Useful second‐tier tests in expanded newborn screening of isovaleric acidemia and methylmalonic aciduria

Common use of pivalate-generating antibiotics in newborns in Japan and low cutoff value of C5-acylcarnitine (C5) to detect mild forms of isovaleric acidemia (IVA) led to 1,065 positive results from IVA screening among 146,000 newborns tested by tandem mass spectrometry over the last 3 years. Using our method to determine isovalerylglycine (IVG) levels in dried blood spots (DBS) as a second-tier test with IVG cutoff value of 0.5 nmol/ml in DBS, one patient with severe IVA was identified, and no recall of the second DBS was needed. Retrospective analysis revealed that most patients with moderate to severe forms of IVA have decreased free-carnitine levels shortly after birth and higher levels of IVG than those of C5, which suggests that this method is useful in evaluating the severity of IVA. Another second-tier test, to measure methylmalonic acid (MMA) levels in DBS by gas chromatography/mass spectrometry (GC/MS), has been developed to overcome difficulties in screening methylmalonic aciduria (MMAU) and propionic acidemia. Methanol extract from DBS was dried and derivatized using N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide. GC/MS was performed using splitless injection, electron-impact ionization, and selected ion monitoring for data recording. MMAU patients had much higher DBS concentrations of MMA (24.2-321.9 nmol/ml) than control newborns (0.34 ± 0.11 nmol/ml). MMA measurement in DBS was thought to provide useful information about the severity of MMAU, as MMAU patients with high levels of MMA had decreased levels of free carnitine and mildly increased levels of propionylcarnitine.