Association between the calcium-to-phosphorus ratio and early hypophosphatemia in preterm infants receiving parenteral nutrition in Korea: a retrospective cohort study

Purpose: This study aimed to evaluate the association between the calcium-to-phosphorus (Ca/P) ratio and serum phosphate levels in preterm infants receiving total parenteral nutrition (TPN) and to explore Ca/P ratio ranges associated with hypophosphatemia.Methods: We retrospectively reviewed the medical records of preterm infants admitted to the neonatal intensive care unit at Ajou University Hospital between January 2022 and June 2024. The review focused on TPN composition and serum electrolyte changes during the first week of life. Based on the cumulative Ca/P ratio during this period, infants were categorized into two groups: Ca/P-Low (mass ratio ≤1.3) and Ca/P-High (mass ratio >1.3).Results: A total of 117 preterm infants were included in the analysis (Ca/P-Low group, n=46; Ca/P-High group, n=71). During the first week of life, the cumulative phosphorus deficit was significantly greater in the Ca/P-High group (61.4 mg/kg vs. 8.5 mg/kg; P<0.001). By day of life (DOL) 7, both hypophosphatemia and severe hypophosphatemia had increased markedly in the Ca/P-High group compared with DOL 3. In the Ca/P-High group, the incidence of hypophosphatemia increased from 44.4% on DOL 3 to 75.0% on DOL 7, while the incidence of severe hypophosphatemia increased from 4.8% to 23.1%.Conclusion: These findings suggest that parenteral nutrition strategies may benefit from phase-adapted Ca/P ratios rather than a fixed ratio during the first week of life. A higher ratio may be considered during DOL 0–2, whereas a lower ratio may be considered during DOL 3–6, particularly when overall calcium and phosphorus supply is suboptimal.