Influence of three types of automated coagulometers on the international sensitivity index (ISI) of rabbit, human, and recombinant human tissue factor preparations - A multicenter study

Five tissue factor reagents and three types of automated instruments for pr othrombin time (PT) determination were studied in an international multicen ter collaborative exercise. The purpose of this work was to determine the i nternational sensitivity index (ISI) for each combination of reagent and in strument against the international reference preparation RBT/90. Each type Of instrument was used by 3 or 4 centers to assess the interlaboratory vari ation of the ISI. The interlaboratory variation of the ISI for each combina tion of reagent and instrument ranged between 0.4% and 7.8% coefficient of variation. For three reagents. the mean ISI values for ACL (nephelometric) and STA (mechanical) were practically identical, but the mean ISI values fo r MLA (photo-optical) were at least 10% higher. For two other reagents prep ared from rabbit tissue, the mean ISI values increased in the order ACL, ST A. MLA. The widest range of mean ISI values was noted with one rabbit tissu e factor reagent: 1.68 for ACL and 2.00 for MLA. Exclusion of patient speci mens with INR <1.5 and INR >4.5 determined by the international reference p reparation resulted in a slight decrease of the mean ISI. The interlaboratory variation of the International Normalized Ratio (INR) w as assessed from the results obtained with common lyophilized and deep-froz en plasmas. The use of instrument-specific ISI values resulted in reduced i nterlaboratory variation of the INR. It is recommended that thromboplastin manufacturers provide instruments-specific ISI values.