The prothrombin time, commonly referred to on the show as a PT is a measurement of how long certain processes in clotting take.  It is used to measure the dosage of warfarin, to assess liver damage and to measure the amount of Vitamin K in the body.

The test is performed by taking a sample of blood from the patient which is then mixed with a citrate to prevent clotting.  The sample is then spun in a centrifuge to separate out the plasma.  The sample is then heated to normal body temperature (98.6F or 37C) and then calcium is added to reverse the effect of the citrate and allow clotting to begin again.  Tissue factor is then added and the technician or doctor measures how long it takes for clotting to occur. This result is adjusted based on the instructions of the tissue factor supplier to provide an international normalized ratio or INR.  The normal range is 12-15 seconds.  A longer time can indicate Vitamin K deficiency, the existence of anti-clotting drugs in the bloodstream, liver failure, or DIC

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