Survival rates in extremely low birthweight infants depend on the denominator: avoiding potential for bias by specifying denominators

OBJECTIVE: The objective of the study was to assess whether recent data reporting survival of preterm infants introduce a bias from the use of varying denominators. STUDY DESIGN: We performed a systematic review of hospital survival of infants less than 1000 g or less than 28 weeks. Included publications specified the denominator used to calculate survival rates. RESULTS: Of 111 eligible publications only 51 (46%) specified the denominators used to calculate survival rates: 6 used all births, 25 used live births, and 20 used neonatal intensive care unit admissions. Overall rates of survival to hospital discharge ranged widely: from 26.5% to 87.8%. Mean survival varied significantly by denominator: 45.0% (±11.6) using a denominator of all births, 60.7% (±13.2) using live births, or 71.6% (±12.1) using used neonatal intensive care unit admissions (P ≤ .009 or less for each of 3 comparisons). CONCLUSION: Variations in reported rates of survival to discharge for extremely low-birthweight (<1000 g) and extremely low-gestational-age (<28 weeks) infants reflect in part a denominator bias that dramatically affects reported data.