Functional Determination of Factor XII in Plasma

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Abstract
Clinical assays for factor XII determination are based on measurements of coagulation time in a mul tifactorial assay requiring the availability of factor XII deficient plasma, and the determination is affected by the presence of heparin. Another way of determining the amount of functionally active factor XII in plasma is to measure the amidolytic activity of factor XIIa in a chro mogenic assay. We describe such an assay, which in con trast to previously described chromogenic assays, is in dependent of the presence of prekallikrein. To prevent kallikrein from interfering with the assay, soy bean trypsin inhibitor was added to the plasma. Factor XII was autoactivated by addition of ZnCl2 and sulfatide. Sul fatide could be exchanged with acidic phospholipids but not with dextran sulfate. Linear standard curves were obtained whether the plasma was diluted with factor XII deficient plasma or a physiological buffer. The presence of a pharmacologic concentration of heparin did not affect the assay. In 37 plasma samples from healthy donors, a linear correlation was found between factor XII activity and the amount of factor XII antigen. Key Words: Factor XII—Hageman factor—Determination—Plasma.