page_title: Prothrombin time
section_title: Methodology
hierarchical_section_title: Prothrombin time / Laboratory measurement / Methodology
caption_attribution_description: English: Blue Top Vacutainer tube used for PT and PTT blood tests
context_page_description: The prothrombin time – along with its derived measures of prothrombin ratio and international normalized ratio – are assays evaluating the extrinsic pathway and common pathway of coagulation. This blood test is also called protime INR and PT/INR. They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage, and vitamin K status. PT measures the following coagulation factors: I, II, V, VII, and X.
  PT is often used in conjunction with the activated partial thromboplastin time which measures the intrinsic pathway and common pathway of coagulation.
context_section_description: 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a relatively high concentration of citrate—the standardized dilution of 1 part anticoagulant to 9 parts whole blood is no longer valid).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. Some laboratories use a mechanical measurement, which eliminates interferences from lipemic and icteric samples.
caption: Vacutainer tube used for PT and PTT blood tests
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