The Value of Plasma Calibrants in Correcting Coagulometer Effects on International Normalized Ratios: An International Multicenter Study

A study of lyophilized plasma calibrants in correcting for coagulometer effects on International Normalized Ratios (INR) has been conducted in an international survey. Prothrombin times were performed with the calibrants on 2 "common" thromboplastins and in-house thromboplastins in 3 brands of coagulometer at 37 centers. The International Sensitivity Indices (ISI) of 95 local systems with the calibrants were derived. The "true" INR of 10 test plasmas from warfarin-treated patients were established centrally using thromboplastin International Reference Preparations (IRP). International Normalized Ratios of these plasmas for each center were calculated using both the manufacturers' stated and the local ISI. The mean deviation of the 95 systems from the "true" INR of the warfarin plasmas was +14.4% with the manufacturers' ISI, but reduced to +1.04% with the local ISI. Local ISI determination with the calibrants avoids many of the difficulties of conventional thromboplastin calibrations. Plasmas from patients on warfarin and parallel manual PT with thromboplastin IRP are not required.