A physician suspects a qualitative platelet defect in a young child. What may be the most useful screening test listed below?

• A. PT
• B. aPTT
• C. PFA-100 EPI/COLL
• D. TT

Answer: (C)

When a qualitative platelet defect is suspected, you want a test that reflects how platelets actually function to form a primary plug, not one that only looks at the coagulation cascade. The PFA-100 using a collagen cartridge with epinephrine (EPI/COLL) measures the time it takes for a platelet plug to form in whole blood as it flows through a small aperture under shear. A prolonged closure time suggests a problem with platelet function or with von Willebrand factor, making this screen particularly sensitive to qualitative platelet disorders.

In contrast, the PT and aPTT assess the extrinsic and intrinsic coagulation pathways, respectively, and are not reliable indicators of platelet function. The thrombin time evaluates the conversion of fibrinogen to fibrin and can be affected by fibrinogen abnormalities or certain inhibitors, but it does not test platelet function either. So, for a first look at suspected platelet function defects, the PFA-100 EPI/COLL is the best screening choice.

Keep in mind that PFA-100 results can be influenced by hematocrit, platelet count, and recent antiplatelet drug exposure; abnormal results should be followed by targeted tests such as platelet aggregation studies and von Willebrand testing to confirm the diagnosis.