Neutrophil/Lymphocyte Ratio as a Diagnostic Tool to Distinguish Infection From Disease Reactivation in Systemic Lupus Erythematosus: An Analytical Observational Study

INTRODUCTION: Differentiating between infection and flare in patients with systemic lupus erythematosus (SLE) presents a clinical challenge due to the overlap of clinical manifestations and laboratory findings. The neutrophil-to-lymphocyte ratio (NLR) has been proposed as an accessible marker of systemic inflammation. The aim was to evaluate the association and diagnostic performance of NLR in identifying infection and differentiating it from a flare in patients with SLE. METHODS: A longitudinal analytical observational study was conducted at the Víctor Lazarte Echegaray Hospital (Peru) between January 2019 and June 2025. 213 patients with SLE were included, of whom 136 presented with flares and 77 with infection. The NLR was calculated from the complete blood count at the time of clinical evaluation. Its association with infection was assessed using logistic regression, and its diagnostic performance was evaluated using receiver operating characteristic ROC curves, comparing it with C-reactive protein (CRP). RESULTS: The NLR was significantly higher in patients with infection. In logistic regression analysis, elevated NLR values were associated with infection (OR: 1.25; 95% CI: 1.15-1.36; p<0.001), remaining significant after adjustment (adjusted OR: 1.18; 95% CI: 1.08-1.30; p<0.001). The NLR showed excellent discriminative ability (AUC: 0.97), with a cutoff point of 6.8, sensitivity of 90%, and specificity of 92%. C-reactive protein (CRP) showed similar performance (AUC: 0.97). The combined model increased sensitivity to 97% and reduced the negative likelihood ratio to 0.03. CONCLUSIONS: NLR is a simple, accessible, and low-cost marker that allows differentiation between infection and flare in patients with SLE, with diagnostic performance comparable to that of CRP. Its combined use can improve clinical decision-making in hospital settings.