External quality assessment (EQA) trials of conventional newborn screening tests for phenylketonuria, galactosemia, congenital adrenal hyperplasia, maple syrup urine disease, homocystinuria, and congenital hypothyroidism, as well as extended newborn screening tests using tandem mass spectrometry, were performed twice in 2018 and 2019. A total of 44 dried blood spot specimens were analyzed in each trial in 14–16 laboratories. Median, mean, standard deviation, minimum and maximum values, and cut-off values were evaluated for each analyte in the newborn screening tests. The proportion of correct answers was 95%–100%. EQA trials for the analyses of methylmalonic acid, vanillylmandelic acid, catecholamines, metanephrines, organic acids, and amino acids were also performed using two or three specimens per trial. A well-designed EQA program and continuous education can help improve the performance of metabolite testing.

Keywords: External quality assessment, Newborn screening, Tandem mass spectrometry, Amino acid, Organic acid, Metabolite