To evaluate the usefulness of C3 and C4 fractions, CH50, sC5b-9, and anti-dsDNA and anti-C1q antibodies in the immunological monitoring of systemic lupus erythematosus (SLE), with or without renal involvement. This cross-sectional study included 74 SLE patients, 32 with renal involvement and 42 without. Immunological parameters were correlated with overall disease activity and with lupus nephritis (LN) activity. Significant differences were observed between the two groups. Renal involvement was strongly associated with anti-dsDNA (P = 0.001), anti-C1q (P = 0.001), low C3 levels (P < 0.0001), and elevated plasma levels of sC5b-9 (P = 0.002). In non-renal SLE, no correlation was found between disease activity and C3, C4, CH50, sC5b-9, anti-C1q, only anti-dsDNA correlated with disease activity (P = 0.02). In contrast, in renal SLE, LN activity correlated positively with anti-dsDNA (P = 0.012) and anti-C1q (P < 0.0001), and negatively with C3 (P = 0.008) and CH50 (P = 0.003). Elevated sC5b-9 levels were also significantly associated with active LN (P = 0.018). Receiver operating characteristic (ROC) curve analysis identified anti-C1q as the best marker of renal involvement, with an area under the curve of 0.929 and a negative predictive value (NPV) of 92% for and active LN. Immunological monitoring of SLE should be tailored to the clinical phenotype. Complement exploration is of limited value in SLE without renal involvement due to moderate activation during active disease. In renal SLE, complement markers prove particularly useful, with anti-C1q emerging as the best marker of lupus nephritis activity.

To evaluate the usefulness of C3 and C4 fractions, CH50, sC5b-9, and anti-dsDNA and anti-C1q antibodies in the immunological monitoring of systemic lupus erythematosus (SLE), with or without renal involvement. This cross-sectional study included 74 SLE patients, 32 with renal involvement and 42 without. Immunological parameters were correlated with overall disease activity and with lupus nephritis (LN) activity. Significant differences were observed between the two groups. Renal involvement was strongly associated with anti-dsDNA (P = 0.001), anti-C1q (P = 0.001), low C3 levels (P < 0.0001), and elevated plasma levels of sC5b-9 (P = 0.002). In non-renal SLE, no correlation was found between disease activity and C3, C4, CH50, sC5b-9, anti-C1q, only anti-dsDNA correlated with disease activity (P = 0.02). In contrast, in renal SLE, LN activity correlated positively with anti-dsDNA (P = 0.012) and anti-C1q (P < 0.0001), and negatively with C3 (P = 0.008) and CH50 (P = 0.003). Elevated sC5b-9 levels were also significantly associated with active LN (P = 0.018). Receiver operating characteristic (ROC) curve analysis identified anti-C1q as the best marker of renal involvement, with an area under the curve of 0.929 and a negative predictive value (NPV) of 92% for and active LN. Immunological monitoring of SLE should be tailored to the clinical phenotype. Complement exploration is of limited value in SLE without renal involvement due to moderate activation during active disease. In renal SLE, complement markers prove particularly useful, with anti-C1q emerging as the best marker of lupus nephritis activity.