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Key Points Question What is the analytical and clinical validity of a mass spectrometric method evaluating very long-chain fatty acyl-lysophosphatidylcholine species for the detection of X-linked adrenoleukodystrophy among newborns in North Carolina? Findings In this newborn diagnostic screening study of 52 301 dried blood spot specimens, 3 male infants were confirmed to have X-linked adrenoleukodystrophy, 3 female infants were identified as heterozygous for X-linked adrenoleukodystrophy (carriers), 1 female infant had a peroxisome biogenesis disorder, 1 female infant had Aicardi-Goutières syndrome, 1 male infant received an indeterminate diagnosis, and 3 female infants had false-positive results. Meaning The newborn screening results suggested that detecting increased concentrations of very long-chain fatty acyl-lysophosphatidylcholine species in dried blood spots is an effective method for identifying infants with X-linked adrenoleukodystrophy., Importance X-linked adrenoleukodystrophy (X-ALD) is a peroxisomal genetic disorder in which an accumulation of very long-chain fatty acids leads to inflammatory demyelination in the central nervous system and to adrenal cortex atrophy. In 2016, X-ALD was added to the US Recommended Uniform Screening Panel. Objective To evaluate the performance of a single-tier newborn screening assay for X-ALD in North Carolina. Design, Setting, and Participants This diagnostic screening study was of all newborn dried blood spot specimens received in the North Carolina State Laboratory of Public Health between January 2 and June 1, 2018, excluding specimens of insufficient quantity or quality. A total of 52 301 specimens were screened for X-ALD using negative ionization high-performance liquid chromatography tandem mass spectrometry to measure C24:0- and C26:0-lysophosphatidylcholine concentrations. Sanger sequencing of the adenosine triphosphate–binding cassette subfamily D member 1 (ABCD1) gene was performed on screen-positive specimens. Exposures A medical and family history, newborn physical examination, sequencing of ABCD1 on dried blood spot samples, and plasma analysis of very long-chain fatty acids were obtained for all infants with screen-positive results. Main Outcomes and Measures The prevalence of X-ALD in North Carolina and the positive predictive value and false-positive rate for the first-tier assay were determined. Results Of 52 301 infants tested (47.8% female, 50.6% male, and 1.7% other or unknown sex), 12 received screen-positive results. Of these 12 infants, 8 were confirmed with a genetic disorder: 3 male infants with X-ALD, 3 X-ALD–heterozygous female infants, 1 female infant with a peroxisome biogenesis disorder, and 1 female infant with Aicardi-Goutières syndrome. Four infants were initially classified as having false-positives results, including 3 female infants who were deemed unaffected and 1 male infant with indeterminate results on confirmatory testing. The positive predictive value for X-ALD or other genetic disorders for the first-tier assay was 67%, with a false-positive rate of 0.0057%. Conclusions and Relevance This newborn screening pilot study reported results on 2 lysophosphatidylcholine analytes, identifying 3 male infants with X-ALD, 3 X-ALD–heterozygous female infants, and 3 infants with other disorders associated with increased very long-chain fatty acids. These results showed successful implementation in a public health program with minimal risk to the population. The findings will support other state laboratories planning to implement newborn screening for X-ALD and related disorders., This diagnostic study evaluates the performance of a single-tier screening assay for X-linked adrenoleukodystrophy among newborns in North Carolina.