This study states that Congenital B12 deficiency is associated with neurologic delay, impaired neurodevelopment, developmental delay and hypotonia, and lifelong increased risk of chronic disease.1,2 Although many cases of congenital B12 deficiency are diagnosed clinically, B12 deficiency has been incidentally diagnosed by newborn screening in many countries using markers designed to detect methylmalonic aciduria.3,4 However, the sensitivity of this approach is unknown. Making a timely diagnosis of B12 deficiency is key. as symptoms may be reversible with early B12 supplementation. As part of a pilot study extending the German Newborn Screen, a method has been developed to sensitively detect congenital B12 deficiency on newborn screening by using a second-tier algorithm that integrates both of the biochemical pathways that are cobalamin dependent: methionine synthetase and methylmalonyl-CoA mutase.5 That initiative was the first effort to make B12 deficiency a primary target of newborn screening and demonstrated the feasibility of such an approach. Expanding on that work, in this volume of The Journal, Mütze et al present the outcomes of children who screened positive using the diagnostic strategy that they had developed.

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