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