Diagnostic value of serum complement C3 and C4 levels in Chinese patients with systemic lupus erythematosus.

In 2009, hypocomplementemia involving C3, C4, and total hemolytic complement (CH50) was proposed as an immunologic criterion to enhance the sensitivity of systemic lupus erythematosus (SLE) classification criteria. This study evaluated the diagnostic value of low serum complement C3 and C4 levels in Chinese patients with SLE. In total, 2452 patients were enrolled in this study (158 with SLE and 2294 with other diseases). Receiver operating characteristic analysis showed that the optimal C3 and C4 cut-off levels for a diagnosis of SLE were 0.785 g/L (sensitivity, 77.9%; specificity, 81.5%) and 0.145 g/L (sensitivity, 80.1%; specificity, 83.2%), respectively. The prevalence of a low C3 or C4 level alone was similar between patients with SLE and those with other diseases, while the prevalence of simultaneously low C3 and C4 levels was higher in patients with SLE (73.42%). Antinuclear antibody had a high sensitivity (96.64%) and low negative likelihood ratio (0.04). Hypocomplementemia with positive antinuclear antibody had a high positive likelihood ratio. Inclusion of hypocomplementemia as a classification criterion for SLE resulted in a 16.18% increase in the number of patients assigned to the SLE group (from 136 to 158 patients). Hypocomplementemia was highly prevalent in patients with hematological disease (41.94%). These results suggest that hypocomplementemia has important diagnostic value for SLE by improving the sensitivity of the diagnosis of SLE. C3 and C4 should be tested simultaneously because a low C3 or C4 level alone is not a suitable immunological criterion.