Newborn screening for primary carnitine deficiency using a second-tier genetic test

Objectives: Newborn screening (NBS) for primary carnitine deficiency (PCD) exhibits suboptimal performance. This study proposes a strategy to enhance the efficacy of second-tier genetic screening by adjusting the cutoff value for free carnitine (C0).Methods: Between January 2021 and December 2022, we screened 119,898 neonates for inborn metabolic disorders. Neonates with C0 levels below 12 mu mol/L were randomly selected for second-tier genetic screening, employing a novel matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) assay.Results: In total, 2,515 neonates with C0 <12 mu mol/L underwent further screening, including 206 neonates with C0 <8.5 mu mol/L and 320 neonates with 8.5<C0<12 mu mol/L. Genetic screening identified positive results in 12.36 % (65) of neonates, with one being homozygous, 10 compound heterozygotes, and 54 heterozygotes. Sanger sequencing revealed a second SLC22A5 variant in three of the 54 neonates. Ultimately, 14 patients were diagnosed with PCD; all 14 patients exhibited low C0 levels, though two had normal C0 levels during the recall review. The MALDI-TOF MS assay demonstrated detection and diagnostic rates of 89.29 % and 78.57 %, respectively. Eleven distinct SLC22A5 variants were identified, with the most common variant being c.51C>G, accounting for 25 % (7/28) of allelic frequencies.Conclusions: A novel MALDI-TOF MS assay targeting 21 SLC22A5 variants in a Chinese population was successfully established. This assay exhibits a high detection and diagnostic rate, making it suitable for population-based genetic screening. Combined genetic screening is recommended to enhance the efficiency of PCD-NBS.