Prothrombin time (PT)

Last updated: Wednesday, 31, March, 2004

MBS Online Search Page: Click here for details

Item Process
Specimen

4.5 mL blood added to 0.5 mL citrate.

Method

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.

Application

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).

Interpretation

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.

Reference

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