The PT test is used to monitor patients taking certain medications as well as to help diagnose clotting disorders.
Patients who have problems with delayed blood clotting are given a number of tests to determine the cause of the problem. The prothrombin test specifically evaluates the presence of factors VIIa, V, and X, prothrombin, and fibrinogen. Prothrombin is a protein in the liquid part of blood (plasma) that is converted to thrombin
bin as part of the clotting process. Fibrinogen is a type of blood protein called a globulin; it is converted to fibrin during the clotting process. A drop in the concentration of any of these factors will cause the blood to take longer to clot. The PT test is used in combination with the partial thromboplastin time (PTT) test to screen for hemophilia and other hereditary clotting disorders.
The PT test is also used to monitor the condition of patients who are taking warfarin (Coumadin). Warfarin is a drug that is given to prevent clots in the deep veins of the legs and to treat pulmonary embolism. It interferes with blood clotting by lowering the liver's production of certain clotting factors.
A sample of the patient's blood is obtained by venipuncture. The blood is collected in a tube that contains sodium citrate to prevent the clotting process from starting before the test. The blood cells are separated from the liquid part of blood (plasma). The PT test is performed by adding the patient's plasma to a protein in the blood (thromboplastin) that converts prothrombin to thrombin. The mixture is then kept in a warm water bath at 37°C for one to two minutes. Calcium chloride is added to the mixture in order to counteract the sodium citrate and allow clotting to proceed. The test is timed from the addition of the calcium chloride until the plasma clots. This time is called the prothrombin time.
The doctor should check to see if the patient is taking any medications that may affect test results. This precaution is particularly important if the patient is taking warfarin, because there are a number of medications that can interact with warfarin to increase or decrease the PT time.
The primary risk is mild dizziness and the possibility of a bruise or swelling in the area where the blood was drawn. The patient can apply moist warm compresses.
The normal prothrombin time is 11-15 seconds, although there is some variation depending on the source of the thromboplastin used in the test. (For this reason, laboratories report a normal control value along with patient results.) A prothrombin time within this range indicates that the patient has normal amounts of clotting factors VII and X.
A prolonged PT time is considered abnormal. The prothrombin time will be prolonged if the concentration of any of the tested factors is 10% or more below normal plasma values. A prolonged prothrombin time indicates a deficiency in any of factors VII, X, V, prothrombin, or fibrinogen. It may mean that the patient has a vitamin K deficiency, a liver disease, or disseminated intravascular coagulation (DIC). The prothrombin time of patients receiving warfarin therapy will also be prolonged—usually in the range of one and one half to two times the normal PT time. A PT time that exceeds approximately two and a half times the control value (usually 30 seconds or longer) is grounds for concern, as abnormal bleeding may occur.
Jandl, J. H. Blood: Textbook of Hematology. New York: Little, Brown and Co., 1996.
Merck Manual of Medical Information, ed. Robert Berkow, et al. Whitehouse Station, NJ: Merck Research Laboratories, 1997.
John T. Lohr, PhD
Disseminated intravascular coagulation (DIC)—A condition in which spontaneous bleeding and clot formation occur throughout the circulatory system. DIC can be caused by transfusion reactions and a number of serious illnesses.
Fibrinogen—A type of blood protein called a globulin that interacts with thrombin to form fibrin.
Plasma—The liquid part of blood, as distinct from blood cells.
Prothrombin—A protein in blood plasma that is converted to thrombin during the clotting process.
Thrombin—An enzyme in blood plasma that helps to convert fibrinogen to fibrin during the last stage of the clotting process.
Thromboplastin—A protein in blood that converts prothrombin to thrombin.
Warfarin—A drug given to control the formation of blood clots. The PT test can be used to monitor patients being treated with warfarin.