Background and aims: Critically ill patients are frequently at risk of malnutrition. However, there is no single universal method of quantifying nutritional status in the intensive care unit (ICU). The aim of this study was to create and validate an objective tool to measure body composition as an indicator of nutritional status in critically ill patients.Method: Retrospective chart review conducted of patients admitted to the surgical ICU (SICU) from January 2012 through July 2014. Radiologic Interpretation at Base of Spine (RIBS) scores generated from the ratio of fat and muscle on CT scans of the spine in the axial and coronal planes. Demographic data, RIBS axial and coronal scores, laboratory markers of nutrition, length of stay (LOS), ventilator days, and in-hospital mortality were recorded and analyzed for statistical significance.Result: Data were collected on 58 SICU patients admitted for at least one month. Each patient had a minimum of two CT A/P scans performed one month apart. RIBS axial scores were found to have statistically significant correlation with BMI, BSA, and albumin. The intraclass correlation coefficients for inter-rater and intrarater assessment ranged from 0.9037 to 0.9966 (p ≤ 0.05) and from 0.9577 to 0.9989 (p ≤ 0.05), respectively. There was no significant association between baseline RIBS axial or coronal scores and postoperative outcomes (SICU LOS, ventilation days, in-hospital mortality). Conclusion:RIBS axial scoring is a reliable and reproducible tool that correlates with BMI, BSA and albumin and therefore can be used as an independent marker of nutrition.