Anti-C1q antibodies in nephritis: correlation between titres and renal disease activity and positive predictive value in systemic lupus erythematosus

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Abstract
Objective: To investigate antibodies to complement 1q (anti-C1q) and investigate the correlation between anti-C1q titres and renal disease in systemic lupus erythematosus (SLE). Methods: 151 SLE patients were studied. In patients with biopsy proven lupus nephritis (n = 77), activity of renal disease was categorised according to the BILAG renal score. Sera were tested for anti-C1q by enzyme immunoassay. Serum samples were randomly selected from 83 SLE patients who had no history of renal disease, and the positive and negative predictive value of the antibodies was studied. Results: Patients with active lupus nephritis (BILAG A or B) had a higher prevalence of anti-C1q than those with no renal disease (74% v 32%; relative risk (RR) = 2.3 (95% confidence interval, 1.6 to 3.3)) (pv 53%, p = 0.06) or between active and non-active nephritis (74% v 53%, p = 0.06). Patients with nephritis had higher anti-C1q levels than those without nephritis (36.0 U/ml (range 4.9 to 401.0) v 7.3 U/ml (4.9 to 401.0)) (pConclusions: Anti-C1q in SLE are associated with renal involvement. Monitoring anti-C1q and their titres in SLE patients could be important for predicting renal flares.