Determination of C1s-C1 inhibitor complexes in plasma by means of an enzyme linked immunosorbent assay.

An enzyme linked differential antibody immunosorbent assay for the quantitation of the C1s-C1 inhibitor complex has been developed. A study of the assays' performance under various conditions has shown that before use in the assay, it is imperative to remove competing forms of C1s from the samples to be tested. This is conveniently achieved in human plasma or serum by polyethylene glycol precipitation of the C1qrs, since the C1s-C1 inhibitor complex remains soluble and can be assayed in the supernatant solution. The detection limit of the assay in the plasma milieu is 0.1 mg/l, and the concentrations of the C1s-C1 inhibitor complex were found to be 1 mg/l in citrated plasma and 2 mg/l in serum. Activation of the fibrinolytic system in vivo does not seem to result in any appreciable C1 activation, since there was no concomitant major change in the plasma concentration of the C1s-C1 inhibitor complex.