EFFECT OF THROMBOPLASTIN AND COAGULOMETER INTERACTION ON THE PRECISION OF THE INTERNATIONAL NORMALIZED RATIO

Aims - To examine the magnitude of thromboplastin and coagulometer int eractions on the precision of International Normalised Ratio (INR) val ues when the manufacturers' recommended instrument specific Internatio nal Sensitivity Index (ISI) values are adopted for the INR calculation . Methods - The variability of INR values obtained from four automated photo-optical coagulometers frequently used in North American laborat ories was studied. When used with five commercial thromboplastins of m oderate to high sensitivity (ISI values 0.92-1.97), 20 prothrombin tim e results were generated for each of a population of 98 patients on es tablished warfarin treatment. Results - The mean INR values of the pat ients ranged from 2.05 to 2.81, depending on which reagent/coagulomete r combination was used. Within patient variation increased as the medi an INR value increased. The mean coefficient of variation of within pa tient INR values was 10%; the mean coefficient of variation of the pro thrombin time results in seconds and prothrombin time ratio were 21 an d 18%, respectively. Conclusions - There was considerable bias in the estimated ISI values of the thromboplastins compared with the manufact urers' instrument specific ISI value. Despite this apparent imperfecti on, our study clearly showed that the INR is preferable to other proth rombin time reporting formats for assessing the degree of anticoagulat ion for patients on warfarin treatment.