This study is a one year prospective observational cohort study designed to compare the performance of APACHE II and APACHE IV severity scoring systems in predicting ICU mortality and key clinical outcomes in adult critically ill patients. Accurate risk stratification is central to ICU benchmarking, prognostication, and resource allocation, yet the applicability of widely used scoring systems varies across populations and healthcare settings. While APACHE II remains extensively used due to its simplicity, APACHE IV incorporates updated physiological variables, diagnostic categories, and predictions for both mortality and ICU length of stay, potentially offering improved prognostic accuracy. However, evidence from Indian ICUs has shown inconsistent calibration and discrimination of these models, underscoring the need for local validation.

All consecutive adult patients (equal or more than 18 years) admitted to the adult intensive care unit of a tertiary care hospital over a 12 month period will be enrolled. Demographic data, comorbidities, admission characteristics, and physiological and laboratory parameters required to calculate APACHE II and APACHE IV scores will be prospectively collected within the first 24 hours of ICU admission using standardized case report forms. Patients admitted solely for comfort-focused or end of life care at entry will be excluded.

The primary outcome is ICU mortality. Secondary outcomes include ICU length of stay, requirement and duration of mechanical ventilation, vasopressor support, and renal replacement therapy. The predictive performance of APACHE II and APACHE IV will be evaluated using measures of discrimination (area under the receiver operating characteristic curve) and calibration (calibration plots, calibration-in-the-large, and goodness of fit testing). Comparative analyses between the two models will be performed using appropriate statistical methods for paired prognostic scores.

As a non-interventional study using routinely collected clinical data, this study poses minimal risk to participants and does not alter standard clinical care. The findings are expected to provide context specific evidence regarding the utility and limitations of APACHE II and APACHE IV in an Indian ICU setting, thereby informing clinical practice, quality benchmarking, and future model recalibration efforts.