Reference values for serum calcium in neonates should be established in a population of vitamin D-replete subjects.

Serum calcium is frequently measured during the neonatal period, and is known to be influenced by the vitamin D status. We hypothesized that the 25OHD concentration may influence the lower limit of the serum calcium normal range in neonates. We included in our prospective cohort study 1002 mother-newborn pair recruited from April 2012 to July 2014, in two centers located in the neighborhoods of Paris, France, whose serum calcium was measured at 3 days of life. We established, after exclusion of outliers, a 95% confidence interval (CI) for serum calcium 1) in our whole population of 1002 neonates, 2) in neonates with a cord blood 25OHD concentration ≥ 30 nmol/L, and 3) in those with a 25OHD ≥ 50 nmol/L. The mean serum total calcium was 2.46 ± 0.13 nmol/L [95% CI: 2.19-2.72 mmol/L], 2.47 ± 0.25 mmol/L [95% CI: 2.22-2.72 mmol/L], and 2.50 ± 0.25 mmol/L [95% CI: 2.25-2.75 mmol/L] in the whole group, in the 514 neonates with 25OHD ≥ 30 nmol/L, and in the 202 neonates with 25OHD ≥ 50 nmol/L respectively. The lower limit of the 95% range was significantly higher in neonates with 25 OHD ≥ 30 nmol/L (p<0.05) and ≥ 50 nmol/L (p<0.001) than in the entire cohort. We show that the lower limit of the normal serum calcium range is higher in groups with a higher 25OHD than in unselected subjects. We propose that the reference range for serum calcium in neonates is 2.25 to 2.75 mmol/L.

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