Immunonephelometric quantitation of complement split product (C3d) plasma levels in rheumatic disease

A simple method is described for routine quantitative determination of C3d by laser nephelometry after precipitation of intact C3 by polyethylene glycol. Plasmas from 292 patients with different types of rheumatic disease was tested. Elevated C3d levels were a far more common finding than low levels of complement components C3 or C4, but despite the increased sensitivity, the test only seemed diagnostically useful for systemic lupus erythematosus and MCTD. Though a better test than the quantitation of C3 and C4, the value of C3d determination in rheumatic disease seems to be limited.