Urinary mineral excretion in preterm neonates during the first month of life.

BACKGROUND

Estimation of urinary parameters in preterm infants is a useful method for identifying metabolic derangements.

OBJECTIVES

A prospective, longitudinal, hospital-based study was designed to examine the variability and the associations in renal excretion of calcium (Ca), magnesium (Mg), phosphate (P) and sodium (Na) in formula-fed preterm infants during the first month of life.

PATIENTS

Thirty-four infants <32 weeks gestational age, clinically stable, not receiving nephrotoxic drugs.

METHODS

Measurements of serum and 8-hour urinary mineral and creatinine (Cr) concentrations were made in all infants during three periods (at 7-10, 14-17 and 21-26 days postnatally). The urinary parameters, FENa, FEP, UCa/UCr, UMg/UCr were calculated. 24- hour urinary excretion was estimated by extrapolation of the 8-hour values.

RESULTS

The 24-hour excretion values (median and range) (mmol/kg) during the three study periods were, respectively, for Ca: 0.027 (0.015-0.15), 0.030 (0.007-0.12), 0.031 (0.008-0.12), for P: 0.26 (0.07-0.83), 0.29 (0.06-0.67), 0.41 (0.22-0.70), for Mg: 0.025 (0.007-0.14), 0.025 (0.008-0.16), 0.027 (0.010-0.10) and for Na: 2.85 (0.8-15), 1.45 (0.3-17), 1.56 (0.6-4.3). Na excretion declined while the excretion of the other minerals remained stable. A positive correlation was observed between excretion of Ca, Mg and Na (Ca vs. Na r = 0.63, p < 0.0001; Ca vs. Mg r = 0.65, p < 0.0001; Na vs. Mg r = 0.38, p = 0.012) as well as between the renal parameters FENa, FEP, UCa/UCr, UMg/UCr and the respective 24-hour excretion values (r = 0.80, 0.86, 0.84, 0.81) CONCLUSIONS: Urinary excretion of Ca, P, and Mg in preterm formula-fed infants is stable during the first month of life while urinary Ca, Na and Mg are closely correlated during the same period.