A blood test measuring how long it takes blood to clot via the extrinsic pathway, normally 11-13 seconds. It's used to monitor warfarin therapy and assess liver function.
From Greek 'pro' (before) + 'thrombos' (clot), named because prothrombin is the precursor to thrombin, the key clotting enzyme. First developed in 1935 by Armand Quick to study bleeding disorders.
This test literally recreates a blood clot in a test tube by adding tissue factor and calcium to plasma - it's like conducting a mini bleeding emergency in the lab! The PT/INR is so crucial for warfarin dosing that it has prevented countless strokes and bleeding deaths.
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