Prothrombin time

Keywords: PT


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:

Refer to laboratory. 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 warfarin therapy.

See INR.


An abnormal result is most often due to liver disease, vitamin K deficiency, warfarin therapy, dilutional coagulopathy or DIC.

To allow better standardisation in the case of warfarin 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.


Bain et al. Dacie and Lewis Practical Haematology. 11th Ed. 2012.

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

Go Back

Page last updated:

Copyright © 2021 RCPA. All rights reserved.