Background/Purpose: We previously established the value of cell-bound complement activation products (CBCAPS) in the diagnosis of systemic lupus erythematosus (SLE) (CAPITAL study). The purpose of this study was to enhance the performance of this diagnostic method for SLE patients compared with non-SLE rheumatic disease patients. For this, we combined CBCAPs together with autoantibodies to extractable nuclear antigens (ENA: anti-Smith [Sm], SS-B, Centromere, Scl-70, Jo-1), and mutated citrullinated vimentin (MCV).

Methods: The study population consisted of 678 subjects (291 SLE, 272 other diseases and 115 healthy) enrolled in CAPITAL (n=503) and follow-up studies (n=175). All SLE patients met the 1982 ACR SLE classification criteria and presented with active and non-active disease. The group of non-SLE patients consisted of subjects with rheumatoid arthritis, systemic sclerosis, primary Sjogren’s, polymyositis/dermatomyositis and various other rheumatic diseases. ANA, autoantibodies to ENA and MCV were measured using solid phase immunoassays. C4d fragment deposited on erythrocytes (EC4d) and B-lymphocytes (BC4d) were determined using flow cytometry. The analysis involved two consecutive “tiers” of analyses. In tier 1 the diagnosis of SLE relied on anti-dsDNA positivity (base model) with the addition of positivity for anti-Sm and elevated CB-CAPS (EC4d>35 units or BC4d>200 units). Tier 2 was determined among subjects negative in tier1 and consisted of a weighted index score of ANA positivity (base model), with the addition of EC4d/BC4d levels and positivity for antibodies to ENA/MCV. Positivity for the index score (>0) was indicative of SLE, and the two-tier combination resulted in the overall performance characteristics. Statistical analyses utilized area under receiver operating characteristic (ROC) curves (SLE vs. non-SLE patients), and calculations of diagnostic sensitivity and specificity.

Results: Positivity for ANA was sensitive for SLE (89%); specificity for non SLE subjects was low (54%). Conversely, anti-dsDNA and anti-Sm were less sensitive (32% and 11%, respectively) but highly specific (96% and 100%, respectively). EC4d and BC4d levels were 2.6 and 3.2-fold higher in SLE than in non SLE subjects (p<0.01). The model combining anti-dsDNA with ANA positivity (base model) yielded poor performances (AUC=0.689) (Table). However, the stepwise addition of antibodies to ENA/MCV and CBCAPs in two-tier analysis improved the overall specificity (AUC=0.887; p<0.01 vs. base model; p<0.01 vs. base model with ENA/MCV). The overall performance of the best model consisted of 80% sensitivity for SLE, and 85% specificity in distinguishing SLE from other rheumatic diseases. Specificity against healthy individuals was 97%.

Conclusion: An assay panel combining CBCAPS with ANA, anti-dsDNA, and autoantibodies to ENA and MCV is sensitive and specific for SLE.