What does PT measure?

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Prothrombin time measures how long it takes for a fibrin clot to form after adding tissue factor (thromboplastin) and calcium to citrated plasma. This test specifically assesses the extrinsic coagulation pathway and the common pathway—primarily reflecting the function of factor VII and the downstream factors II, V, X, plus fibrinogen. Clinically, it’s used to monitor warfarin therapy and to evaluate conditions affecting vitamin K–dependent factors or liver synthetic function. It is not a measure of plasma volume, which would involve volume assessment; nor does it test platelet function, which is assessed by platelet aggregation tests or bleeding time. It also isn’t a direct measure of fibrinogen level, which is evaluated with dedicated fibrinogen assays. The result is reported in seconds and often standardized as INR.