prothrombin time
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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
Last reviewed 01/2018
Links:
- target INRs in different disease managements
- reference range (prothrombin time)
- prolonged INR (whilst on warfarin)
- 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
- NICE - indication for prothrombin complex concentrate