Prothrombin time (PT) measures how quickly the extrinsic coagulation pathway — activated by tissue factor released from damaged blood vessel walls — leads to clot formation. It assesses clotting factors VII, X, V, prothrombin (factor II), and fibrinogen. Results are often reported as an INR for standardisation.
PT/INR is prolonged in liver disease (which impairs clotting factor synthesis), vitamin K deficiency, warfarin use, and clotting factor deficiencies. It is a key component of liver function assessment — a prolonged PT indicates severe hepatic dysfunction. PT is also used to assess clotting status before surgical procedures and to monitor warfarin therapy.
FAQs
What is the difference between PT and INR?
PT is measured in seconds and varies between laboratories depending on the reagent used. INR is a standardised ratio that corrects for these laboratory differences, making results comparable across different centres. INR is the preferred way to express PT for clinical and monitoring purposes.
Can PT be used to assess liver function?
Yes. PT/INR is an important marker of liver synthetic function, as all extrinsic pathway clotting factors are produced by the liver. A prolonged PT in the absence of anticoagulants or vitamin K deficiency indicates impaired liver synthetic capacity and is used in prognostic scoring systems for liver disease.
How quickly does PT respond to vitamin K?
After oral vitamin K administration, PT/INR typically begins to fall within 6-12 hours and reaches maximum effect at 24-48 hours. IV vitamin K acts faster (response within 4-6 hours). Response is limited in severe liver failure, as the liver cannot produce factors regardless of vitamin K availability.
Should I take my warfarin before a PT test?
No. Take your warfarin after the blood test. Taking it before will cause a transient spike that does not reflect your steady-state level and may lead to inappropriate dose adjustments.
