prothrombin time

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Also known as PT and Pro Time, this test measures how long it takes blood to clot. This coagulation test measures the presence and activity of five different blood clotting factors. This test can screen for bleeding abnormalities, and may also be used to monitor medication treatments that prevent the formation of blood clots


Prothrombin time is typically analyzed by a laboratory technologist on an automated instrument at 37 °C (as a nominal approximation of normal human body temperature).

Blood is drawn into a test tube containing liquid sodium 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 (as prothrombin time is most commonly measured using blood plasma). In newborns, a capillary whole blood specimen is used.

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a relatively high concentration of citrate—the standardized dilution of 1 part anticoagulant to 9 parts whole blood is no longer valid).

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.

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 using calorimeter or photometer.
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