Editing Prothrombin time (section)

===Methodology===
[[File:Blue Top.JPG|thumb|Vacutainer tube used for PT and PTT blood tests]]
Prothrombin time is typically analyzed by a laboratory technologist on an automated instrument at 37&nbsp;°C (as a nominal approximation of normal human body temperature).{{citation needed|date=July 2020}}
* 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 [[newborn]]s, a capillary whole blood specimen is used.<ref name="fritsma">Fritsma, George A. (2002). "Evaluation of Hemostasis." Hematology: Clinical Principles and Applications . Ed. Bernadette Rodak. W.B. Saunders Company: Philadelphia, 2002. 719-53. Print</ref>
* 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a relatively high concentration of citrate—the standardized dilution of 1 part anticoagulant to 9 parts whole blood is no longer valid).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 [[lipemia|lipemic]] and [[icteric]] samples.