Calcium, Sodium and Potassium Urinary Excretion during the First Five Days of Life in Very Preterm Infants

Abstract

Background: Hypercalciuria has been associated with the risk of nephrocalcinosis and renal stones in both adults and children. Renal calcifications are frequently encountered in preterm infants because this particular population presents most of the risk factors of increased urinary calcium excretion. Urinary calcium excretion has been shown to correlate with sodium and potassium excretions in adult patients, but these correlations have not been demonstrated in the early neonatal period yet. Objective: To define the relationship between calcium urinary excretion and sodium or potassium excretions in the first 5 days of life in preterm babies. Methods: A prospective study was conducted in 16 preterm infants born before 32 weeks of gestation (body weight 1,373 ± 310 g; gestational age 29.1 ± 1.6 weeks). Fifteen consecutive 8-hour urine collections were performed for each infant from the 8th hour of life. A plasma sample was obtained at the end of each urine collection. Sodium, potassium, calcium and creatinine were measured in urine and blood samples as often as possible. Results: (1) Urine sodium excretion was 6.56 ± 4.35 mmol/kg per day. (2) Urinary calcium excretion was 5.9 ± 5.4 mg/kg per day and the urinary calcium/creatinine ratio was 0.48 ± 0.39 mg/mg. (3) Urinary calcium and sodium excretion were positively correlated (r = 0.65, p = 0.0001), while an inverse correlation was found between calcium and potassium excretion (r = 0.31, p = 0.004). Conclusion: The mean values of urinary calcium excretion and calcium/creatinine ratio observed in our population were higher than 4 mg/kg per day and 0.4 mg/mg, respectively, i.e. boundary values previously associated with an increased risk of nephrocalcinosis. We hypothesize that an increase in urinary sodium excretion in this population may facilitate calcium excretion.