Quantification of contact system activation using a radiometric assay for activated factor IX.

A radiometric assay to quantify activation of the contact system of human plasma is described. The assay utilizes tritiated bovine factor IX as a probe and measures release of its radiolabeled activation peptide. which is soluble in trichloracetic acid. Using celite as an activator. rate of factor IX activation increased asymptomatically as celite concentration increased from 0 to 5 mg/mI plasma. changing over 100-fold. Parallel activated partial thromboplastin times showed a sixfold change. Fitting our data to an equation for a hyperbola. yielded a K112 of 1 .24 ± 0.21 mg celite/mI plasma and a V,,, of 1125 ± 65 cpm/min. Contact activation of various materials can be compared using this system. either by determining their respective K112 values, or by relating the contact activating ability of a substance to that of a standard such as celite. using the substance’s measured reaction velocity and the above equation. Using the latter method we determined that celite is 345-fold more potent than Dacron aortic graft in activating the contact system. The radiometric assay can quantify activation of the contact system more precisely than a clotting test and can be used to compare the contact activating ability of various materials. This suggests its potential usefulness in the search for biocompatible. nonthrombogenic artificial surfaces and prostheses.