Evaluation of plasma C3d and immune complex determinations in the assessment of disease activity of patients with rheumatoid arthritis, systemic lupus erythematosus, and spondylitis ancylopoetica.

Patients with rheumatoid arthritis, systemic lupus erythematosus, and spondylitis ancylopoetica were examined, along with healthy controls, for C3d plasma levels, circulating immune complexes, C3 serum levels, and CRP. Immune complexes were determined using a Clq binding assay, a 2.75% PEG precipitation technique, including the analysis of IgG and C3, and a new laser nephelometric latex test. C3d plasma levels were significantly (P less than 1%) elevated in all groups of patients as compared to controls. With regard to the demonstration of circulating immune complexes, the PEG precipitation method discriminated best between patients and the control population. It was not possible to differentiate between the different disease entities with neither C3d serum levels or immune complexes. Concerning the assessment of disease activity, none of the evaluated parameters alone appears to be of clinical relevance. The individual application of more than one immune complex assay in combination with the measurement of C3d serum levels must be recommended if disease activity is to be assessed.