Ionized calcium is the more physiologically significant fraction of total serum calcium. Current guidelines on the treatment of early hypocalcemia of prematurity make use of existing total serum calcium standards in the preterm infant, however, the ionized fraction is the more appropriate indicator of the need for calcium therapy. Method We reviewed twenty-two nonasphyxiated, AGA premature infants (< 33 weeks gestation) within the past year who had simultaneous ionized and total serum calcium measurements within 48 hours of life. Ionized Ca++ was measured on a Ciba Corning 288 simultaneously with an arterial blood gas using a minimum sample size of 0.25 ml. Total Ca++ was measured on a Beckman Synchron CX7 using a minimum sample size of 0.50 ml. Results We calculated a Pearson correlation coefficient (r) for ionized (mmol/L) versus total (mg/dL) calcium of 0.53(ionized Ca++=0.51 + 0.074*(total Ca++)). We found that 8/22 and 4/22 had an acceptable ionized Ca++ >1.0 mmol/L despite low total Ca++ of < 8.0 mg/dL and < 7.5 mg/dL, respectively. The correlation coefficient for ionized Ca++ versus pH was -0.50 (ionized Ca++=8.32 + -0.984*(pH)) in the pH range of 7.24-7.45. The correlation for total Ca++ versus pH was not statistically significant. We also found no statistically significant correlation between ionized or total Ca++ and gestational age from 23.5 to 32 weeks. Conclusion A decreased total calcium is often associated with a normal ionized calcium in the preterm infant. Ionized calcium rather than total calcium better identifies preterm infants with early hypocalcemia, avoiding excessive treatment and the morbidity associated with calcium therapy. Table

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