The prothrombin time (PT) and its derived measures of prothrombin ratio (PR) and international normalized ratio (INR) are measures of the extrinsic pathway of coagulation. They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage, and vitamin K status. The reference range for prothrombin time is usually around 12–15 seconds; the normal range for the INR is 0.8–1.2. PT measures factors II, V, VII, X and fibrinogen.

It is used in conjunction with the activated partial thromboplastin time (aPTT) which measures the intrinsic pathway. The prothrombin time is most commonly measured using blood plasma. Blood is drawn into a test tube containing liquid citrate, which acts as an anticoagulant by binding the calcium in a sample.

The blood is mixed, then centrifuged to separate blood cells from plasma. In newborns, whole blood is used.[citation needed] The plasma is analyzed by a Biomedical scientist on an automated instrument at 37°C, which takes a sample of the plasma. An excess of calcium is added (thereby reversing the effects of citrate), which enables the blood to clot again.

For an accurate measurement the proportion of blood to citrate needs to be fixed; many laboratories will not perform the assay if the tube is underfilled and contains a relatively high concentration of citrate. If the tube is underfilled or overfilled with blood, the standardized dilution of 1 part anticoagulant to 9 parts whole blood is no longer valid. For the prothrombin time test the appropriate sample is the blue top tube, or sodium citrate tube, which is a liquid anticoagulant.

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