The effects of gestational age and birth weight on false-positive newborn-screening rates.

Objective: Newborn-screening false-positive rates (FPRs) are disproportionately increased in preterm infants. The objective of this study was to determine variation in newborn screening FPRs according to birth weight and gestational age. Our secondary objective was to examine the effect of postnatal age on FPRs in preterm infants.

Methods: The Ohio State Newborn Screening Program Database was analyzed to determine the overall and birth weight-specific FPRs for 18 analytes. Data were stratified into birth weight categories (<1000 g, 1000-1499 g, 1500-2499 g, 2500-3999 g, and >4000 g). In addition, to examine the effect of postnatal age on FPRs, we examined the 2 analytes with the highest FPRs, thyrotropin with back-up thyroxine and 17-hydroxyprogesterone, in infants whose gestational age was <32 weeks, determined on the basis of postnatal age at screening.

Results: Data from 448 766 neonates were reviewed. Infants with very low birth weight (VLBW) comprised 1.9% of the study cohort, but accounted for 18% of false-positive results. For 14 of 18 analytes studied, FPRs increased with decreasing birth weight/gestational age and were significantly increased in infants with VLBW compared with infants who weighed 2500 to 3999 g (P < .001). Thyrotropin/back-up thyroxine and 17-hydroxyprogesterone accounted for 62% of total false-positive results in VLBW infants. When blood specimens were collected at a postnatal age of ≥ 48 hours in infants born at <32 weeks, a 44% relative reduction in 17-hydroxyprogesterone false-positive results was detected.

Conclusions: False-positive newborn-screening rates are disproportionately increased in VLBW infants. FPRs may be reduced by delaying screening of <32 weeks' gestation, preterm infants until 24 to 48 hours' postnatal age.