prothrombin time

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Prothrombin Time



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.


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 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.


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.


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.
The blood clotting process.
(Illustration by Hans & Cassady.)



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.


a glycoprotein present in the plasma that is converted into thrombin by extrinsic thromboplastin during the second stage of blood clotting; called also factor II.
prothrombin consumption a clinical laboratory test done to determine thromboplastin generating capacity, which provides information about the first stage of blood clotting. When clotting of a normal blood sample occurs, prothrombin is converted to thrombin; thus there should be little or no prothrombin in the serum after the clot is formed. If, however, there is deficiency of blood clotting (coagulation), some of the prothrombin will not be utilized (consumed). Abnormal results of the test are found in deficiencies of the first-stage coagulation factors (factors VIII and IX), and in the presence of circulating anticoagulants, thrombocytopenia, and any other condition leading to inadequate generation of thromboplastin.
prothrombin consumption test a test to measure the formation of intrinsic thromboplastin by determining the residual serum prothrombin after blood clotting is complete.
prothrombin time a test to measure the activity of coagulation factors I, II, V, VII, and X, which participate in the extrinsic pathway of coagulation; abbreviated Pro time or PT. Called also one-stage prothrombin time and Quick's test. Deficiency of any of these factors leads to a prolongation of the one-stage prothrombin times, as will circulating anticoagulants that are active against factors V and VII or against thromboplastin.

The test is considered basic to any study of the clotting process and is also widely used for guidance in establishing and maintaining anticoagulant therapy. Test results are best understood when both the patient's and the control times are reported. The therapeutic range for coagulation therapy is usually 2 to 3 times that of the normal (12 to 15 sec.) control.


a measure of duration. See under adjectives for specific times, such as bleeding time.
activated partial thromboplastin time (APTT, aPTT) the period required for clot formation in recalcified blood plasma after contact activation and the addition of platelet substitutes such as brain cephalins or similar phospholipids; used to assess the coagulation pathways. A prolonged aPTT can indicate a deficiency of any of various coagulation factors, including factors XII, XI, IX, VIII, X, V, and II, and fibrinogen.
AEC minimal response time the shortest duration at which x-ray exposure can be terminated by automatic exposure control.
atrioventricular sequential time a fixed nonprogrammable interval that extends from the atrial stimulus to the ventricular stimulus.
bleeding time the time required for a standardized wound to stop bleeding; used as a test for platelet disorders; see also bleeding time.
circulation time the time required for blood to flow between two given points; see also circulation time.
clotting time (coagulation time) the time required for blood to clot in a glass tube; see also clotting.
cold ischemia time the time between the placement of a traumatically amputated body part in ice and the time of surgical replantation.
inertia time the time required to overcome the inertia of a muscle after reception of a stimulus.
ischemia time the total time between traumatic amputation of a limb or portion of a limb and its surgical reimplantation; it is the sum of warm and cold ischemia times.
minimal response time in radiology, the shortest possible exposure time for an x-ray film to be exposed automatically.
one-stage prothrombin time prothrombin time.
prothrombin time see prothrombin time.
real time a term used to describe a recording device that shows events simultaneously to their occurrence.
thrombin time the time required for plasma fibrinogen to form thrombin; see also thrombin time.
warm ischemia time the time interval between traumatic amputation of a limb or part and its placement on ice.

pro·throm·bin time (PT),

the time required for clotting after thromboplastin and calcium are added in optimal amounts to blood of normal fibrinogen content; if prothrombin is diminished, the clotting time increases; used to evaluate the extrinsic clotting system.
See also: prothrombin test.

prothrombin time

The clotting time of blood plasma and sodium citrate in the presence of thromboplastin and calcium chloride, used to determine the level of prothrombin and several other clotting factors in the blood.

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.

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.

pro·throm·bin time

(PT) (prō-throm'bin tīm)
The time required for clotting after thromboplastin and calcium are added in optimal amounts to blood of normal fibrinogen content; if prothrombin is diminished, the clotting time increases; used to evaluate the extrinsic clotting system.
See also: prothrombin test

prothrombin time

A test for blood clotting defect due to deficiency of clotting factors I (fibrinogen), II (prothrombin) V (proaccelerin), VII (serum prothrombin conversion acclelerator), or X (Stuart-Prower factor). The test determines the ratio between the time taken for the treated patient's blood sample to clot and the time taken for a standard control sample. Prothrombin time estimation is important in bleeding disorders and when oral ANTICOAGULANT drugs (which are antagonists of vitamin K) are being used. Vitamin K is necessary for the formation of several of the clotting factors. A deficiency of platelets does not affect the prothrombin time.

prothrombin time

time required for clot formation, after thromboplastin and calcium are added to blood with normal fibrinogen levels; low prothrombin levels = increased clotting time

pro·throm·bin time

(PT) (prō-throm'bin tīm)
Time required for clotting after thromboplastin and calcium are added in optimal amounts to blood of normal fibrinogen content; if prothrombin is diminished, clotting time increases.
References in periodicals archive ?
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Outsourcing prothrombin time tests was risking both legal liability and patient health.
The Quick (1) and Owren (2) prothrombin time (PT) tests remain the basis for monitoring anticoagulant therapy worldwide.
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The prothrombin time (PT) [1] assay was introduced to monitor chronic warfarin therapy.

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