title: Systemic lupus erythematosus and primary fibromyalgia can be distinguished by testing for cell-bound complement activation products.
abstract: OBJECTIVE
  We sought to establish the performance of cell-bound complement activation products (CB-CAPs) as a diagnostic tool to distinguish primary fibromyalgia (FM) from systemic lupus erythematosus (SLE).
  
  
  METHODS
  A total of 75 SLE and 75 primary FM adult subjects who fulfilled appropriate classification criteria were enrolled prospectively. CB-CAPs (erythrocyte-C4d (EC4d) and B-lymphocyte-C4d (BC4d)) were determined by flow cytometry. Antinuclear antibodies (ANAs) were determined using indirect immunofluorescence while other autoantibodies were determined by solid-phase assays. The CB-CAPs in a multi-analyte assay with algorithm (MAAA) relied on two consecutive tiers of analysis that was reported as an overall positive or negative assessment. Test performance was assessed using sensitivity, specificity, positive and negative likelihood ratio (LR).
  
  
  RESULTS
  ANAs yielded 80% positives for SLE and 33% positives for FM. High CB-CAP expression (EC4d >14 units or BC4d >60 units) was 43% sensitive and 96% specific for SLE. The CB-CAPs in MAAA assessment was evaluable in 138 of the 150 subjects enrolled (92%) and yielded 60% sensitivity (CI 95% 48% to 72%) for SLE with no FM patient testing positive (100% specificity). A positive test result was associated with a strong positive LR for SLE (>24, CI 95%; 6 to 102), while a negative test result was associated with a moderate negative LR (0.40; CI 95% 0.30 to 0.54).
  
  
  CONCLUSIONS
  Our data indicate that CB-CAPs in MAAA can distinguish FM from SLE.
authors: Cedars Sinai Medical Center, Los Angeles, California, USA; Wallace Rheumatology Study Center, Los Angeles, California, USA.
authors: Daniel J
authors: DJ
authors: Wallace
authors: Cedars Sinai Medical Center , Los Angeles, California , USA.
authors: Stuart L
authors: SL
authors: Silverman
authors: Exagen Diagnostics , Vista, California , USA.
authors: John
authors: J
authors: Conklin
authors: Exagen Diagnostics , Vista, California , USA.
authors: Derren
authors: D
authors: Barken
authors: Exagen Diagnostics , Vista, California , USA.
authors: Thierry
authors: T
authors: Dervieux
keywords: Fibromyalgis/Pain Syndromes; Systemic Lupus Erythematosus; complement