Prothrombin time

* A crude preparation of tissue factor (usually an extract of brain) is added to citrate-anticoagulated plasma. The plasma is recalcified and clotting time is measured.

* Usually both the thromboplastin and calcium chloride are added in a single step.

* The thromboplastins are calibrated against an international reference preparation to derive an international sensitivity index (ISI). Once the ISI of the thromboplastin is assigned, the results can be reported as the international normalised ratio (INR).

* A prolonged prothrombin time is due to deficiencies of factors II, V, VII or IX, or to the presence of a factor inhibitor.

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