Surprising causes of C5-carnitine false positive results in newborn screening.

During an 18-month period, we noticed an alarming increase of newborn screening false positivity rate in identifying isovaleric acidemia. In 50 of 50 newborns presenting elevated C5-carnitine, we confirmed the presence of pivaloylcarnitine. Exogenous pivalate administration had been previously identified as the causal agent of this concern. No pivalic-ester prodrug is commercially available in Belgium, but pivalic derivates are also used in the cosmetic industry as emollient under the term "neopentanoate". We have identified neopentanoate-esters in a nipple-fissure unguent that was provided to young mothers. Ceasing distribution of this product hugely reduced the C5-carnitine false positivity rate.