Differentiate PT/INR and aPTT in terms of pathways assessed and clinical uses?

• A. PT/INR assess extrinsic and common pathways (factor VII) and monitor warfarin; aPTT assesses intrinsic and common pathways and monitor heparin.
• B. PT/INR assess intrinsic pathways and monitor heparin; aPTT assess extrinsic pathways and monitor warfarin.
• C. PT/INR measure platelet function; aPTT measure vitamin K levels.
• D. PT/INR and aPTT both assess only the common pathway.

Answer: (A)

PT/INR and aPTT test different coagulation pathways and are used for different anticoagulant therapies. PT/INR primarily reflects the extrinsic pathway through factor VII and the common pathway, which is why it is the standard test for monitoring vitamin K–antagonist therapy like warfarin. The INR standardizes PT results so they are comparable between laboratories, which is important for adjusting warfarin dosage.

aPTT, on the other hand, evaluates the intrinsic pathway plus the common pathway, involving factors VIII, IX, XI, XII and the shared factors II, V, X, I. This makes it the test of choice for monitoring unfractionated heparin therapy, since heparin mainly prolongs the intrinsic pathway.

The other statements don’t fit because they misstate which pathways each test assesses or what they monitor—PT/INR does not reflect the intrinsic pathway or platelet function, and aPTT does not measure vitamin K levels.