Determination of coagulation factor VIII activity by a chromogenic substrate method on STA, an automated coagulation analyzer

This study was aimed at evaluating the performance of a chromogenic factor VIII assay on STA, an automated coagulation analyzer. Additionally, a correlation study was conducted with an aPTT-based one-stage factor VIII clotting assay. Throughout the study the performance of the chromogenic assay was tested in two ranges of factor VIII activity: a high range with activity between 20% and 150% and a low range with activity below 20%. Inter-assay coefficient of variation ranged from 1.9% to 8.9% and intra-assay coefficient of variation from 0.5% to 11.4%, depending on factor VIII concentration. Day-to-day reproducibility was tested over a 5-day period; between-day imprecision ranged from 7.1% to 9.4%. The chromogenic factor VIII assay showed a good correlation with the clotting assay in both ranges. The correlation coefficients were 0.924 and 0.792 for the high and low range, respectively. A statistically significant difference in mean values was observed in the high range (p < 0.0001). Comparison between the chromogenic assay on STA versus on microplates showed a high correlation (0.991), which was highly significant (p < 0.0001). In conclusion, the chromogenic assay for factor VIII on STA shows good analytical performance. It correlates well with the one-stage factor VIII clotting assay, although significant differences between individual samples occur. Probably these are partly related to differences in measurement principle and standardization. Altogether, this precise and rapid assay is suitable for determination of factor VIII by an automated procedure.