The interference of fibrinogen and heparin with the determination of circulating immune complexes in the C1q-binding assay.

When citrate plasma and serum of the same individual were tested simultaneously in the C1q-binding assay (C1qBA), binding levels in plasma were found to be 90-400% higher than in serum. The difference in 125I-Clq binding was due to the presence of fibrinogen in plasma. It was shown that complex formation between fibrinogen and 125I-Clq occurs and that this complex precipitates in the presence of polyethylene glycol, leading to the false positive results in the ClqBA. When heparin plasma was used to the assay, heparin itself also induced an increase in 125I-C1q binding that was not based on the presence of immune complexes. The effect of both fibrinogen and heparin could be inhibited by addition of protamine sulphate. Therefore, pretreatment of plasma with protamine sulphate makes it possible to use plasma samples for a reliable determination of C1q-binding levels. However, serum that is well clotted should be used preferentially.