The Acute Illness, Not Cumulative Comorbidity, Dictates Short-Term Functional Recovery in An ACE Unit: A retrospective cohort study

Abstract Background Accurate pre-intervention assessment in an ACE unit is crucial, yet heavily reliant on comorbidity indices like the Charlson Comorbidity Index (CCI), which often fail to explain significant outcome heterogeneity. This study aimed to determine whether the primary admission diagnosis, representing the acute physiological insult, is a more powerful predictor of short-term functional recovery than the CCI. Methods The primary outcome was short-term functional recovery, measured as the change in the Barthel Index (BI) from admission to discharge (ΔBI). Hierarchical multiple linear regression was used to create three models: a base model (demographic/clinical covariates), a CCI model, and a primary diagnosis model. Model performance was compared using Adjusted R², Area Under the Curve (AUC) from ROC analysis, Net Reclassification Improvement (NRI), and information criteria (AIC/BIC). Results Compared to matched controls, ACE unit patients had significantly shorter lengths of stay (9.82 vs. 11.15 days, p = 0.004), lower hospitalization costs (p < 0.001), higher mean discharge BI (80.49 vs. 75.70, p = 0.033), and lower 15-day readmission rates (1.41% vs. 9.86%, p < 0.001). In the multivariable analysis of the ACE cohort, the primary admission diagnosis was a strong predictor of ΔBI (e.g., Neurological vs. Renal disease, B = 5.091, p = 0.001), while the CCI was not a significant predictor (p = 0.628). The Diagnosis Model demonstrated superior performance over the CCI Model, with a higher Adjusted R² (0.85 vs. 0.78), a significantly better AUC for predicting clinically significant recovery (0.928 vs. 0.888, p = 0.031), and a positive NRI (0.38, p = 0.004). Conclusions The short-term functional recovery of hospitalized older adults is dictated by the nature of the acute illness, not the accumulation of chronic diseases. This suggests the acute diagnosis reflects a unique pathophysiological stress and recovery potential not captured by comorbidity indices. Clinicians should prioritize the primary diagnosis for risk assessment and tailoring interventions in ACE units.