Hemoglobin-to-Red Cell Distribution Width Ratio Predicts Mortality in Chronic Critical Illness: A MIMIC-IV Cohort Study

Chronic critical illness (CCI) is a common ICU syndrome associated with persistent inflammation, immunosuppression, and high mortality. The hemoglobin-to-red blood cell distribution width ratio (HRR) is a novel biomarker reflecting oxygenation and inflammatory status, but its prognostic significance in CCI remains unclear. We conducted a retrospective cohort study using the MIMIC-IV database, including 2,855 ICU patients with CCI. Patients were categorized into quartiles according to HRR measured within 24 h of ICU admission. The primary outcomes were 28-day, 90-day, and 365-day mortality. Kaplan–Meier analysis, multivariable Cox proportional hazards models, generalized additive models (GAM), and subgroup analyses were performed. Patients with lower HRR had greater illness severity and comorbidity burden. Higher HRR was associated with improved survival at all time points (log-rank P < 0.001). After full adjustment, patients in the highest HRR quartile had 25%, 37%, and 48% lower risks of 28-day, 90-day, and 365-day mortality, respectively. GAM showed linear negative associations between HRR and short-term mortality and a nonlinear association with 365-day mortality. Subgroup analyses demonstrated consistent associations across most strata, but the protective effect was attenuated in patients with renal disease (P for interaction = 0.017). HRR is an independent predictor of short- and long-term mortality in CCI and may serve as a simple tool for risk stratification, particularly in patients without renal disease.