It can be very difficult to definitively rule out some inborn errors of metabolism once an infant has screened positive, and it is helpful to understand what circumstances may contribute to false positive results. An interesting situation was reported by Boemer et al (Mol Genet Metab. 2014 Jan;111(1):52-4.), in which a pivalate derivative was used in skin emollients provided to some new mothers in Belgium. This derivative was leading to false positive C5 screens in a series of infants, who were probably ingesting this emmolient while breastfeeding. Stopping the distribution of this emmolient reduced the C5 false positive rate.
This case emphasizes how astute observations and simple interventions can affect the accuracy of the newborn screening process.
Hilary Vernon, MD PhD