Disease activity in the nephritis of systemic lupus erythematosus in relation to serum complement concentrations. DNA-binding capacity and precipitating anti-DNA antibody.

Serum C4 and C3 concentration and binding of double-stranded-DNA (ds-DNA) were measured in sera from ninety-nine patients with systemic lupus erythematosus and clinical evidence of nephritis. C3 and C4 concentrations correlated poorly with ds-DNA binding. In sera from fifty-three patients, precipitating antibody was sought using the counterimmunoelectrophoretic technique. Precipitating antibody was detected on at least one occasion in 44% of the patients, and these sera with precipitating antibody showed higher binding of ds-DNA and lower C4 concentrations than those without precipitating antibody. In thirty-two patients, serial assessments of the activity of the renal disease were made using decline or improvement in glomerular filtration rate, degree of proteinuria, oedema and hypertension as indices of "activity". All patients were receiving immunosuppressive drugs. Active nephritis was rarely found in patients showing, at that time, a normal serum C4 or normal ds-DNA binding; but a raised ds-DNA binding or lowered serum C4 were found in both active and inactive nephritis. There was no correlation of activity with serum concentrations of C3, or the presence or absence of precipitating antibody. We conclude that measurements of serum-complement concentrations and binding of ds-DNA are of most use in the diagnosis of systemic lupus erythematosus, and that in patients with nephritis and taking immunosuppressive drugs, these tests are of limited use in guiding treatment.