USMLE STEP 3 MCQ 30: A middle aged woman is suspected of having a coagulation disorder affecting the intrinsic pathway

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Question:
A middle aged woman is suspected of having a coagulation disorder affecting the intrinsic pathway before the prothrombin to thrombin stage. Which of the tests would be most appropriate in this patient ?

 

a) Activated partial thromboplastin time
b) Venous clotting time
c) Activated whole blood clotting time
d) Thrombin time
e) Plasma fibrinogen


Correct Answer: A

Explanation:
The APTT is very sensitive to coagulation factor deficiencies within the intrinsic system before the prothrombin to thrombin stage. The precursor of the activated partial thromboplastin time (APTT) was the partial thromboplastin time (PTT). An incomplete thromboplastin reagent plus calcium is added to patient plasma, and the time necessary to form a fibrin clot is measured. The partial thromboplastin reagent is only a phospholipid platelet substitute without any of the other components of thromboplastin. The PTT was useful in detecting intrinsic factor abnormalities but was relatively insensitive to effects of heparin. Adding certain contact activators (usually chemicals or particulate matter, such as kaolin) to the PTT reagent was found to activate factor XII (contact factor) swiftly and uniformly and thus eliminate another variable in the clotting process. In addition, the activated APTT was found to be sensitive to heparin. The APTT is very sensitive to coagulation factor deficiencies within the intrinsic system before the prothrombin to thrombin stage. It may also be abnormal in prothrombin or fibrinogen deficiencies but only if the defect is relatively severe (prothrombin or fibrinogen/fibrin abnormalities may affect the test because the test depends on fibrin clot formation as the reaction end point).


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