Association between monocyte-to-lymphocyte ratio and mortality in critically ill patients with acute respiratory failure: a retrospective cohort study.

Background: Accumulating clinical evidence suggests that the monocyte-to-lymphocyte ratio (MLR) may serve as a hematological biomarker with prognostic significance for unfavorable outcomes in cardiovascular diseases, cancers, infections, respiratory ailments, and endocrine disorders. Nonetheless, research examining the prognostic value of MLR in acute respiratory failure (ARF) remains scarce. The present investigation seeks to clarify the prognostic association between MLR and adverse outcomes among critically ill individuals with ARF. Methods: Critically ill individuals diagnosed with ARF were extracted from the Medical Information Mart for Intensive Care IV (MIMIC-IV) database. Baseline MLR was calculated using complete blood count data obtained within the first 24 hours of intensive care unit (ICU) admission. Patients were stratified into four categories based on MLR quartiles (Q1: MLR ≤0.36, Q2: 0.36< MLR ≤0.68, Q3: 0.68< MLR ≤1.20, Q4: MLR >1.20). The principal endpoint of interest was 28-day all-cause mortality, while extended follow-up endpoints included 90- and 365-day mortality. To characterize the association between MLR values and mortality risk, we applied Cox proportional hazards regression together with restricted cubic spline (RCS). Differences in survival probabilities across quartile categories were further illustrated through Kaplan-Meier analyses. Additionally, we executed a battery of subgroup and sensitivity analyses to validate stability of our results. Results: In total, 3,889 patients diagnosed with ARF were incorporated into the cohort. The observed all-cause mortality rates were 26.69% at 28 days, 30.21% at 90 days, and 31.70% at 1 year. Multivariate Cox proportional hazards regression analysis demonstrated that increased MLR values were robustly associated with elevated risks of all-cause mortality across all follow-up intervals. RCS analysis revealed a significant nonlinear relationship between MLR levels and the risk of all-cause mortality (P for nonlinearity <0.001). Furthermore, Kaplan-Meier analysis showed significant variations in survival outcomes among MLR quartile groups (log-rank P<0.001). A significant interaction effect between MLR levels and mortality risk was noted solely in the subgroup stratified by ischemic heart disease (IHD) comorbidity status (P for interaction <0.05). Conclusions: In critically ill individuals with ARF, the MLR exhibited a nonlinear relationship with mortality risk across both short- and long-term mortality risks. Elevated MLR levels remained independently linked to unfavorable clinical outcomes, underscoring its relevance as a prognostic biomarker and its potential role in risk stratification of ARF populations.