Multiplex MSMS measurement of lysosomal enzymes enables incidental diagnosis of acid sphingomyelinase deficiency in patients evaluated for Gaucher disease.

BACKGROUND: Acid sphingomyelinase deficiency (ASMD) is a rare lysosomal storage disorder caused by pathogenic variants in the SMPD1 gene, resulting in deficient acid sphingomyelinase activity and progressive lipid accumulation in multiple organs. Owing to its clinical heterogeneity and lower prevalence compared with Gaucher disease (GD), ASMD remains underrecognized, particularly given the substantial overlap in clinical features such as hepatosplenomegaly and cytopenias. We evaluated the contribution of multiplex measurement of enzyme activities by tandem mass spectrometry (MS/MS) on dried blood spots (DBS) for the incidental detection of ASMD in patients referred for suspected GD. METHODS: All DBS samples referred to our center between 2018 and 2025 underwent multiplex MS/MS analysis, measuring six lysosomal enzyme activities, including acid sphingomyelinase and glucocerebrosidase. Patients with decreased enzyme activity, together with an abnormal enzyme-to-control enzyme activity ratio (using α-L-iduronidase (IDUA) as the control enzyme) underwent disease-specific biomarker followed by molecular analyses for diagnostic confirmation, when biomarkers were abnormal. RESULTS: Among 37,277 DBS samples analyzed, 5,765 were referred for suspected GD and/or ASMD. ASMD was confirmed in 56 patients and GD in 56 patients. Notably, 8 ASMD cases (14.3%) were incidentally detected among patients initially suspected of having GD, highlighting the diagnostic yield of parallel lysosomal enzyme testing. Three of these patients subsequently initiated enzyme replacement therapy. CONCLUSION: Multiplex MS/MS analysis on DBS enables systematic and simultaneous evaluation of GD and ASMD in routine practice. This strategy improves differential diagnosis, reduces diagnostic delay, and facilitates timely therapeutic decision-making, ultimately enhancing patient management.