prothrombin time (redirected from Prothrombin ratio)

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 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.

The blood clotting process.

(Illustration by Hans & Cassady.)

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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time (tīm) a measure of duration. Symbol t.

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activated partial thromboplastin time  (APTT) (aPTT) (PTT) the period required for clot formation in recalcified blood plasma after contact activation and the addition of platelet substitutes; used to address the intrinsic and common pathways of coagulation.

bleeding time  the duration of bleeding after controlled, standardized puncture of the earlobe or forearm; a relatively inconsistent measure of capillary and platelet function.

circulation time  the time required for blood to flow between two given points.

clotting time , coagulation time the time required for blood to clot in a glass tube.

inertia time  the time required to overcome the inertia of a muscle after reception of a stimulus from a nerve.

one-stage prothrombin time  prothrombin t.

prothrombin time  (PT) the rate at which prothrombin is converted to thrombin in citrated blood with added calcium; used to assess the extrinsic coagulation system of the blood.

reaction time  the time elapsing between the application of a stimulus and the resulting reaction.

stimulus-response time  reaction t.

thrombin time  (TT) the time required for plasma fibrinogen to form thrombin, measured as the time for clot formation after exogenous thrombin is added to citrated plasma.

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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 sec. Compare International Normalized Ratio and partial thromboplastin time. See also blood clotting.

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prothrombin (prōthrom´bin),

n a glycoprotein precursor of thrombin that is produced in the liver and is necessary for the coagulation of blood. A prothrombin deficiency is uncommon but may occur in liver disease. Vitamin K is essential for the synthesis of prothrombin.

prothrombin, B,

n See factor II.

prothrombin, component A of,

n See factor V.

prothrombin, component B of,

n See factor VII.

prothrombin time,

n 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. A prolonged prothrombin time indicates deficiency in one of the factors. Normal findings are 11 to 12.5 seconds.

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time,

n a measure of duration.

time, clot retraction,

n the time required for a given quantity of blood to separate in the tube in which it has been placed. For 3 ml of blood at room temperature, 1 hour is normal. It is very slow in thrombocytopenia.

time, coagulation,

n the time required for blood clotting to begin in a capillary tube, normally 2 to 8 minutes. A coagulation time three times normal is a definite danger sign.

time, gel,

n (gelation time), the interval of time required for a colloidal solution to become a solid or semisolid jelly or gel. Usually refers to the working time of a hydrocolloid or alginate impression material.

time, gelation,

n See time, gel.

time limits,

n.pl the periods within which a notice of claim must be filed.

time, median lethal,

n (LD₅₀ time, MLT), the time required for 50% of a large group of animals or organisms to die after administration of a specified dose of radiation.

time, prothrombin

n (one-stage test), a gross but useful screening test of the completeness of the second and third stages of blood coagulation. Normal prothrombin time by the Quick method is 12 to 15 seconds. The time is affected by deficiencies of factor V or VII as well as of prothrombin. See also test, prothrombin consumption.

time, serum prothrombin,

n See test, prothrombin consumption.

time, setting,

n the length of time for a mixed preparation of materials to reach a state of hardness, measured from the start of the mixing. The end point for dental materials is usually determined by a penetration test.

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prothrombin time

PT, protime Hematology A coagulation test used to monitor oral anticoagulant therapy, in particular with warfarin, to maintaining the PT at 2-2.5 times > the normal control; if PT is < 2-fold normal, anticoagulation is inadequate, if > 2.5 times normal, anticoagulation is excessive; PT is ↓ in thrombophlebitis, PTE, MI, drugs–barbiturates, OCs, digitalis, diphenhydramine, diuretics, metaproterenol, vitamin K; PT is ↑ in afibrinogenemia, drugs–anticoagulants, antibiotics, chlorpromazine, chlordiazepoxide, methyldopa, reserpine, salicylates, sulfonamides, erythroblastosis fetalis, coagulation factor deficiencies Ref range 11.5–13.7 secs. Cf Partial thromboplastin time.