MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 35
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
Some Kidd antibodies are missed in the antiglobulin test when only anti-IgG reagent is used. Why?
Kidd antibodies often bind complement, and some examples are detected mainly through C3 on the cells. Polyspecific AHG containing anti-C3 may therefore detect them better.
A D-negative mother with anti-D delivers a baby whose cord cells type D-negative but have a strongly positive DAT with anti-IgG. The most likely explanation is:
Heavy coating with maternal IgG anti-D can block D sites so monoclonal anti-D cannot bind (blocked D). Gentle heat elution or RHD genotyping shows the true type.
Which laboratory finding may be seen transiently soon after TRALI?
Neutrophils are sequestered in the lung, so a sudden transient drop in white cell count can occur. A high BNP suggests TACO rather than TRALI.
Why may a vapor pressure osmometer miss a raised osmolal gap in methanol poisoning?
Volatile compounds like methanol and ethanol escape into the vapor phase and are not counted by vapor pressure osmometry. Freezing point depression osmometry detects them.
A patient on biotin shows suppressed TSH with high free T4 on streptavidin–biotin assays but no symptoms. The correct approach is:
Biotin interference can mimic hyperthyroidism (falsely low TSH, falsely high free T4). Stopping biotin for a suitable period (often at least 2–3 days for high doses) or using another platform resolves it.
Total CK is 5000 U/L and CK-MB is 40 U/L. What is the relative index, and the likely source?
Relative index = CK-MB ÷ total CK × 100 = 40 ÷ 5000 × 100 = 0.8%. Values below about 2.5–3% point to a skeletal muscle source despite a raised CK-MB.
A patient with stage 5 chronic kidney disease has chest discomfort. hs-cTnT is 40 ng/L (99th percentile 14 ng/L) at 0 h and 41 ng/L at 3 h. The best interpretation is:
Acute MI needs a rising and/or falling troponin pattern with clinical evidence of ischemia. A stable value above the 99th percentile indicates chronic myocardial injury, common in CKD.
An asymptomatic patient has troponin I 600 ng/L with a normal ECG. The same sample gives <5 ng/L on another manufacturer's assay. The most likely cause is:
Large disagreement between immunoassays in a clinically unlikely setting suggests interfering antibodies (heterophile or macro-troponin). Blocking reagents or dilution studies can confirm this.
A heart failure patient is started on sacubitril/valsartan. Which natriuretic peptide test is preferred for monitoring?
Sacubitril inhibits neprilysin, which breaks down BNP, so BNP rises from the drug itself. NT-proBNP is not a neprilysin substrate and better reflects cardiac status.
Direct spectrophotometric (bilirubinometer) measurement of neonatal bilirubin reads absorbance at 454 nm and 540 nm. The 540 nm reading is used to:
Bilirubin absorbs strongly at 454 nm, but hemoglobin also absorbs there; measuring at 540 nm allows correction for hemoglobin. This method suits neonates, whose serum lacks carotenoids that would interfere.
A serum sample with marked in-vitro hemolysis is used for protein electrophoresis. Which region is most likely to show a false increase?
Free hemoglobin and hemoglobin–haptoglobin complexes migrate in the alpha-2 to beta area and can look like an abnormal band there. Hemolysis does not add to the gamma fraction.
A patient with pancreatic cancer has an undetectable CA 19-9. Which explanation is most likely?
CA 19-9 is a sialylated Lewis a antigen; Lewis-negative people cannot make it. Biliary obstruction usually raises CA 19-9, not lowers it.
Which test helps separate iron deficiency from anemia of chronic disease because it rises in iron deficiency but is little affected by inflammation?
Iron-deficient erythroid cells express more transferrin receptors, raising sTfR; inflammation has little effect. Ferritin, by contrast, rises in inflammation.
Which hemoglobin derivative is NOT converted to hemiglobincyanide in the reference hemoglobin method?
All common hemoglobin forms except sulfhemoglobin are converted to hemiglobincyanide. Carboxyhemoglobin converts slowly but is still measured.
A Pakistani patient has a band in the HbS position on alkaline electrophoresis, but a negative solubility test. On acid (citrate agar, pH 6.0–6.2) electrophoresis the band moves with HbA. The most likely variant is:
HbD and HbG move with HbS at alkaline pH but with HbA at acid pH, and they do not sickle. HbS keeps a separate position on acid agar and gives a positive solubility test.
Using an acid-elution (Kleihauer) stain or anti-HbF flow cytometry, HbF is spread evenly in all red cells of a healthy adult with HbF 25%. This pattern suggests:
Deletional HPFH shows a pancellular HbF distribution, while delta-beta thalassemia and hydroxyurea therapy give a heterocellular pattern. Fetomaternal hemorrhage shows a small separate population of strongly stained cells.
A patient with marked eosinophilia and raised serum tryptase has a cryptic 4q12 deletion producing FIP1L1::PDGFRA. Why is this finding important?
FIP1L1::PDGFRA myeloid/lymphoid neoplasms with eosinophilia are highly sensitive to low-dose imatinib, which inhibits the PDGFRA kinase. It is a clonal, not reactive, process.
In the Grocott methenamine silver (GMS) stain for fungi, what is the role of chromic acid?
Chromic acid oxidizes fungal wall polysaccharides to aldehydes, which then reduce methenamine silver to black metallic silver. Light green is the usual counterstain.
In radial immunodiffusion read at endpoint (Mancini method), antigen concentration is proportional to:
At completion (Mancini), diameter squared is directly proportional to antigen concentration. In the timed Fahey method, diameter is proportional to the log of concentration.
A patient with post-streptococcal glomerulonephritis after impetigo has a normal ASO. Which antibody test is more likely to be elevated?
Streptolysin O is partly inactivated by skin lipids, so ASO response after skin infection is weak. Anti-DNase B rises after both pharyngeal and skin infections.