An effect of predilution on VIII:C determination by the one-stage assay

Factor VIII concentrates are usually prediluted in Owrens buffer, before use of the one-stage assay for VIII:C determinations. We found that predilution of high purity Factor VIII concentrates in fresh VIII:C deficient plasma gave VIII:C estimates about 3 times higher than predilution in Owrens buffer. Predilution in reconstituted lyophilized VIII:C deficient plasma gave VIII:C estimates about 1.6 times higher. Likewise the VIII:C in a plasma sample can be estimated to e.g. 0.03, 0.06 or 0.10 IU/ml depending on whether the reference curve is established by predilution of the reference plasma in fresh VIII:C deficient plasma, in reconstituted lyophilized VIII:C deficient plasma or in Owrens buffer, respectively. It is shown that the major part of the differences between the effects of the various prediluents on the VIII:C determinations, can be accounted for by differences in the content of Factor V, Fibrinogen, and vitamin K dependent coagulation factors in the assay mixtures.