Elevated Monocyte-to-Lymphocyte Ratio at Peritoneal Dialysis Initiation Predicts Long-Term Peritonitis Risk: A Retrospective Cohort Study

Introduction: Peritoneal dialysis (PD)-associated peritonitis is a major complication in PD patients, leading to increased morbidity and technique failure. Identifying reliable biomarkers for predicting peritonitis risk is crucial for early intervention. Monocyte-to-lymphocyte ratio (MLR) is an emerging inflammatory marker associated with adverse outcomes in end-stage renal disease, but its predictive value for peritonitis remains unclear. Methods: This retrospective cohort study included PD patients from a single center who had undergone PD for at least 3 months. MLR was assessed at the time of PD catheter insertion, and patients were followed for 36 months. Peritonitis was defined according to the International Society for Peritoneal Dialysis criteria. Cox proportional hazards models were used to analyze the association between MLR (continuous and tertile-based) and peritonitis, adjusting for demographic, clinical, and laboratory factors. Restricted cubic spline (RCS) regression was applied to evaluate nonlinearity, and subgroup analysis was conducted to examine whether the association between MLR and peritonitis was consistent across different subgroups. Results: A total of 108 patients were included, with 33 (30.6%) developing peritonitis. MLR was significantly higher in the peritonitis group (p = 0.032). Cox regression showed that higher MLR was independently associated with an increased risk of peritonitis (adjusted hazard ratio = 1.85, 95% confidence interval: 1.01–3.40, p = 0.048). Patients in the highest MLR tertile had a sixfold increased peritonitis risk compared to those in the lowest tertile (p for trend = 0.002). RCS analysis revealed a nonlinear association, with a threshold at natural logarithm-transformed MLR = −0.9. Subgroup analysis suggested a stronger association in patients with lower body mass index (<24 kg/m2). Conclusion: Higher MLR at PD initiation is an independent predictor of long-term peritonitis risk. MLR may serve as a simple, cost-effective biomarker for early peritonitis risk stratification, particularly in leaner patients.