Assay of total factor XII and of activated factor XII in plasma with a chromogenic substrate

Abstract

The chromogenic substrate H-D-Pro-Phe-Arg-pNA (S-2302 KABI) is split by plasma kallikrein and by plasmin. Since kallikrein is activated from its precursor prekallikrein by the action of active factor XII, a photometric assay method for this factor was developed. A possible action of plasmin on the chromogenic substrate was prevented by addition of epsilonaminocaproic acid to the activation mixture. Comparable results were obtained when factor XII was simultaneously assayed by a coagulation method based on aPTT and by the photometric method with S-2302. Moreover, a direct assay of active factor XII in plasma of patients suffering from different forms of intravascular coagulation was carried out. The presence of active factor XII in plasma was found to be a sensitive indicator for intravascular activation of the clotting mechanism.