A method to differentiate between anti-C1q antibodies and C1q-binding immune complexes using collagenase-digested solid phase C1q.

A method for the detection of circulating immune complexes in the presence of autoantibodies to C1q is described. Solid phase C1q-digestion with bacterial collagenase results in the elimination of the collagen-like region of C1q. Binding of model immune complexes to this modified solid phase C1q is practically unaltered, while reactivity of anti-C1q antibodies is abolished by this procedure. In conjunction with an ELISA using the collagen-like region of C1q as antigen this modified C1q solid phase assay may be used to determine immune complexes and anti-C1q antibodies in the sera of patients with autoimmune rheumatic diseases.