Biochemistry of Prothrombin Time (PT)

The exogenous coagulation system is activated by addition of tissue-thromboplastin and calcium to citrated plasma. Prothrombin time is measured as the time until clotting. In analogy to activated partial thromboplastin time (aPTT) the prothrombin time is the global test for the extrinsic system. The speed of the fibrin formation is thus above all dependent of the activity of the factors II, VII and X. Factor V and fibrinogen are of less importance.

Thromboplastins are tissue extracts of various origins. They consist of a protein and a phospholipid part. Thromboplastins are now also recombinantly produced. The sensitivities of the reagents differ considerably. For this reason results originating from assays made with different thromboplastins are not comparable.

Analyses by the Quick-method are mostly carried out in a geometrical range of dilutions with physiological sodium chloride using a normal plasma pool to create a reference curve. The resulting clotting time (prothrombin time) of the subject sample is then related to the calibration curve and reported in percent of normal value.

In order to obtain a better comparability of the assay results WHO 1983 introduced the INR (International Normalized Ratio). The ISI-value (International Sensitivity Index) shows the sensitivity of the used reagent against a standard-thromboplastin.

Clinical or Research use of Prothrombin Time (PT)

  • Monitoring of therapy with Vitamin K-antagonists
  • Suspected factor deficiency in the extrinsic system

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See also: activated partial thromboplastin time (aPTT)