Quantification of 2-methylcitric acid in dried blood spots improves newborn screening for propionic and methylmalonic acidemias

Abstract

Background Newborn screening for propionic acidemia and methylmalonic acidurias using the marker propionylcarnitine (C3) is neither sensitive nor specific. Using C3 to acetylcarnitine (C3/C2) ratio, together with conservative C3 cut-offs, can improve screening sensitivity, but the false positive rate remains high. Incorporating the marker 2-methylcitric acid has been suggested, to improve the positive predictive value for these disorders without compromising the sensitivity. Methods Between July 2011 and December 2012 at the Newborn Screening Ontario laboratory, all neonatal dried blood spot samples that were reported as screen positive for propionic acidemia or methylmalonic acidurias based on elevated C3 and C3/C2 ratio were analyzed for 2-methylcitric acid, using liquid chromatography tandem mass spectrometry. Results Of 222,420 samples screened, 103 were positive for methylmalonic acidurias or propionic acidemia using C3 and C3/C2 ratio as markers. There were nine true positives: propionic acidemia (n = 3), Cobalamin (Cbl) A (n=1), and Cbl C (n = 5). Among false positives there were 72 neonates not affected, 20 with maternal B12 deficiency, and two incidental finding (transcobalamin II and unclassified Cbl defect). 2-Methylcitric acid was analyzed in all 103 samples and ranged between 0.1 and 89.4 µmol/l (reference range 0.04–0.36). Only 14 samples exceeded the set 2-methylcitric acid cut-off of 1.0 µmol/l, including the samples from all nine true positives. Conclusion By including 2-methylcitric acid in the screening algorithm, the positive predictive value of our primary and secondary screening targets improved from 8.7 to 64.3%. This would have eliminated 89 unnecessary referrals while maintaining 100% sensitivity.