The prothrombin time (PT) and its derived measures of prothrombin ratio (PR) and international normalized ratio (INR) are measures of the extrinsic pathway of coagulation . They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage, and vitamin K status. The reference range for prothrombin time is usually around 12–15 seconds; the normal range for the INR is 0.8–1.2. PT measures factors II , V, VII, X and fibrinogen . It is used in conjunction with the activated partial thromboplastin time (aPTT) which measures the intrinsic pathway.

Laboratory measurement

1. METHODOLOGY

The prothrombin time is most commonly measured using blood plasma . Blood is drawn into a test tube containing liquid citrate , which acts as an anticoagulant by binding the calcium in a sample. The blood is mixed, then centrifuged to separate blood cells from plasma. In newborns , whole blood is used.

The plasma is analyzed by a medical technologist on an automated instrument at 37°C, which takes a sample of the plasma. An excess of calcium is added (thereby reversing the effects of citrate), which enables the blood to clot again. For an accurate measurement the proportion of blood to citrate needs to be fixed; many laboratories will not perform the assay if the tube is underfilled and contains a relatively high concentration of citrate. This is because Vacutainer test tubes generally contain a powdered anticoagulant to prevent blood from clotting. For the prothrombin time test the appropriate sample is the blue top tube, or citrate tube, which is a liquid anticoagulant. Just as adding solvent to any solution will dilute it, adding liquid anticoagulant to blood will dilute it. This dilution will cause a falsely long prothrombin time. So, all analysis takes this dilution into account by multiplying the result by 1.1 to account for the dilution. If a tube is underfilled or overfilled with blood, the standardized dilution of 1.1 is no longer valid.

Tissue factor (also known as factor III or thromboplastin) is added, and the time the sample takes to clot is measured optically. Some laboratories use a mechanical measurement, which eliminates interferences from lipemic and icteric samples.

The prothrombin ratio is the prothrombin time for a patient, divided by the result for control plasma.

2. INTERNATIONAL NORMALIZED RATIO

Because of differences between different batches and manufacturers of tissue factor (it is a biologically obtained product), the INR was devised to standardize the results.

Each manufacturer gives an ISI (International Sensitivity Index) for any tissue factor they make. The ISI value indicates how the particular batch of tissue factor compares to an internationally standardized sample. The ISI is usually between 1.0 and 1.4.

The INR is the ratio of a patient’s prothrombin time to a normal (control) sample, raised to the power of the ISI value for the control sample used.

3. INTERPRETATION

The prothrombin time is the time it takes plasma to clot after addition of tissue factor (obtained from animals). This measures the quality of the extrinsic pathway (as well as the common pathway) of coagulation .

The speed of the extrinsic pathway is greatly affected by levels of factor VII in the body. Factor VII has a short half-life and its synthesis requires vitamin K . The prothrombin time can be prolonged as a result of deficiencies in vitamin K, which can be caused by warfarin , malabsorption , or lack of intestinal colonization by bacteria (such as in newborns ). In addition, poor factor VII synthesis (due to liver disease ) or increased consumption (in disseminated intravascular coagulation ) may prolong the PT.

A high INR level such as INR=5 indicates that there is a high chance of bleeding, whereas if the INR=0.5 then there is a high chance of having a clot. Normal range for a healthy person is 0.9–1.3, and for people on warfarin therapy, 2.0–3.0, although the target INR may be higher in particular situations, such as mechanical heart valves .

4. FACTORS DETERMINING ACCURACY

Lupus anticoagulant , a circulating inhibitor predisposing for thrombosis, may skew PT results, depending on the assay used. Variations between various thromboplastin preparations have in the past led to decreased accuracy of INR readings, and a 2005 study suggested that despite international calibration efforts (by INR) there were still statistically significant differences between various kits, casting doubt on the long-term.