What does the prothrombin time (PT) test assess, and what does prolongation of PT/INR indicate?

• A. Measures the intrinsic coagulation pathway; prolongation indicates factor XII deficiency.
• B. Assesses the extrinsic coagulation pathway; prolongation can indicate liver dysfunction, vitamin K deficiency, or warfarin effect.
• C. Reflects platelet function; prolonged indicates thrombocytopenia.
• D. Evaluates kidney function; prolonged indicates nephron damage.

Answer: (B)

The prothrombin time assesses the extrinsic pathway of coagulation and the common pathway. It uses tissue factor (thromboplastin) and calcium to trigger clot formation and primarily reflects the function of factors II, V, VII, and X (with VII being vitamin K–dependent and having a short half-life). A prolonged PT or INR means there is reduced activity of these factors. This can occur with liver dysfunction (where clotting factors are synthesized), vitamin K deficiency (needed to activate these factors), or warfarin therapy (which inhibits vitamin K–dependent factor activation). INR is used to standardize PT results across laboratories. It’s not a test of the intrinsic pathway (that’s measured by the aPTT) and it doesn’t directly assess platelets or kidney function.