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

Department of Pediatrics, Children's Hospital of Philadelphia, Philadelphia, PA 19104, USA.
American journal of obstetrics and gynecology (Impact Factor: 4.7). 05/2011; 205(4):329.e1-7. DOI: 10.1016/j.ajog.2011.05.032
Source: PubMed


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.
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.
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).
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.

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