Evaluation of Chromogenic Factor VIII Assay Compared with One-Stage Clotting Assay

Congenital factor VIII (FVIII) deficiency causes hemophilia A due to different types of defects in the FVIII gene. Although the chromogenic measurement is the reference method and shows less variability, a one-stage assay is the most commonly preferred method for measurement of FVIII. In this study, we aimed to evaluate the analytical performances of chromogenic and one-stage assays, and compare the results prior to introduction of newly developed extended half-life recombinant FVIII products.

Sixty-six blood samples from residual material of Istanbul Faculty of Medicine, Central Laboratory workflow comprised the study group. Samples were classified; plasma FVIII > 40 IU and FVIII < 40 IU. FVIII activities were measured using one-stage clotting and chromogenic assays on a CS-2500 analyzer. Analytical performances were determined through precision, linearity, carryover, and comparability studies.

The within-run CV% of the one-stage assay on the CS-2500 had 1.6%, 2.6%, the between day CV% were 8.5%, 4.9 % for low and high controls, respectively. The within-run CV% of chromogenic method had 1.2% and 0.9%. Both methods demonstrated good linearity (R2 > 0.998), and the comparisons of both assays exhibited good agreement with minor bias for FVIII activity > 40 IU. However, a significant bias was obtained for FVIII activity < 40 IU.

We obtained higher results using the one-stage assay compared with the chromogenic assay, and a significant bias was found for the samples lower than 40 IU. The discrepancy can explained by the presence of a weak agreement for samples lower than 10 IU due to the lower detection limit of the chromogenic assay used in this study (1.5%).