LOCAL INR CORRECTION - JUSTIFICATION FOR A SIMPLIFIED APPROACH

Aims-Errors in reporting International Normalised Ratios (INR) may be corrected by assignment of a System International Sensitivity Index (S ystem ISI). This 57 centre study tests the validity of several procedu res for INR correction. Methods-Prothrombin times of eight lyophilised coumarin calibrants, a lyophilised normal pool calibrant, and eight f rozen coumarin plasmas were determined at each centre. The calibrants were calibrated using international reference preparations. The eight frozen coumarin plasmas were calibrated in a four centre international exercise. The relations tested were: (a) the logarithm of local proth rombin time against the logarithm of reference prothrombin time; (b) r eference INR against local prothrombin time; and (c) logarithm of refe rence INR against logarithm of local prothrombin time. These methods w ere analysed by both linear and orthogonal regression. Results-All sys tem groups required correction, the mean percentage deviation of the u ncorrected data from the calibrated values was 19.0%. There was also c onsiderable variation in INR, with the coefficient of variance (CV) ra nging from 11.30 to 17.29%. Correction of INR was possible with all me thods (CV reduced to < 7%). However, only when a plot of the logarithm of local prothrombin time against the logarithm of reference prothrom bin time was fitted by orthogonal regression, or a plot of logarithm o f reference INR against logarithm of local prothrombin time was fitted by either type of regression analysis, did the best fit line through the calibrant plasmas also pass close to the local mean normal prothro mbin time. Conclusions-While INR correction may be achieved by all the above methods, that relating log reference INR to log local prothromb in time by linear regression analysis is the simplest to perform.