There is an interesting article in the February issue of the Journal of Thrombosis and Haemostasis, Trippodi A, Chantarangkul V, Mannucci PM. The international normalized ratio to prioritize patients for liver transplantation: problems and possible solutions. J Thrombos haemostas 2008;6:243-8. I’m led to provide a clinical discussion of reporting conventions for the prothrombin time test in seconds and INR.
The prothrombin time (PT)-derived international normalized ratio (INR) is valid for monitoring stable Warfarin (Coumadin) oral anticoagulant therapy only. It is likely to generate inaccurate results during the initiation of anticoagulant therapy, in liver disease or when screening for extrinsic or common factor deficiencies. Point of care PT/INRs are most often used to monitor Warfarin therapy in an anticoagulation clinic or in home care, so in point of care applications, the INR is perhaps the only number necessary. In the central laboratory we seldom know the purpose for the PT/INR order, consequently we are constrained to report both the PT in seconds and the INR. The PT in seconds is necessary for monitoring Warfarin during the first days of anticoagulation, and for screening for Coagulopathies (deficiency of factors VII, X, V or II). The central laboratory-generated results should be accompanied by a brief narrative describing the purpose for both the PT and INR. Geo.