International Normalized Ratio determination using calibrated reference plasmas

We have compared the conventional method of International Normalized Ratio (INR) determination with an alternative method involving extrapolation from a calibration curve using freeze-dried 'reference' plasmas. The latter approach does not require the determination of a mean normal prothrombin time (MNPT) or local system International Sensitivity Index (ISI). Calibration curves were constructed by plotting local prothrombin time (PT) against assigned INR values for a normal plasma and either two plasma pools from patients on oral anticoagulants or two artificially depleted plasmas. Six laboratories determined the INR of a freeze-dried test plasma and frozen patient plasma samples using the conventional method and by extrapolation. Similarities in the results with the freeze-dried test plasma and the frozen plasmas were encouraging for the projected use with fresh plasma samples. INR values by the conventional method for the test plasma gave an overall mean of 2.73 and inter-laboratory variability (gcv%) of 8.92%, whereas estimates by extrapolation against the normal and patient plasmas or the normal and artificially depleted plasmas gave identical overall mean INR values of 2.70 with inter-laboratory variability (gcv%) of 3.44% and 4.92% respectively. The results indicate that INR determination by extrapolation is associated with reduced interlaboratory variability.