Prothrombin time (PT)

Last updated: Wednesday, 31, March, 2004

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Item Process

4.5 mL blood added to 0.5 mL citrate.


Recalcification of platelet-poor plasma at 37°C in the presence of a reagent with 'tissue factor' activity.

Measure time to clot.

Reference Interval

Reagent dependent; prothrombin time generally 11-15 seconds.


More sensitive than the APTT for the detection of coagulation factor deficiencies due to vitamin K deficiency, liver disease.

Screen for deficiency of factor VII and, with APTT, factors X, V, II, I.

The results are expressed as an INR when the test is used to monitor oral anticoagulant therapy.
See  International normalised ratio (INR).


An abnormal result is most often due to liver disease, vitamin K deficiency or oral anticoagulant therapy.

To allow better standardisation in the case of oral anticoagulant therapy and comparison amongst laboratories, the PT is commonly adjusted according to the formula INR =PTISI, where the ISI is the international sensitivity index of the thromboplastin used by the laboratory.


Kovaks MJ et al. Thromb Haemost 1994; 71: 727-730.