Urine sodium to urine creatinine ratio as a marker of total body sodium in infants with intestinal failure

Purpose: Urine sodium (UNa) is a measure of total body sodium in infants with intestinal failure (IF) but can be misleading as it does not reflect volume status. Urine sodium to urine creatinine ratio (UNa:UCr) may offer a more accurate measure, but is not routinely used. This study compares UNa:UCr to UNa as a maker of sodium status in infants with IF. Methods: A retrospective review of infants with IF, from a single center, from 2018 to 2020 was conducted (REB H20–00,816). IF etiology, intestinal anatomy, nutritional intake, urine electrolytes and anthropometrics were collected. Linear mixed effects models adjusting for repeated measures were used to associate UNa and UNa:UCr with weight gain and sodium intake. Results: Twenty-two infants with a median gestational age of 31 weeks were included. IF etiology included gastroschisis (41%), necrotizing enterocolitis (23%), and intestinal perforation (14%). Infants had an average of 3 paired UNa and UNa:UCr measures for a total of 74 paired measurements. UNa:UCr more strongly correlated with sodium intake compared to UNa (R = 0.25, p = 0.032 vs. R = 0.10, p = 0.38). Overall, neither UNa (p = 0.21) nor UNa:UCr (p = 0.16) were significantly correlated with weight gain. However, for infants receiving ≤50% nutrition enterally, weight gain correlated with UNa (p = 0.01) and UNa:UCr (p = 0.01). UNa:UCr >35 predicted adequate growth regardless of enteral intake (92% sensitivity, 59% specificity). Conclusion: UNa:UCr is a measure of total body sodium that correlates with sodium intake in infants with IF. Our study indicates UNa:UCr >35 is associated with adequate growth and can be used to guide further validation studies.