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international normalised ratio

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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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