Simplified Method for Local Correction of System International Normalized Ratio

Abstract Background International normalized ratio (INR) derivation is dependent on the international sensitivity index (ISI) of thromboplastin. It varies with instrument and reagents used. Objective To evaluate the role of a correction factor in the derivation of INR. Methods We studied prothrombin time (PT) and INR from patients using 3 thromboplastins of varying ISI values. The correction factor was applied to the observed INR to obtain a corrected INR. Results The difference between corrected INR and observed INR values varies from −0.8 through 0.96. Conclusions Corrected INR is dependent on PT only. It can be applied to all patients irrespective of cause for elevated PT.