A North Carolina newborn screening pilot for mucopolysaccharidosis II: Evaluating endogenous nonreducing end glycosaminoglycan analysis and IDS sequencing as higher-tier testing options

PURPOSE: Mucopolysaccharidosis II (MPS II, OMIM 309900) is a lysosomal disorder recommended for newborn screening (NBS) in the United States. This study evaluated outcomes of high-throughput NBS for MPS II and use of 2 reflex testing methods to improve sensitivity and specificity. METHODS: We implemented a first-tier liquid chromatography tandem mass spectrometry laboratory-developed test measuring iduronate-2-sulfatase (I2S) enzyme activity in dried blood spots. Newborns with activity ≤10% of the daily median underwent reflex testing for endogenous nonreducing end (NRE) glycosaminoglycan (GAG) and IDS sequencing. Screen-positive infants were referred for clinical follow-up with urinary GAG testing and repeat dried blood spots I2S activity measurement. RESULTS: Among approximately 220,000 newborns screened, 33 tested positive with low I2S activity. Two were confirmed with MPS II. Results of the remaining 31 newborns were indicative of pseudodeficiency. The NRE GAG ratios correlated with confirmatory urinary GAG measurements and were elevated exclusively in newborns with MPS II. CONCLUSION: NBS for MPS II can be efficiently achieved with liquid chromatography tandem mass spectrometry measuring I2S activity. The NRE GAG biomarker successfully differentiated between newborns with MPS II and those with pseudodeficiency. Utilization of NRE GAG analysis as a 2nd-tier test significantly reduces the false-positive rate. IDS sequencing provides additional information for clinical evaluation and follow-up.