Physiologic Basis
PT evaluates the extrinsic and common coagulation pathways.The time required for fibrin clot formation is determined. It is most sensitive to deficiencies in the vitamin K-dependent clotting factors II, VII, and X. It is also sensitive to deficiency of factor V. It is less sensitive to fibrinogen deficiency and heparin. PT/INR tests are increasingly being used for monitoring warfarin therapy.
Interpretation
Increased in: Warfarin, liver disease, DIC, vitamin K deficiency, hereditary deficiency in factors VII, X, V and II, fibrinogen abnormality (eg, hypofibrinogenemia, afibrinogenemia, dysfibrinogenemia), circulating anticoagulant affecting the PT system (rarely lupus anticoagulant), massive transfusion.
Decreased in: Recombinant factor F VII treatment
Comments
Warfarin therapeutic range is INR 2.0 - 3.0. Bleeding has been reported to be 3 × more common in patients with INRs of 3.0–4.5 than in patients with INRs of 2.0–3.0.