Assay discrepancy in mild haemophilia A: entire population study in a National Haemophilia Centre

Assay discrepancy in mild haemophilia, here defined by a significantly higher factor VIII (FVIII):C response by the one-stage procoagulant assay as compared with a two-stage enzymatic method, has repeatedly been reported in literature. The purpose of this study was to determine the overall prevalence of this phenomenon amongst mild haemophilia families from a population of 2.95 million inhabitants in the Western Danish region. Information was collected retrospectively through a thorough search of archives of the National Haemophilia Centre in Aarhus. We identified 109 patients with mild haemophilia A amongst whom 92 were eligible to enter the study. These represent a total of 53 unrelated families. Our data illustrate that this assay discrepancy pattern is found quite frequently amongst our mild haemophilia A families. While the ratio of FVIII:C chromogenic/FVIII:C clot values was quite consistent amongst patients belonging to same family pattern, ratios in the entire cohort of families ranged from 0.18 to 1.00. Selecting a cut-off level for the FVIII:C chromogenic/FVIII:C clot ratios at 0.7, 0.6 and 0.5, respectively, we found that 38 (72%), 27 (51%) and 19 (36%) of families, respectively, displayed this assay discrepancy. In 10 patients, the FVIII:C chromogenic level was inside the category of moderate haemophilia at >0.01-<0.05 IU mL(-1), pointing to a class-shift in the biochemical phenotype. In conclusion, our data illustrate a substantial prevalence of the assay discrepancy phenomenon amongst mild haemophilia A patients in our geographical area.