Nutrient balance in the preterm infant. 3. Effect of balance duration on outcome measurements.

To determine the effect of balance duration on outcome measurements 18 84-h balances were performed in a group of formula-fed preterm infants (birth weight = 1,362 +/- 125 g; gestational age = 32.3 +/- 1.7 weeks). Each 84-h balance was broken into contiguous 48- and 36-h periods. Comparisons (paired t tests, paired variance tests, and linear regression analysis) were made between the 48-h and 36-h (paired t and F tests) and between the 48-h and 84-h collections (simple linear regression). Nitrogen absorption (mean +/- SD) during 48-h collection (365 +/- 58 mg/kg/day and 87 +/- 3%) was not significantly different from that of the 36-h collection (381 +/- 66 mg/kg/day and 84 +/- 4%). Nitrogen retention during the 48-h collection (301 +/- 59 mg/kg/day, 72 +/- 6%) also was not significantly different from that of the 36-h collection (318 +/- 64 mg/kg/day, 72 +/- 6%). Moreover, nitrogen absorption and retention during the 48-h collection were significantly related (p less than 0.0001 for linear regression comparisons) to that of the 84-h collection. Similar observations were noted for calcium, phosphorus, magnesium, zinc, and copper absorption and retention. We conclude that a 48-h collection may be used in place of an 84-h collection without compromising the accuracy of the balance data in the enterally fed preterm infant.