Neonatal screening for isovaleric aciduria: Reducing the increasingly high false‐positive rate in Germany

Newborn screening (NBS) for isovaleric acidemia (IVA) is performed by flow injection tandem mass spectrometry quantifying C5 carnitines (C5). Isovalerylcarnitine, however, is isomeric with pivaloylcarnitine which can be present in blood due to maternal use of pivaloylester‐containing antibiotics, available in Germany since late 2016. During a 36‐month period (January 19–December 21), all newborns screened in Hamburg with a C5 above cutoff (NeoGram®: 0.50 μmol/L or Neobase®2: 0.45 μmol/L) were included in the study. As a second‐tier test, a simple ultra performance liquid chromatography‐tandem mass spectrometry (UPLC‐MS/MS) method was developed to differentiate the C5 isomers pivaloyl‐, 2‐methylbutyryl‐, isovaleryl‐, and valerylcarnitine. Out of 156 772 newborns tested, one turned out to have genetically proven IVA while 99 were false positive (C5: 0.5–8.2 μmol/L) due to the presence of pivaloylcarnitine. These cases have increased year by year and show local clusters. Retrospective analysis of another 39 cases from 287 206 neonates tested at the NBS center in Heidelberg with C5 elevation (0.9–10.6 μmol/L) but clinical and biochemical exclusion of IVA yielded evidence of pivaloylcarnitine in all cases. Inclusion of a second‐tier test into NBS significantly reduces the high and increasing false‐positive rate of IVA screening. This avoids further diagnostic steps, prevents unnecessary stress and anxiety of parents in a remarkably high number of cases. If Hamburg data of 2021 are extrapolated to all of Germany, one can assume around 800 (1‰) false‐positive cases in comparison to an average of two classic IVA cases per year. Unless licensing of pivaloylester‐containing drugs for use during pregnancy is reconsidered, a second‐tier test for C5 determination is indispensable.