Factor VIII activity as measured by an amidolytic assay compared with a one-stage clotting assay

Factor VIII amidolytic activity was measured by a commercially available method and compared with clotting activity measured in a one-stage assay. Parallel assays with both methods were used for plasma samples from 40 blood donors with normal or high Factor VIII levels, 22 patients with hemophilia before or after treatment with Factor VIII concentrates, 10 patients with chronic liver disease, and 10 normal subjects with high Factor VIII levels after treatment with desmopressin. The results obtained with the amidolytic assay were highly correlated (r = 0.97) with those obtained in the one-stage clotting assay. There were no significant differences in the results with the two methods in any of the patient groups, although in two cases of mild hemophilia the amidolytic assay gave lower values than the clotting assay. The reproducibility of the amidolytic assay (coefficient of variation = 6%) was better than that of the clotting assay (12%) at both normal and low levels of Factor VIII.