Prothrombin Time

The Prothrombin Time (PT) is one of the oldest coagulation tests. It measures the combined activity of the clotting factors of the so-called 'Extrinsic Pathway. These are shown in yellow in the diagram to the right. Typically, the PT is used for one for three reasons:

1. To screen for coagulation factor deficiencies in a patient with a possible bleeding disorder. The PT should always be performed in parallel with the APTT to aid interpretation of the results.
2. To monitor oral anticoagulants such as coumadin. In these cases, the result of the PT is converted to an International Normalized Ratio (INR) to facilitate comparison with other laboratories.
3. As a screening test of liver function. The PT - especially using modern reagents - is very sensitive to the plasma levels of Factor VII. Because this protein has a very short half-life (~3 hours) it falls quickly when hepatic function is impaired and the PT is therefore an early indicator of hepatic insufficiency.

An unexplained prolonged PT should be followed up with a mixing study in which the patient's plasma is mixed in equal olumes with normal plasma. Failure of the PT of the mixture to correct to a time close to that of the normal plasma alone suggests the presence of an inhibitor. It should be noted that patients on coumadin may appear to have a mild inhibitor in mixing studies. This is because their plasma contains decarboxylated forms of the Vitamin K-dependent clotting factors (II, VII and X) that are relatively non-functional - this is the so-called PIVKA (Proteins Induced by Vitamin K Absence or Antagonism) effect.

In the Florida Hospital Center for Thrombosis Research, the normal range for PT is 7.9 - 10.3 seconds.