Some sources of error in the one-stage assay of factor VIII

The error of the one-stage assay of factor VIII activity is determined by the quality of reference and substrate (factor VIII-deficient) plasmas, as well as by the accuracy of dilution and clot observation time. Storage of the fresh normal plasma used as reference and/or of the substrate plasma for 2--3 h at 0 degrees C may cause considerable errors, which can be eliminated by the immediate use of plasma or by the determination of an appropriate correction factor. With all precautions taken into account the average error of the assay in our hands is 20%.