INTERNATIONAL NORMALIZED RATIO DETERMINATION USING CALIBRATED REFERENCE PLASMAS

We have compared method of International Normalized Ratio (INR) determ ination with an alternative method involving extrapolation from a cali bration curve using freeze-dried 'reference' plasmas. The latter appro ach does not require the determination of a mean normal prothrombin ti me (MNPT) or local system International Sensitivity Index (ISI). Calib ration 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 deplet ed plasmas. Six laboratories determined the INR of a freeze-dried test plasma and frozen patient plasma samples using the conventional metho d and by extrapolation. Similarities in the results with the freeze-dr ied test plasma and the frozen plasmas were encouraging for the projec ted use with fresh plasma samples. INR values by the conventional meth od for the test plasma gave an overall mean of 2.73 and inter-laborato ry variability (gcv%) of 8.9%, whereas estimates by extrapolation agai nst the normal and patient plasmas or the normal and artificially depl eted plasmas gave identical overall mean INR values of 2.70 with inter -laboratory variability (gcv%) of 3.44% and 4.92% respectively. The re sults indicate that INR determination by extrapolation is associated w ith reduced inter-laboratory variability.