THE CORRECTION OF COAGULOMETER EFFECTS ON INTERNATIONAL NORMALIZED RATIOS - A MULTICENTER EVALUATION

One hundred and twenty-nine centres in the U.K. participated in a stud y to test the reliability of the three methods of correction for coagu lometer effects on international normalized ratios (INR). Results from 37 centres which tested three warfarinized plasmas by the manual meth od were taken as the 'true' INR for the assessment of coagulometers. 6 3 centres (11 manual and 52 using coagulometers) determined their loca l International Sensitivity Index (ISI) in a calibration exercise. Thi s was performed with a set of 20 lyophilized plasma calibrants with ce rtified manual prothrombin times for the thromboplastin used in the st udy. The following methods of INR derivation were compared by assessin g the percentage deviation from the three INK values established by 37 manual users: I. No coagulometer correction, i.e. (local PT/reference manual normal PT)(manual ISI) . II. Coagulometer ratio correction, i. e. (local coagulometer PT/local coagulometer MNPT)(manual ISI) . III. Local system ISI, i.e. (local coagulometer PT/local coagulometer MNPT) (local system ISI) . IV. System ISI, i.e. (local coagulometer PT/local coagulometer MNPT)(system ISI) . The local system ISI with the plasma calibrants (method III) gave the most reliable correction (mean devia tion from 'true' INR 4.87%). The method which gave the least was with the coagulometer ratio correction, i.e. the manual ISI and local coagu lometer MNPT (mean 11.25%). The system ISI tested with ACL coagulomete rs gave less correction than the local ISI calibration. The local syst em calibration with lyophilized plasmas also avoids some of the constr aints on conventional thromboplastin calibrations.