Urinary excretion of calcium and phosphate in preterm infants

Abstract

The aims of this study were to determine reference ranges for the urinary calcium (UCa/Cr) and phosphate (UPO(4)/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 UPO(4)/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, P0(4), urea, and electrolytes and on the use of drugs (frusemide, dexamethasone, and theophylline). Data from infants treated with any of these three drugs were analyzed separately and not included in establishing the reference ranges for UCa/Cr and UPO(4)/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 per-centile for UPO(4)/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 UPO(4)/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 UPO(4)/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 UPO(4)/Cr. We report a reference range for UCa/Cr and UPO(4)/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 guide-lines.

Changes in red cell phosphate metabolism of preterm and fullterm infants with perinatal problems during their first month of life

“…At the same time calcium excretion (UCa/UCr) was found higher. Phosphate depletion is reported to cause hypercalciuria in preterms and it must be due to decreased utilization of calcium for bone growth, since urinary Ca in preterm infants originates mainly from bone turnover [2]. The increased calcium excretion in the preterms with problems persisted during the entire month of the study, which might have some effect on their bone metabolism.…”

Nutrition Management in Childhood Kidney Disease: an Integrative and Lifecourse Approach

“…In the adult, renal excretion of ingested phosphorus results in a net zero phosphorus balance and maintains homeostasis filtered through the kidney is reabsorbed. In the neonate calcium excretion in the urine is higher, especially in the premature infant [13,14]. Despite this, there is net calcium accretion throughout childhood.…”

Urinary Mineral Excretion in Preterm Neonates during the First Month of Life

“…Glomerular and tubular function and renal handling of several ions are immature in preterm infants at birth [9,10] . This immaturity may lead to Na, bicarbonate and glucose wasting, while P and K renal handling appears to be relatively effective at this age [7,8] . During the first few weeks postnatally acceleration in renal function occurs, the glomerular filtration rate increases and the serum Cr level falls [9,11] .…”

“…During the first few weeks postnatally acceleration in renal function occurs, the glomerular filtration rate increases and the serum Cr level falls [9,11] . Studies examining the effects of postnatal age on the comparative renal excretion of various minerals in stable preterm infants are few [7,8] .…”

“…Very few studies have investigated the variability in the urinary excretion of Ca, magnesium (Mg), P and Na, and their associations during the first few weeks of life in preterm low-birth-weight infants [7,8] .…”