Reference values for urinary calcium and phosphorus to prevent osteopenia of prematurity

The prevention of osteopenia of prematurity is an important issue in the care of preterm infants. Fetal bone mineral accretion has been achieved in preterm infants by establishing and maintaining a simultaneous slight excretion of calcium (Ca) and phosphorus (P) (urine concentrations of 1–2 mmol/l) by means of an individual supplementation with Ca and/or P, resulting in a slight surplus supply (SSS). In this issue, Aladangady et al. present associations between urinary Ca/Cr and PO₄/Cr ratios of preterm infants and biochemical variables of bone mineral metabolism. However, to date it has not been proven that these variables are a reliable substitute for direct measurement of bone mineral content (BMC). Before Ca/Cr and PO₄/Cr ratios can be recommended as a new reference for improving BMC, the following steps are required: (1) direct measurement of BMC, (2) a prospective interventional trial to test and compare this new reference with the existing one (SSS, urinary Ca and P of 1–2 mmol/l) investigating BMC as primary outcome, and (3) adequate proof that Ca and P/Cr ratios are superior to simple urinary Ca and P concentrations.