Prothrombin

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Prothrombin Time

Definition
Purpose
Description
Preparation
Aftercare
Risks
Normal results
Abnormal results
Morbidity and mortality rates
Alternatives Resources
Precautions
Side effects

Definition

The prothrombin time test belongs to a group of blood tests that assess the clotting ability of blood. The test is also known as the pro time or PT test.

Purpose

The PT test is used to monitor patients taking certain medications as well as to help diagnose clotting disorders.

Diagnosis

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 Vila, V, and X, prothrombin, and fibrinogen. Prothrombin is a protein in the liquid part of blood (plasma) that is converted to thrombin 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.

Monitoring

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.

Description

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.

Preparation

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

KEY TERMS

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.

Fibrin— The protein formed as the end product of the blood clotting process when fibrinogen interacts with thrombin.

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.

medications that can interact with warfarin to increase or decrease the PT time.

Aftercare

Aftercare consists of routine care of the area around the puncture site. Pressure is applied for a few seconds and the wound is covered with a bandage.

Risks

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.

Normal results

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.

Abnormal results

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.

Morbidity and mortality rates

Morbidity rates are excessively miniscule. The most common problems are minor bleeding and bruising. Since neither are reportable events, morbidity can only be estimated. Mortality is essentially zero.

Alternatives Resources

There are no alternatives to a prothrombin time.

Precautions

The only precaution needed is to clean the venipuncture site with alcohol.

Side effects

The most common side effects of a prothrombin time test are minor bleeding and bruising.

Resources

BOOKS

Fischbach, F. T. and M. B. Dunning. A Manual of Laboratory and Diagnostic Tests. 8th ed. Philadelphia: Lippincott Williams & Wilkins, 2008.

McGhee, M. A Guide to Laboratory Investigations. 5th ed. Oxford, UK: Radcliffe Publishing Ltd, 2008.

Price, C. P. Evidence-Based Laboratory Medicine: Principles, Practice, and Outcomes. 2nd ed. Washington, DC: AACC Press, 2007.

Scott, M.G., A. M. Gronowski, and C. S. Eby. Tietz’s Applied Laboratory Medicine. 2nd ed. New York: Wiley-Liss, 2007.

Springhouse, A. M.. Diagnostic Tests Made Incredibly Easy!. 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2008.

PERIODICALS

Aksungar, F. B., A. E. Topkaya, Z. Yildiz, S. Sahin, and U. Turk. “Coagulation status and biochemical and inflammatory markers in multiple sclerosis.” Journal of Clinical Neuroscience 15, no. 4 (2008): 393–397.

Awan, M. S., M. Iqbal, and S. Z. Imam. “Epistaxis: when are coagulation studies justified?” Emergency Medicine Journal 25, no. 3 (2008): 156–157.

Mueck, W., B. I. Eriksson, K. A. Bauer et al. “Population pharmacokinetics and pharmacodynamics of rivaroxaban - an oral, direct factor Xa inhibitor - in patients undergoing major orthopaedic surgery.” Clinical Pharmacokinetics 47, no. 3 (2008): 203–216.

Rosenkrantz, A., M. Hinden, B. Leschmik, et al. “Calibrated automated thrombin generation in normal uncomplicated pregnancy.” Thrombosis and Hemostasis 99, no. 2 (2008): 331–337.

ORGANIZATIONS

American Association for Clinical Chemistry. http://www.aacc.org/AACC/.

American Society for Clinical Laboratory Science. http://www.ascls.org/.

American Society of Clinical Pathologists. http://www.ascp.org/.

College of American Pathologists. http://www.cap.org/apps/cap.portal.

OTHER

American Clinical Laboratory Association. Information about clinical chemistry. 2008 [cited February 24, 2008]. http://www.clinical-labs.org/.

Clinical Laboratory Management Association. Information about clinical chemistry. 2008 [cited February 22, 2008]. http://www.clma.org/.

Lab Tests On Line. Information about lab tests. 2008 [cited February 24, 2008]. http://www.labtestsonline.org/.

National Accreditation Agency for Clinical Laboratory Sciences. Information about laboratory tests. 2008 [cited February 25, 2008]. http://www.naacls.org/.

L. Fleming Fallon, Jr, MD, DrPH

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Prothrombin Time

Definition

The prothrombin time test belongs to a group of blood tests that assess the clotting ability of blood. The test is also known as the pro time or PT test.

Purpose

The PT test is used to monitor patients taking certain medications as well as to help diagnose clotting disorders.

Diagnosis

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 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.

Monitoring

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.

Description

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.

Preparation

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.

Aftercare

Aftercare consists of routine care of the area around the puncture mark. Pressure is applied for a few seconds and the wound is covered with a bandage.

Risks

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.

Normal results

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.

Abnormal results

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 prolongedusually 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.

Resources

BOOKS

Berktow, Robert, et al., editors. Merck Manual of Medical Information. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

KEY TERMS

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.

Fibrin The protein formed as the end product of the blood clotting process when fibrinogen interacts with thrombin.

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.

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prothrombin (PT) (proh-throm-bin) n. a substance, present in blood plasma, that is the inactive precursor from which the enzyme thrombin is derived during the process of blood coagulation. See also coagulation factors. p. time (PT) the time taken for blood clotting to occur in a sample of blood to which calcium and thromboplastin have been added; a prolonged PT indicates a deficiency of coagulation factors. Measurement of PT is used to control anticoagulant therapy (e.g. with warfarin).

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prothrombin (Factor II) One of the blood clotting factors. It is the precursor of the enzyme thrombin, which catalyses the formation of the fibrin matrix of the blood clot. Prothrombin synthesis occurs in the liver and is dependent on adequate supplies of vitamin K. See also blood clotting.

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prothrombin The enzymatically inactive precursor of thrombin, which is activated by a complex cascade of events requiring the presence of Ca2+ (calcium), of proteinaceous thromboplastins, and of a number of additional blood factors. See also BLOOD CLOTTING.

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prothrombin Protein in plasma involved in coagulation of blood. The prothrombin time is an index of the coagulability of blood (and hence of vitamin K nutritional status) based on the time taken for a citrated sample of blood to clot when calcium ions and thromboplastin are added.