The impact of early transfer bias in a growth study among neonatal intensive care units.
OBJECTIVE
Transfer of infants between hospitals or their discharge home may bias comparisons of the performance across neonatal intensive care units (NICUs). This study attempts to show the potential size of transfer bias in the context of a large cohort study and describe strategies for minimizing this type of bias.


METHODS
To limit transfer bias in a neonatal growth study of extremely premature infants in six tertiary NICUs, we restricted eligibility to infants <30 weeks gestation at birth and substituted matched replacements for early transfers (infants transferred or discharged prior to day of life 16).


RESULTS
The restriction strategy was successful, reducing the overall early transfer rate from 16.4 to 3.6% and the range of transfer rates among individual NICUs from 0.6-32.7% to 0-11.0%. Replacement by matched substitutes had a much smaller effect because of the small number of early transfers and our inability to match on all factors distinguishing early transfers.


CONCLUSIONS
Sampling strategies to minimize infants lost to follow-up were more successful than replacement strategies in limiting transfer bias in a NICU growth study. Although complete elimination of bias is likely impossible, valid studies require efforts to minimize, quantify, and test the effect of transfer bias.
Olsen
Irene E
IE
Department of Nutrition, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02212, USA. olseni@email.chop.edu
Richardson
Douglas K
DK
Schmid
Christopher H
CH
Ausman
Lynne M
LM
Dwyer
Johanna T
JT
D015982:Bias; D015331:Cohort Studies; D005260:Female; D006302:Health Services Research; D006801:Humans; D007231:Infant, Newborn; D007234:Infant, Premature / Q000254:growth & development*; D007363:Intensive Care Units, Neonatal / Q000706:statistics & numerical data*; D008297:Male; D009511:New England; D010360:Patient Transfer / Q000706:statistics & numerical data*; D012494:Sampling Studies; D016896:Treatment Outcome; D015430:Weight Gain