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abnormalities in coagulation factors VII, X, II or I. Disease processes expected to prolong the PT include Vitamin
abnormalities in coagulation factors VII, X, II or I. Disease processes expected to prolong the PT include Vitamin
K antagonists, liver disease and DIC.
K antagonists, liver disease and DIC.
Definition
The PT measures the time necessary to generate fibrin after activation of factor VII. It measures the integrity of the "extrinsic" and "common" pathways (factors VII, V, X, prothrombin, and fibrinogen).
Technique
Citrated plasma and an activating agent (usually thromboplastin extracted from animal brain) are incubated at 37°C. The plasma is recalcified and the time is measured until fibrin filaments are observed. Each laboratory has its own normal value, usually between 12 and 15 seconds.
Basic Science
As with the interpretation of a prolonged aPTT, a prolonged PT may reflect either factor deficiency or a circulating inhibitor of coagulation. The distinction is made by repeating the test after a 1:1 mix with normal plasma.
The test is more sensitive than the aPTT for deficient levels of factors, and a relatively small drop in factor VII levels may prolong the PT.
Clinical Significance
Inherited deficiency of factor VII is a rare bleeding disorder characterized by a prolonged PT and a normal aPTT. The PT completely corrects when mixed with normal plasma. Acquired deficiencies are usually related to liver disease, warfarin therapy, or depletion secondary to consumptive coagulopathy, severe bleeding, or massive transfusion.
Circulating inhibitors are most often directed at factor X or thrombin. Most common are heparin or products of fibrinolysis. In their presence the prolonged PT cannot be completely corrected to normal in a 1:1 mixing study.
===(A)PTT===
===(A)PTT===