Peripheral neutrophil CD64 index combined with complement, CRP, WBC count and B cells improves the ability of diagnosing bacterial infection in SLE

Objective To investigate the diagnostic role of complement C3, complement C4, C-reactive protein (CRP), procalcitonin (PCT), white blood cell count (WBC), neutrophil CD64 (nCD64) index, lymphocyte subsets and their combination in differentiating bacterial infection from disease relapse in systemic lupus erythematosus (SLE). Methods The above biomarkers in 36 hospitalized SLE patients with bacterial infection and 45 with lupus flare without infection were retrospectively studied. Bacterial infection was proven by positive cultures or typical clinical symptoms and signs combined with positive response to antibiotics. Lupus flare was defined as three points greater than their previous SLE disease activity index score. The diagnostic value for bacterial infection was evaluated by the areas under the receiver operating characteristic curves (AUC) and a novel bioscore system combining multiple biomarkers. Results Increased CRP (p=0.049), WBC (p=0.028) and nCD64 index (p=0.034) were observed in the infected group and C3 (p=0.001), C4 (p=0.016) and B cells levels (p=0.010) were significantly reduced. The AUC for the above six biomarkers had no significant difference. Interestingly, the combination of nCD64 index, CRP, WBC, C3 and C4 improved significantly the diagnostic potential of SLE infection (AUC 0.783 (interquartile range 0.672, 0.871), p<0.001; sensitivity 85.29% specificity 62.50%). In the bioscore system including the above six biomarkers, the bacterial infection rate in patients with bioscore 2, 3, 4, 5 and 6 were 0.00, 39.29, 59.10, 61.54 and 100.00%, respectively. Conclusion The combination of nCD64 index, C3, C4, CRP, WBC and B cells in a bioscore is useful to diagnose bacterial infection in SLE.