OBJECTIVES: Positive fluid status has been consistently associated with worse prognosis in critically ill patients. In view of the potential for errors in the calculation of fluid balance (FB) totals and the problem of accounting for insensible fluid losses, measurement of body weight (BW) changes is an alternative non-invasive method commonly used for estimating body fluid status. The objective of the study was to compare measurements of FB and BW over time and to assess their correlation with ICU mortality.

METHODS: Prospective observational study of adult patients consecutively admitted to the 34-bed multidisciplinary ICU of a tertiary teaching hospital in southern Brazil from June to October 2018, eligible if their expected length of stay was superior to 24 hours and no oral nutritional was being offered. Agreement between daily FB and BW changes, and between cumulative FB and total BW change was calculated. A ROC curve was plotted for each technique according to ICU survival (classification variable).

RESULTS: Among survivors and non-survivors, cumulative FB and total BW change were, respectively, +2.67L versus +5.6L (p= 0.01) and -2.95kgs versus -1.1kgs (p= 0.02). The average daily difference between measured FB and BW was 0.79 (1L = 1kg), with range of values far beyond limits of agreement: -6.89 to 5.78. Correlation between variables was moderate (r= 0.56). Areas under ROC curve to distinguish mortality for cumulative FB, cumulative FB with insensible losses and total BW change were, respectively, 0.65, 0.56 and 0.65 (p= 0.14).

CONCLUSIONS: Both FB and BW changes were significantly different among survivors and non-survivors, however neither measure discriminated ICU mortality even fairly. Poor agreement between methods might be partially explained by average length of ICU stay, since muscle and fat loss as well as bone demineralization could play a role on BW after 5-7 days. Clinical decision-making should be based on more objective techniques.