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Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

The prothrombin time (PT) and international normalized ratio are ways of measuring of the extrinsic system in the coagulation pathway - factors II, V,VII and X

  • PT measures the extrinsic system (factor VII) as well as factors common to both the intrinsic and extrinsic systems (factors X, V, prothrombin and fibrinogen)
  • the normal clotting time is 10-14 seconds

Thromboplastin and plasma are mixed at 37 deg.C. and the time taken for a clot to form after the addition of calcium is measured; this is the prothrombin time.

The time to clot is compared to a control - this is the international normalised ratio (INR). The normal range of the INR is 0.9-1.2.

Conditions which cause a prolongation of the prothrombin time include:

  • liver disease
  • use of anticoagulants e.g. warfarin

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