Differences between one stage clotting and chromogenic factor VIII assay results

Introduction: Hemophilia A patients can be treated with various FVIII concentrates. Treatment is monitored by the commonly used one-stage clotting assay, the two-stage clotting assay or a chromo-genic assay. The aim of this study was to analyze possible differences between the one-stage clotting and chromogenic assay in healthy volunteers and Hemophilia A patients untreated or treated with FVIII concentrates. Methods: We analyzed samples from healthy volunteers (27), untreated hemophiliacs (23), and treated hemophiliacs (130). These hemophiliacs received recombinant FVIII concentrates: (Refacto, 34; Kogenate, 27; Advate, 27; Helixate, 11), purified concentrate (Aafact, 2) or desmopressin (Minrin, 29). The chromogenic and clotting FVIII assays (total CV 6.1 and 4.3%, resp. 2.1 and 6.3% for normal pool and 40% FVIII concentration) were determined using a CA-7000 system with accompanying reagents (Siemens). Results: In healthy volunteers we found a ratio in assay results (chro-mogenic over one stage clotting) of 0.8 (range 0.6-0.9) indicating lower results obtained with the chromogenic assay. In hemophiliacs treated with Refacto using a human plasma calibration we found a ratio of 1.3 (0.4-1.7). The manufacturer recommends the Refacto Laboratory Standard for monitoring Refacto treatment, considering this we found a ratio 1.0 (0.3-1.3). With other factor VIII concentrates the ratios are Advate 1.1 (0.6-1.4), Kogenate 1.3 (0.7-2.2), Helixate 1.1 (0.6-1.4), Aafact 1.0 (0.9-1.1). With the desmopressin-treated patients the ratio was 1.2 (1.0-1.6). Conclusion: The differences in factor VIII results between a one-stage clotting assay and a chromogenic assay can be very large in individual patients, but also on average. Physicians and clinical chemists should be aware of these differences when interpreting factor VIII results in individual patients.