Discrepancy between one-stage and two-stage assay of factor VIII:C

Two methods (one-stage and two-stage) are commonly used for the assay of factor VIII clotting activity (VIII:C). We present collected data from seven separate studies of VIII:C assay which show that these methods do not give the same result when comparing concentrates and plasma. On average, two-stage assays detect 20% more VIII:C activity in concentrates as compared to plasmas than do one-stage assays.