prothrombin time (PT), a one-stage test for detecting certain plasma coagulation defects caused by a deficiency of factors V, VII, or X. Thromboplastin and calcium are added to a sample of the patient’s plasma and simultaneously to a sample from a normal control. The amount of time required for clot formation in both samples is observed. Thrombin is formed from prothrombin in the presence of adequate calcium, thromboplastin, and the essential tissue coagulation factors. A prolonged PT therefore indicates deficiency in one of the factors, as in liver disease, vitamin K deficiency, or anticoagulation therapy with the drug warfarin sodium. Normal findings of prothrombin time are 11 to 12.5 seconds. Compare partial thromboplastin time, International Normalized Ratio. See also blood clotting.
