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 μmol/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 μmol/L underwent further screening, including 206 neonates with C0 <8.5 μmol/L and 320 neonates with 8.5<C0<12 μmol/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.