Arterial Blood Gas-Derived Risk Models for 28-Day Mortality in Critically Ill Adults: A Retrospective Cohort Study

BackgroundAcid-base disturbances are common in critically ill patients and provide immediate physiological insight and prognostic information. We aimed to characterize the epidemiology of specific acid-base patterns and to develop ABG-driven models for 28-day mortality.MethodsWe retrospectively analyzed 1150 critically ill adults with acid-base disorders identified from the first arterial blood gas (ABG) after ICU admission. The primary outcome was 28-day mortality. Three multivariable logistic regression models were developed: (1) a continuous model using physiological variables, (2) a pragmatic threshold-based model with Firth bias-reduced logistic regression, and (3) an ABG-only model. Discrimination (area under the receiver operating characteristic curve [AUC]), calibration, Brier score, and decision curve analysis (DCA) were reported with bootstrap optimism.ResultsMixed acid-base disorders were the most prevalent (35.7%) and associated with high mortality (73.2%), while respiratory alkalosis also carried high mortality (69.6%). Non-survivors showed an apparently "normal" or higher pH (7.40 vs 7.30) despite marked hypocapnia and elevated lactate (4.8 vs 2.5 mmol/L). The continuous model achieved an AUC of 0.94 (optimism-corrected 0.88). The threshold model achieved an AUC of 0.85, with lactate > 4 mmol/L (adjusted odds ratio [aOR] 7.68) and sepsis (aOR 2.78) as dominant predictors. The ABG-only model maintained high discrimination (AUC 0.92; optimism-corrected 0.88) with acceptable calibration.ConclusionsRoutine ABG combined with basic clinical data at ICU admission enables accurate early mortality risk stratification in ICU patients. An apparently normal pH may conceal severe metabolic stress, emphasizing the need for integrated acid-base assessment in critical care.