Urinary excretion of calcium and phosphate in preterm infants

The aims of this study were to determine reference ranges for the urinary calcium (UCa/Cr) and phosphate (UPO₄/Cr) creatinine ratios and to study factors influencing these ratios in a representative population of preterm infants managed according to current nutritional guidelines. Spot urine samples were obtained from 186 preterm infants (gestation 24–34 weeks) for measurement of UCa/Cr and UPO₄/Cr ratios as part of a routine metabolic bone screening program, once every 2–4 weeks from the 3rd to the 18th week of life. Data were also collected on gender, appropriate or small for gestational age (SGA), nutrition [total parenteral nutrition (TPN), preterm or term formula, and breast milk], plasma Ca, PO₄, urea, and electrolytes and on the use of drugs (frusemide, dexamethasone, and theophylline). Data from infants treated with any of these three drugs were analysed separately and not included in establishing the reference ranges for UCa/Cr and UPO₄/Cr. The mean gestational age of the study population was 28 weeks (range 24–34 weeks). The 95th percentile for UCa/Cr at 3 weeks of age was 3.8 mmol/mmol and decreased significantly with increasing postnatal age ( P <0.001). The 95th percentile for UPO₄/Cr was 26.69 mmol/mmol at 3 weeks of age, but this did not change significantly with increasing postnatal age ( P =0.296). On univariate analysis there was no significant association of UCa/Cr and UPO₄/Cr with gender and type of enteral nutrition. The UCa/Cr was lower in infants who were SGA ( P =0.013) and with low plasma Ca ( P =0.008). Infants on TPN had significantly higher UCa/Cr ( P =0.019) and lower UPO₄/Cr ratios ( P ≤0.001). Multivariate analysis confirmed the decrease in UCa/Cr ratio with increasing postnatal age, but the SGA effect was eliminated. The use of furosemide ( P <0.001) and theophylline ( P =0.003) was associated with a significant increase in the UCa/Cr ratio. The use of dexamethasone was also associated with an increase in UCa/Cr ratio, but this did not achieve statistical significance ( P =0.339). The use of furosemide, theophylline, and dexamethasone had no effect on UPO₄/Cr. We report a reference range for UCa/Cr and UPO₄/Cr ratios and factors influencing these ratios in a representative population of preterm infants between 24 and 34 weeks gestation, managed according to current nutritional guidelines.