Circadian variations of urinary electrolyte concentrations in preterm and term infants.

Abstract

To determine whether a circadian variation of urinary excretion of calcium and phosphorus exists in preterm infants. We studied 70 newborn infants (median birth weight 1920 gm, range 660 to 3550 gm; median gestational age 34 weeks, range 25 to 42 weeks) at a median postmenstrual age of 36 weeks (range 32 to 42 weeks). Within a period of 24 hours, four urine specimens were collected during 6-hour periods. The concentrations of calcium, phosphorus, sodium, potassium, and creatinine were determined and creatinine quotients were calculated for each specimen. No clinically relevant circadian variation in urinary excretion for any of these minerals was found. If spot urine specimens are used to monitor calcium and phosphorus balance in preterm infants, the time of the day these are collected is not important.