Discrepancies between the one-stage clotting assay and the chromogenic assay in haemophilia B

Introduction: Assay discrepancy in factor VIII activity between the one-stage and the chromogenic assays has been described in approximately one third of patients with non-severe haemophilia A. Whether assay discrepancy may also occur in patients with haemophilia B remains unknown. Aim: This study compared the results from the one-stage and the chromogenic assays in patients with haemophilia B. Methods: Plasma samples from patients with haemophilia B attending the haemophilia centre in Malmö, Sweden, were collected after a wash-out period of more than 7 days and analysed with both assays. Results: Fifty samples from 36 patients were analysed. No discrepancy was found in patients with severe haemophilia B. Among the 44 plasma samples from patients with non-severe disease, 15 showed a twofold or greater difference between the results of the two methods, with the chromogenic method presenting the higher value (mean FIX:C_one-stage 0.02 vs. FIX:C_chromo 0.06 IU mL^-1). Of these 15 samples, 14 were from seven individuals from five families with the same mutated amino acid at the N-terminal cleaving site of the activation peptide (FIX: c.572G>A; p.Arg191His or FIX: c.571C>T; p.Arg191Cys). These mutations were not observed in any patients with non-discrepant results. The reported bleeding frequency for these patients was low and indicative of a mild bleeding phenotype. Conclusion: Our findings imply that assay discrepancy occurs for factor IX activity and that both type of assays are needed for a correct diagnosis and classification of haemophilia B. The underlying mechanism by which the mutation influences the assays remains to be determined.