Both liver failure and DIC can give low platelets, long PT and low fibrinogen. Which result favors DIC?
Factor VIII is made largely outside hepatocytes and is often normal or high in liver disease, but it is consumed in DIC. Factor VII falls in both.
For assigning myeloid lineage in mixed-phenotype acute leukemia, which marker is the most specific?
MPO (by flow or cytochemistry) is the key marker for myeloid lineage in MPAL. CD13, CD33 and CD117 can be expressed aberrantly in lymphoblastic leukemias.
The Bethesda System notes the presence of a transformation zone component when at least how many well-preserved endocervical or squamous metaplastic cells are seen?
At least 10 well-preserved endocervical or metaplastic cells, singly or in groups, indicate sampling of the transformation zone. Its absence is reported as a quality indicator.
During suspected anaphylaxis, which serum marker is most useful to confirm mast cell activation if collected within about 1–3 hours?
Tryptase is released by activated mast cells, peaks around 1 hour and remains elevated for several hours, so it helps confirm anaphylaxis. Total IgE does not rise acutely.
Which IgG subclass is least able to activate complement by the classical pathway?
IgG3 and IgG1 activate complement strongly, IgG2 weakly, and IgG4 essentially not at all. IgG4 is also known for Fab-arm exchange.