Kinetic determination of blood coagulation Factor XIII in plasma.

We have designed a new kinetic assay for estimating Factor XIII in plasma. Plasma fibrinogen is removed by treatment with bentonite (colloidal aluminum silicate) before measurement. During the lag phase, Factor XIII is transformed by thrombin and Ca2+ into active transglutaminase (EC 2.3.2.13), which attaches the substrate ethylamine to a glutamine residue in acetylated, dephosphorylated beta-casein. During the reaction, ammonia is released, which can be continuously monitored in an NADPH-dependent indicator reaction catalyzed by glutamate dehydrogenase (EC 1.4.1.4). We determined the optimal concentrations of substrate and activator and found that, to eliminate the clottable fibrinogen from the plasma samples, bentonite treatment was more advantageous than the traditional heat treatment. Results by the method correlate well with those by the most widely used amine incorporation and immunoinhibition assays for Factor XIII. We established a reference interval of 12.1-22.7 U/L; at optimal conditions, the variance of the method was less than 3% within this range. The method has several theoretical and practical advantages over traditional determinations of Factor XIII.