Renal phosphate handling of premature infants of 23–25 weeks gestational age

Premature infants with low serum phosphate concentrations (<2 mmol/l) are at risk for osteopenia. Therefore, serum phosphate levels in premature infants should be kept above 2 mmol/l. Premature infants of 26–31 weeks gestational age (GA) have renal phosphate threshold concentrations (Tp/GFR) in the range of normal serum phosphate values (2 mmol/l). Therefore, these infants show significant urinary phosphate excretion only when serum phosphate levels are normal, and urinary phosphate excretion can be used to monitor phosphate supplementation. However, few data are available on extremely premature infants of 23–25 weeks GA. The objective of this study was to compare Tp/GFR levels in infants of 23–25 weeks GA to those in infants of 26–31 weeks GA. We retrospectively evaluated case notes of 12 infants of 23–25 weeks GA and compared them to 19 infants of 26–31 weeks GA. Tp/GFR was calculated from simultaneous measurements of urinary phosphate, urinary creatinine, serum phosphate, and serum creatinine. Tp/GFR values 3–5 weeks postnatally were lower in infants of 23–25 weeks GA (1.06±0.36 mmol/l, p<0.001) than in infants of 26–31 weeks GA (1.76±0.26 mmol/l). Near term (35–37 weeks postmenstrual age), there was no significant difference between Tp/GFR values in infants of 23–25 weeks GA (1.83±0.32 mmol/l) and in infants of 26–31 weeks GA (2.05±0.22 mmol/l). We conclude that at 3–5 weeks postnatally, infants of 23–25 weeks GA are at risk for low Tp/GFR values, leading to urinary phosphate excretion even in the presence of low serum phosphate levels. In these infants, serum phosphate levels should be monitored, and phosphate supplementation should be adjusted to keep serum phosphate levels above 2 mmol/l.