In 2007, the Dutch newborn screening (NBS) program was expanded to include C5-OH as a marker to screen for three inborn errors of metabolism (IEMs): 3-methylcrotonyl-CoA carboxylase deficiency (3-MCCD), 3-hydroxy-3-methylglutaryl-CoA lyase deficiency (HMGCLD) and holocarboxylase synthetase deficiency (HLCSD). This study evaluates the effectiveness of C5-OH as an NBS marker by analyzing data from neonates screened in the Dutch NBS program from 2007 to 2023 and by reviewing the literature on various IEMs detected by an elevated NBS C5-OH concentration worldwide. Of the 126 neonates referred on the basis of elevated C5-OH concentrations in the Netherlands, 46 were true positive cases. No missed cases in the Netherlands have been reported so far, resulting in a positive predictive value of 38.3% and a negative predictive value of 100%. Strikingly, there was notable overlap between C5-OH concentrations of true and false positive cases. The systematic review included 58 articles and showed that C5-OH concentrations of patients with different IEMs reported in the literature were insufficiently distinctive to differentiate between these diseases. While C5-OH can be used to detect patients with 3-MCCD, HCLSD, and HMGCLD, its value is limited by the overlap of C5-OH concentrations between affected and unaffected neonates and among patients with different diseases. This emphasizes the need for improvement of the screening strategy and potentially the use of additional markers to increase its specificity.

3-Methylcrotonyl-CoA carboxylase deficiency (3-MCCD) is a metabolic disorder with a wide clinical spectrum ranging from asymptomatic individuals to severe metabolic decompensation. Following the introduction of expanded newborn screening, a high number of asymptomatic individuals with 3-MCCD were identified, prompting debates about its inclusion in screening panels. In order to inform policy and healthcare decisions regarding the inclusion of 3-MCCD in newborn screening programs, we evaluated the long-term outcomes for newborns with positive results over a decade of screening experience in North-East Italy, as well as the psychological impact on their parents. Of the 336,668 newborns screened between 2014 and 2025, 9 were confirmed to be affected. These infants underwent annual clinical and biochemical assessments, including dried blood spot acylcarnitine profile, plasma free carnitine, and urinary organic acids assays. An emergency protocol was provided to all affected children to manage intercurrent illnesses. An ad hoc survey was developed to assess the psychological impact of the disease on parents. During follow-up (mean age at last visit: 4.2 years), one patient experienced metabolic decompensation during an intercurrent illness, which was promptly treated. One patient presented with growth retardation and another with transient psychomotor delay. Five patients developed carnitine deficiency, requiring supplementation. Psychological assessments revealed an initial high level of parental psychological impact, which decreased over time. All parents strongly supported the screening program. Newborn screening for 3-MCCD enabled the early identification and management of affected individuals, thereby avoiding severe metabolic decompensation. Although there is an initial psychological burden on parents, it significantly decreases over time. Therefore, the long-term benefits of newborn screening for 3-MCCD seem to outweigh the psychological drawbacks.

Elevated plasma levels of 3-hydroxyisovaleryl-carnitine (C5OH) and impaired leucine catabolism are frequently observed in newborn screening reports, necessitating consideration of various diseases in the differential diagnosis. This study aimed to analyze different forms of C5OH and explore their potential predictive value for diagnosis and outcomes. A retrospective review of newborn screening positive cases for C5OH-related diseases from May 2011 to December 2023 was conducted. Clinical, biochemical, and molecular phenotypes of all confirmed positive cases during this period were examined. A total of 15 true positive cases were diagnosed. No significant correlation was found between the C5OH levels in newborn screening and the diagnosis of specific C5OH-related disorders or the presence of metabolic, neonatal, or developmental abnormalities. Outcomes varied based on the spectrum of diseases. These findings indicate that relying solely on C5OH levels from newborn screening is insufficient for making accurate diagnoses or predictions regarding C5OH-related disorders. Further comprehensive evaluation and consideration of additional factors are essential for accurate diagnosis, management and outcome.

3-Methylcrotonyl-CoA carboxylase deficiency (3-MCCD) is an autosomal recessive leucine catabolism condition caused by 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency due to MCCC1/MCCC2 variants. We investigated its incidence and features in Quanzhou, China. We screened 643,606 newborns (January 2014 to December 2022) for elevated 3-hydroxyisovalerylcarnitine (C5OH) levels using tandem mass spectrometry (MS/MS). Molecular analyses identified MCCC1/MCCC2 variants in suspected 3-MCCD cases. Seventeen neonates, two maternal patients, and one paternal patient had 3-MCCD. Its incidence in the Quanzhou study population was 1/37,859 newborns. All patients and neonates with 3-MCCD exhibited increased C5OH concentrations. Most patients [76.5%(13/17)] had increased urinary 3-methylcrotonylglycine (3-MCG) and 3-hydroxyisovaleric acid (3-HIVA) levels. Eight neonates and all adults with 3-MCCD had secondary carnitine deficiency. We identified seventeen variants, including 6 novel ones.MCCC1and MCCC2 variants were found in 47.1% and 52.9% of patients,with c.1331G > A (31.3%) and c.351_353delTGG (50.0%) being the most prevalent, respectively. Clinical symptoms were observed in 11.8% of patients. We identified six new MCCC1/MCCC2 variants, enhancing our understanding of the 3-MCCD molecular profile. Secondary carnitine deficiency occurred in eight neonates and all adult patients. Although clinical symptoms were observed in 11.8% of patients, whether they were related to 3-MCCD remain unclear. Therefore, further studies are required to decide whether 3-MCCD and C5OH indicators should continue to be used.

3-Methylcrotonyl-CoA carboxylase (3-MCC) deficiency is a rare autosomal recessive disease of leucine catabolism. 3-MCC deficiency may lead to metabolic decompensation under stress; however, outcomes of elective surgery requiring cardiopulmonary bypass (CPB) are unknown. We report a 4-year-old girl with asymptomatic 3-MCC deficiency and atrial septal defect (ASD) who's undergone surgical ASD repair under CPB. She was otherwise normal developmentally and medically. Although patients with 3-MCC may face metabolic crises, the ASD repair under CPB was uneventful.