Prothrombin time
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 degrees 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
Reference:
- Royal Pharmaceutical Society. Liver function tests: indication and interpretation. Feb 2022 [internet publication].
Related pages
- Target INRs in different disease managements
- Reference range (prothrombin time)
- Prolonged INR (whilst on warfarin)
- Warfarin
- Extrinsic pathway of blood coagulation
- 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
Create an account to add page annotations
Annotations allow you to add information to this page that would be handy to have on hand during a consultation. E.g. a website or number. This information will always show when you visit this page.