Serum phosphate, calcium, magnesium and ALP varied with gestational age, cord blood serum phosphate, magnesium and ALP negatively and calcium positively. The existence of these relationships between cord blood bone minerals levels (serum calcium, phosphate, and magnesium), and related laboratory tests (ALP, and albumin-adjusted calcium), with gestational age, and other neonatal variables (multiple birth, maternal PIH, and small size for gestational age) suggests that these factors should be considered for the development of preterm reference intervals.
Our ranges of preterm cord blood phosphate and calcium (Tables 2 and 3) are compatible with expert recommendations [5, 6]. In contrast, our preterm cord blood range of ALP are considerably lower than expert recommendations to maintain preterm infant ALP less 400  or 800 units/L . The cord blood ALP reference range may not be a relevant goal for the care of the preterm infant since the postnatal levels are considerably different from the cord blood levels, and may not be achievable in clinical care of preterm infants.
These results come from a single center, which could be influenced by local events or lab methodology. Therefore these results should be verified and validated by larger multicentre studies.
Albumin-adjusted calcium values are useful to consider for subjects with hypoalbuminemia, since non-adjusted total calcium can be reduced by a lower albumin-bound fraction, while the ionized physiologically active portion may be normal. Therefore, when cord blood serum ionized calcium is not available, or not appropriate, the use of albumin-adjusted calcium is superior to the use of total calcium in hypoalbuminemic patients. Measurement of ionized calcium is altered by changes in cord blood serum pH , and therefore its utility in cord blood is hampered since pH can be variable in cord blood.
Although reference intervals are useful to consider when evaluating the health status of any group, they may or may not define ideal health status. Research is needed to determine whether these reference ranges define the ideal serum levels for preterm infants, or whether the expert recommendations for cord blood serum phosphate (maintained greater than 1.8 mmol/L ), and/or ALP (less than 400  units/L may be superior goals and/or more relevant. Reference intervals from cord blood at preterm births are generated from infants who are born prematurely, and for that reason may not be in a state of ideal health. However, the fetus is considered to be the reference source for the establishment of the goals for normal concentrations of nutrients in the blood and tissue [1, 2] and cord blood provides a convenient opportunity to sample fetal blood.
Serum calcium is usually maintained within a limited range in vivo by homeostatic mechanisms. It is interesting that our preterm lower limits for calcium extended lower than the term lower limits, which could be related to the very rapid calcium deposition in bone mid-gestation. Preterm infants are prone to hypercalcaemia in the neonatal intensive care unit, which can be seen secondary to hypophosphatemia [22, 23]. Our findings suggest that cord blood serum calcium up to 12.4 mg/dl (3.1 mmol/L) is acceptable given these reference intervals.
Serum calcium and phosphate were influenced by multiple births; phosphate was significantly lower for triplets while cord blood serum calcium was significantly lower for twins. These effects were likely due to limited placental supply due to limited nutrient supply to the multiple fetuses.
In comparison to previous reports, our values for cord blood serum phosphate were almost identical to four other studies [7, 8, 12, 13], but were lower than the small study from Japan . Our values for cord blood serum calcium were similar to another cord blood report , and slightly higher than five other studies [8, 9, 11–13]. Our magnesium values were slightly lower than one study , and higher than another . Compared to our ALP values, fetal levels have been reported as similar  or much higher [10, 12].
Some previous work regarding analyte relationships with gestational age agrees with our findings while some others do not. Moniz et al also noted an increase in cord blood serum calcium with increasing gestational age among the foetuses assessed using fetoscopy . Seki et al  reported a more dramatic negative relationship between phosphate and gestational age. In terms of ALP, other researchers [10, 12] also observed decreases in ALP with gestational age among the preterm fetuses.
Although most literature regarding phosphate for preterm infants emphasizes phosphate's role in bone mineralization, phosphate also has roles in glucose metabolism. Serum phosphate decreases in response to infused glucose in adults, , presumably due to it entering cells in response to insulin secretion. Further, low cord blood serum phosphate may limit clinical stability since it is required for glucose tolerance, tissue sensitivity to insulin , as well as glucose-induced insulin secretion .