Prognostic value and nonlinear association of the rdw/albumin ratio with mortality among COVID-19 patients: a MIMIC-IV database analysis

BACKGROUND: RDW and the RDW-to-albumin ratio (RAR) have prognostic relevance in many conditions, including COVID-19. We assessed their prognostic value, examined the potential nonlinear association of RAR with mortality, and tested whether RAR and RDW improve prediction by established ICU scores and COVID-19 prognostic markers. METHODS: In this retrospective cohort of 373 ICU COVID-19 patients from MIMIC-IV, clinical and laboratory data within 24 h of ICU admission were analyzed. RAR and RDW were divided into tertiles. Kaplan–Meier plots and Cox models were used to evaluate mortality associations after confirming model assumptions and checking collinearity. Restricted cubic splines examined RAR nonlinearity. ROC curves were used to compare the predictive accuracy of RAR and RDW. Predictive performance was then further evaluated using ROC curves, Brier scores, calibration plots, and DCA for models with and without RAR or RDW added to LDH, SOFA, SIRS, APS III, and SAPS II. Subgroup and fairness analyses were performed. RESULTS: A total of 31.9% of the cohort died within 28 days of ICU admission. Higher RAR was associated with increased 28-day ICU mortality, with the highest tertile showing the greatest risk. (HR = 2.12, 95% CI 1.19–3.76, P = 0.010). Trend analysis confirmed a dose-response relationship (P for trend = 0.0137). RDW did not show a significant association with 28-day ICU mortality in Model 3. RAR had better predictive performance than RDW in unadjusted and partly adjusted models (Model 1: 0.674 vs. 0.586, DBA = 0.089, P = 0.002; Model 2: 0.700 vs. 0.650, DBA = 0.051, P = 0.005). A nonlinear J-shaped relationship was identified between RAR and 28-day ICU mortality, with a threshold at 4.98. Adding RAR to LDH or SIRS improved mortality prediction across outcomes, whereas SOFA gained mainly for long-term outcomes. Subgroups confirmed more stable performance for RAR across most subgroups. CONCLUSION: RAR outperforms RDW and shows a nonlinear relationship with mortality. RAR enhances risk prediction when added to simpler markers and SOFA, supporting its use in risk stratification among critically ill COVID-19 patients. CLINICAL TRIAL NUMBER: Not applicable.