Prothrombin Time

a.k.a PT is a laboratory test used to measure the integrity of the extrinsic coagulation pathway and commonly used to monitor warfarin (Coumadin) therapy. Normal range: 10-13 seconds. It is sensitive to detecting deficiencies in the extrinsic pathway such as factor VII and common pathway factors I, II, V, and X (Note: Common pathway coagulation factors effect both PT and PTT). The test is performed by taking citrated plasma to clot at 37° C after the addition of calcium. Tissue factor is then added and the time it takes to clot is measured. The resultant PT is normalized with the International Normalized Ratio (INR) (see INR). PT is typically increased in Coumadin therapy, vitamin K deficiency, liver disease, DIC, and deficiencies in extrinsic or common pathway coagulation factor(s).