Clinical and Serological Correlates of Serum C1Q and Anti-C1Q Antibodies in South Africans With Systemic Lupus Erythematosus

Objective: To investigate prevalence and clinical correlates of serum C1q and anti-C1q antibody titres in black South Africans with systemic lupus erythematosus (SLE). Methods: Cross-sectional study of 96 SLE patients, 49 with lupus nephritis (LN). Anti-C1q antibodies were tested by ELISA. Serum C1q was measured as a percentage of normal by immunoelectrophoresis. Disease activity was assessed using the SELENA-SLE disease activity index. Results: Most patients were female (90.7%), mean (SD) age and follow-up period diagnosis of 38.1 (13.0) and 4.2 (4.4) years, respectively. Low serum C1q and positive anti-C1q antibody test were detected in 17 (17.7%) and 12 (12.5%) patients, respectively, overall. There was an inverse correlation between serum C1q and anti-C1q antibodies titres (r = −0.22, p = 0.03) and a direct correlation of anti-C1q antibodies titres with SELENA-SLEDAI scores (r = 0.27, p = 0.008). Patients with an active urine sediment (n = 21) had higher anti-C1q antibodies titres (p = 0.007) and low serum C1q (OR = 4.51, p = 0.01), compared to the remainder of patients. Anti-C1q antibody titres were higher in patients with C3/C4 hypocomplementaemia (n = 14) than those with normal C3/C4 (p = 0.02). A positive Coombs test (without evidence of red cell haemolysis) (n = 17) was associated with a positive anti-C1q antibody test (OR = 4.29, p = 0.02), low serum C1q (OR = 3.37, p = 0.04) and C3/C4 hypocomplementaemia (OR = 4.84, p = 0.02). Conclusion: Our findings broadly confirm the clinical utility of the anti-C1q antibody test in SLE. Raised anti-C1q antibody titres were associated with active LN, as evidenced by an active urine sediment. The association of a positive anti-C1q antibody and low serum C1q with a positive Direct Coombs' test merits further study and confirmation.