Sensitivity and specificity of plasma and urine complement split products as indicators of lupus disease activity

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Abstract
Objective. To determine if measurement of serum complement split products (C4d, Bb, C5b–9) is better than conventional C3 and C4 measurements in distinguishing patients with varying degrees of lupus disease activity, and to determine if the presence of C3d in urine is helpful in distinguishing lupus patients with from those without early lupus nephritis. Methods. Lupus disease activity was prospectively determined at 3 consecutive visits an average of 4 months apart, using the Systemic Lupus Activity Measure (SLAM), the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI), and physician global assessment (PGA). Blood samples were evaluated for the presence of C4d, Bb, and C5b–9 by quantitative microsassay plate enzyme immunoassay at each patient visit. We characterized urinary excretion of C3 fragments (with attention to C3d) by sodium dodecyl sulfate–polyacrylamide gel electrophoresis with Western blotting. Results. Thirty‐one SLE patients were enrolled in the study. The mean SLAM score and the mean SLEDAI score each correlated well with the PGA at all 3 visits. A SLAM score of 6 and a SLEDAI score of 4 had the best overall sensitivity and specificity for predicting moderate‐to‐severe disease activity by PGA (100% and 73%, respectively, for the SLAM and 86% and 94%, respectively, for the SLEDAI). Serum C4d and Bb were more sensitive indicators of current moderate‐to‐severe lupus disease activity at all 3 visits than were serum C5b–9, C3, and C4. C3 and C4 were more specific indicators of moderate‐to‐severe disease activity. Serum C4d and Bb were more sensitive at predicting moderate‐to‐severe disease activity at subsequent visits than were C5b–9, C3, and C4. Urine C3d was better than C3, plasma C4d, Bb, C5b–9 and anti–double‐stranded DNA antibody in distinguishing patients with from those without acute lupus nephritis (P = 0.02). Conclusion. C4d and Bb are sensitive indicators of moderate‐to‐severe lupus disease activity and may be most helpful in situations where conventional measurements are not, such as in lupus patients whose C3 and C4 levels remain normal despite evidence of clinical disease activity. It appears from this study that detection of urine C3d may be a simple way of measuring complement activation in the setting of lupus renal disease. The availability of instruments for clinical disease activity measurement such as the SLAM and the SLEDAI may enable more consistent definition of lupus disease activity and may thus provide a means for better examining the role of complement activation products in predicting lupus disease activity in larger patient populations.