Hidden Metabolic Diseases Found in Unexplained Child Deaths Through Blood Spot and DNA Analysis

Every year, thousands of young children die suddenly or unexpectedly, and in a substantial fraction of these cases, no clear cause of death is ever established. For grieving families, the absence of an answer is a second, quieter tragedy. For clinicians and public health authorities, it represents a persistent blind spot in pediatric medicine. A new retrospective cohort study from Shenzhen, China, now suggests that a meaningful share of these unexplained early childhood deaths may be attributable to inborn errors of metabolism, a group of inherited biochemical disorders that often strike without warning in the first years of life. By combining retrospective biochemical screening of archived newborn blood spots with whole-exome sequencing, the researchers were able to recover diagnoses that had been missed entirely during the children’s lives.

The study, published in BMC Pediatrics, was led by a team at the Women and Children’s Medical Center of Shenzhen Maternity and Child Healthcare Hospital affiliated with Southern Medical University. The investigators started with an unusually complete dataset: 2,652 deaths of children under five years of age recorded in Shenzhen between January 2016 and December 2023. From this mortality cohort, they applied prespecified, record-based cause-of-death exclusions and removed deaths occurring within 72 hours of birth, reasoning that very early neonatal deaths typically involve distinct clinical circumstances and are less likely to reflect undiagnosed metabolic disease. That filtering left 1,982 children eligible for metabolic investigation.

The critical resource was the archived dried blood spots, the small cards of filter paper collected from nearly every newborn in China as part of routine screening programs. These spots preserve a biochemical snapshot of the infant’s blood in the first days of life. Dried blood spots were available for 652 of the eligible children, and the team analyzed each one for 43 metabolites using tandem mass spectrometry, a technique that can precisely quantify tiny amounts of amino acids, acylcarnitines, and other small molecules in a single rapid run. The screening panel was designed to flag 48 different conditions, using both individual metabolite concentrations and prespecified metabolite ratios, since ratios often discriminate true disease from benign variation more reliably than any single value.

Tandem mass spectrometry works by ionizing the compounds extracted from the blood spot and then sorting them by their mass-to-charge ratio in two sequential stages, which allows the instrument to distinguish closely related molecules such as the various acylcarnitines that accumulate when fatty acid oxidation is impaired. When a fatty acid oxidation enzyme is deficient, the body cannot efficiently break down fats for energy, and specific intermediate acylcarnitines build up in the blood in characteristic patterns. Elevated concentrations of these markers, or abnormal ratios between them, serve as biochemical fingerprints of the underlying enzymatic block. In this cohort, 50 of the 652 tested children were flagged as positive by the MS/MS panel, a striking yield given that none of them had received a metabolic diagnosis before death.

To determine whether these biochemical signals reflected genuine disease, the researchers turned to whole-exome sequencing, a next-generation sequencing approach that reads the protein-coding regions of all roughly 20,000 human genes. Because most known inborn errors of metabolism are caused by variants in genes encoding enzymes, transporters, or cofactor proteins, the exome captures the great majority of the relevant disease space. The team sequenced all 50 MS/MS-positive cases and interpreted the variants according to American College of Medical Genetics and Genomics and Association for Molecular Pathology criteria, the standard framework for classifying sequence variants as pathogenic, benign, or uncertain.

The molecular work delivered definitive answers in 13 of the 50 cases, or 26 percent. Twelve of these were inborn errors of metabolism, and one child turned out to have Smith-Magenis syndrome, a genomic disorder caused by a deletion on chromosome 17 that is not a metabolic disease at all but which had produced a misleading biochemical signal. One additional case was classified as probable ornithine transcarbamylase deficiency, a urea cycle disorder that can cause lethal hyperammonemia but which often eludes both standard screening and postmortem testing. Among the definitively diagnosed metabolic cases, the spectrum included fatty acid oxidation disorders, which emerged as the dominant category, consistent with the known tendency of these conditions to present as sudden, catastrophic illness in a previously healthy child, sometimes triggered by fasting or infection.

The diagnostic categories named in the study’s abbreviations read like a catalogue of rare but devastating pediatric disease: very-long-chain acyl-CoA dehydrogenase deficiency, carnitine-acylcarnitine translocase deficiency, systemic primary carnitine deficiency, multiple acyl-CoA dehydrogenation deficiency, methylmalonic acidemia, pyruvate dehydrogenase deficiency, ethylmalonic encephalopathy, and mucolipidosis II or III alpha/beta. Each of these conditions has a distinct biochemical signature and a distinct genetic cause, and each is potentially detectable, and in some cases treatable, if identified in time. The fact that they surfaced only after death underscores the central argument of the paper: expanded newborn screening is not universally implemented, and where it is absent or incomplete, lethal metabolic disease can pass through the health system entirely unrecognized.

One of the most clinically informative findings emerged from the comparison between the 13 definitively diagnosed cases and the 37 MS/MS-positive cases that lacked a definitive molecular diagnosis. The diagnosed children had significantly lower rates of low birth weight, 31 percent versus 84 percent; preterm birth, 15 percent versus 78 percent; and obstetric complications, 46 percent versus 78 percent, with P values below 0.001 for the first two comparisons and 0.040 for the third. In other words, the children whose deaths were ultimately explained by inherited metabolic disease were far more likely to have been born at term with normal birth weight and uncomplicated pregnancies. This pattern suggests that a metabolic cause of death should rise higher on the differential diagnosis when a seemingly healthy, full-term baby deteriorates or dies without an alternative explanation, whereas MS/MS abnormalities in preterm or low-birth-weight infants may more often reflect the physiological turbulence of prematurity rather than true inherited disease.

The study also examined how well the biochemical and genetic evidence agreed with each other, a question of real practical importance because discordant findings complicate diagnosis and family counseling. Among the 13 definitively diagnosed cases, clear metabolite-genotype concordance was observed in nine, or 69.2 percent, meaning the biochemical pattern and the pathogenic variants pointed to the same disease. Partial concordance was seen in two cases and non-concordance in two. These discordant cases illustrate a well-known limitation of newborn screening panels: metabolite levels can be influenced by age, feeding, liver function, and prematurity, and a positive screen is not a diagnosis. The authors argue that this is precisely where sequencing adds decisive value, establishing molecular diagnoses and clarifying atypical, nonspecific, or biochemically confusing findings that MS/MS alone cannot resolve.

The implications reach well beyond Shenzhen. The study provides direct evidence that retrospective analysis of archived dried blood spots can convert unexplained pediatric deaths into explained ones, offering families a diagnosis, clarifying recurrence risk for future pregnancies, and identifying relatives who may carry the same condition. Just as importantly, it strengthens the case for broader access to expanded MS/MS newborn screening worldwide, since every undiagnosed case in this cohort represents a child whose condition was, in principle, detectable at birth. The authors advocate a risk-stratified approach to genomic testing, reserving whole-exome sequencing for cases that remain clinically or biochemically unresolved after screening. As sequencing costs continue to fall and blood spot archives grow, the combination of metabolomics and genomics may become a standard tool not only for diagnosing living children but for understanding why some of the most vulnerable never survive their first years.

Subject of Research: Detection of undiagnosed inborn errors of metabolism in unexplained early childhood deaths using newborn dried blood spot screening and whole-exome sequencing

Article Title: Inborn errors of metabolism in unexplained early childhood deaths: a retrospective cohort study combining MS/MS and WES

Article References: Zhou, L., Yang, X., Wu, D., Zhang, L., Tu, X., Zhan, Z., Xie, S., Xu, Y., Liu, C., Wu, B., & Liu, W. (2026). Inborn errors of metabolism in unexplained early childhood deaths: a retrospective cohort study combining MS/MS and WES. BMC Pediatrics. https://doi.org/10.1186/s12887-026-07740-3

Image Credits: AI Generated

DOI: 10.1186/s12887-026-07740-3

Keywords: inborn errors of metabolism, newborn screening, tandem mass spectrometry, whole-exome sequencing, dried blood spots, early childhood mortality, fatty acid oxidation disorders, pediatrics, metabolomics, genetic diagnosis, urea cycle disorders, Shenzhen