Last reviewed 01/2018
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
target INRs in different disease managements
reference range (prothrombin time)
prolonged INR (whilst on warfarin)
extrinsic pathway of blood coagulation
INR (dental extractions in patients on warfarin)
interpretation of the results of screening tests used in the diagnosis of coagulation disorders