![]() Factor VII has a short half-life and the carboxylation of its glutamate residues requires vitamin K. The speed of the extrinsic pathway is greatly affected by levels of functional factor VII in the body. This measures the quality of the extrinsic pathway (as well as the common pathway) of coagulation. The prothrombin time is the time it takes plasma to clot after addition of tissue factor (obtained from animals such as rabbits, or recombinant tissue factor, or from brains of autopsy patients). PT normal is established as the geometric mean of the prothrombin times (PT) of a reference sample group. 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 analytical system being used. The ISI is usually between 0.94 and 1.4 for more sensitive and 2.0–3.0 for less sensitive thromboplastins. The ISI value indicates how a particular batch of tissue factor compares to an international reference tissue factor. Each manufacturer assigns an ISI value (International Sensitivity Index) for any tissue factor they manufacture. ![]() The INR was devised to standardize the results. This is due to the variations between different types and batches of manufacturer's tissue factor used in the reagent to perform the test. The result (in seconds) for a prothrombin time performed on a normal individual will vary according to the type of analytical system employed. Elevated INR may be useful as a rapid and inexpensive diagnostic of infection in people with COVID-19. The PT ratio varies depending on the specific reagents used, and has been replaced by the INR. The prothrombin time ratio is the ratio of a subject's measured prothrombin time (in seconds) to the normal laboratory reference PT. Some laboratories use a mechanical measurement, which eliminates interferences from lipemic and icteric samples. Finally, in order to activate the extrinsic / tissue factor clotting cascade pathway, tissue factor (also known as factor III) is added and the time the sample takes to clot is measured optically.Next an excess of calcium (in a phospholipid suspension) is added to the test tube, thereby reversing the effects of citrate and enabling the blood to clot again.A sample of the plasma is extracted from the test tube and placed into a measuring test tube (Note: for an accurate measurement, the ratio of blood to citrate needs to be fixed and should be labeled on the side of the measuring test tube by the manufacturing company many laboratories will not perform the assay if the tube is underfilled and contains a relatively high concentration of citrate-the standardized dilution of 1 part anticoagulant to 9 parts whole blood is no longer valid).In newborns, a capillary whole blood specimen is used. The blood is mixed, then centrifuged to separate blood cells from plasma (as prothrombin time is most commonly measured using blood plasma). Blood is drawn into a test tube containing liquid sodium citrate, which acts as an anticoagulant by binding the calcium in a sample.Prothrombin time is typically analyzed by a laboratory technologist on an automated instrument at 37 ☌ (as a nominal approximation of normal human body temperature). Methodology Vacutainer tube used for PT and PTT blood tests In some cases, if more intense anticoagulation is thought to be required, the target range may be as high as 2.5–3.5 depending on the indication for anticoagulation. The target range for INR in anticoagulant use (e.g. The reference range for prothrombin time depends on the analytical method used, but is usually around 12–13 seconds (results should always be interpreted using the reference range from the laboratory that performed the test), and the INR in absence of anticoagulation therapy is 0.8–1.2. PT is often used in conjunction with the activated partial thromboplastin time (aPTT) which measures the intrinsic pathway and common pathway of coagulation. ![]() ![]() PT measures the following coagulation factors: I (fibrinogen), II (prothrombin), V (proaccelerin), VII (proconvertin), and X (Stuart–Prower factor). ![]() They are used to determine the clotting tendency of blood, in such things as the measure of warfarin dosage, liver damage, and vitamin K status. This blood test is also called protime INR and PT/INR. The prothrombin time ( PT) – along with its derived measures of prothrombin ratio ( PR) and international normalized ratio ( INR) – is an assay for evaluating the extrinsic pathway and common pathway of coagulation. ![]()
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