Factors affecting the rate of contact activation in human plasma.

The rate of high molecular weight kininogen cleavage in human plasma treated with dextran sulfate depends largely on the length of preincubation at 37 degrees C and the sample size during preincubation. The longer the preincubation with aliquots < 100 microliters, the more refractory to contact activation the plasma became. This effect is not due to prekallikrein or factor XII consumption/inhibition since both proteins are functional in clotting assays and detectable in immunoblots even after prolonged incubation times at 37 degrees C.