| Diagnostic modality | Sample type | Expected finding in sialidosis type II | Distinguishes from | Notes / pitfalls | Key citations | URL / publication date |
|---|---|---|---|---|---|---|
| Urine oligosaccharide screening by thin-layer chromatography (TLC) | Urine | Abnormal urinary oligosaccharide pattern; increased urinary excretion of sialylated oligosaccharides / glycopeptides is a diagnostic hallmark of NEU1 deficiency | Supports separation from non-oligosaccharidosis causes of hydrops/ascites; helps prioritize sialidosis or galactosialidosis among lysosomal storage diseases | Screening test rather than standalone confirmation; congenital cases may also show oligosaccharides in ascitic fluid in historical reports summarized by review literature | (pqac-00000027, pqac-00000029, pqac-00000030, pqac-00000032) | https://doi.org/10.3390/diagnostics8020029 (2018-04-20); https://doi.org/10.1172/jci.insight.166470 (2023-10-23) |
| Neuraminidase (alpha-N-acetyl neuraminidase / NEU1) enzyme assay | Cultured skin fibroblasts from biopsy (gold standard), also leukocytes in some reports | Deficient lysosomal sialidase activity / neuraminidase deficiency | Confirms primary neuraminidase deficiency and narrows differential within lysosomal storage diseases | Fibroblasts are emphasized as the preferred material for sialidosis/galactosialidosis enzymology; specialized metabolic laboratories are often required | (pqac-00000027, pqac-00000031, pqac-00000029, pqac-00000033) | https://doi.org/10.3390/diagnostics8020029 (2018-04-20); https://doi.org/10.3390/jcm13051465 (2024-03-01) |
| Paired beta-galactosidase assay with neuraminidase testing | Cultured fibroblasts or other enzymology sample used for lysosomal enzyme workup | Beta-galactosidase activity should be normal in primary sialidosis type II | Galactosialidosis (secondary combined NEU1 and beta-galactosidase deficiency due to CTSA/PPCA defects) | Essential differential step because galactosialidosis can phenocopy sialidosis; PPCA deficiency causes secondary NEU1 deficiency | (pqac-00000027, pqac-00000029, pqac-00000043, pqac-00000046) | https://doi.org/10.3390/diagnostics8020029 (2018-04-20); https://doi.org/10.3390/jcm9030695 (2020-03-04) |
| Molecular genetic confirmation: targeted NEU1 sequencing | Blood, saliva, buccal swab, or DNA extracted from clinical specimen | Biallelic pathogenic/likely pathogenic NEU1 variants confirming autosomal recessive sialidosis | Distinguishes primary NEU1-related sialidosis from CTSA-related galactosialidosis and from other lysosomal storage disorders with overlapping hydrops/hepatosplenomegaly phenotypes | Definitive diagnosis requires variant interpretation in clinical context; useful for prenatal testing, carrier testing, and family counseling | (pqac-00000027, pqac-00000029, pqac-00000033, pqac-00000043, pqac-00000048) | https://doi.org/10.3390/diagnostics8020029 (2018-04-20); https://doi.org/10.3390/genes16020151 (2025-01) |
| Exome / genome sequencing (WES/WGS) | Blood-derived DNA or family trio DNA | Identification of biallelic NEU1 variants, including novel missense, splice-site, frameshift, or compound-heterozygous variants | Useful when biochemical phenotype overlaps with other lysosomal disorders or when prenatal/neonatal presentations are nonspecific | Particularly valuable in atypical presentations or when single-gene testing is unrevealing; sequencing results may still require biochemical validation | (pqac-00000027, pqac-00000031, pqac-00000047) | https://doi.org/10.3390/diagnostics8020029 (2018-04-20); https://doi.org/10.3390/jcm13051465 (2024-03-01); https://doi.org/10.3390/genes16020151 (2025-01) |
| Prenatal / family-based molecular diagnosis | Fetal DNA, parental DNA, or prenatal specimen in at-risk pregnancies | Detection of familial NEU1 pathogenic variants before birth | Helps distinguish congenital hydropic sialidosis from other genetic causes of nonimmune hydrops fetalis | Particularly relevant because congenital type II may be lethal in utero or shortly after birth; enables recurrence-risk counseling | (pqac-00000014, pqac-00000030, pqac-00000033) | https://doi.org/10.3390/diagnostics8020029 (2018-04-20) |
| Peripheral blood smear / bone marrow smear | Peripheral blood or bone marrow | Storage granules in lymphocytes may be present | Supports lysosomal storage disease rather than isolated renal/hepatic disease | Ancillary, not disease-specific; should not replace enzyme or molecular confirmation | (pqac-00000027) | https://doi.org/10.3390/diagnostics8020029 (2018-04-20) |
| Integrated LSD second-line workup | Plasma, leukocytes, fibroblasts, urine, DNA | Combination of low neuraminidase activity, abnormal urine oligosaccharides, and biallelic NEU1 variants | Helps distinguish from mucolipidoses, multiple sulfatase deficiency, and other hepatosplenomegaly-associated LSDs | Serrano 2024 recommends molecular testing accompanied by enzymatic testing when feasible; biopsy should generally be last resort | (pqac-00000031, pqac-00000034) | https://doi.org/10.3390/jcm13051465 (2024-03-01) |
| Interpretation safeguard for VUS / pseudodeficiency | DNA plus matched biochemical testing | Variants of uncertain significance should be corroborated by enzymology and/or biomarkers; pseudodeficiency may show low in vitro activity without true disease | Prevents overcalling sialidosis when lab abnormalities are non-pathogenic or inconclusive | Important pitfall from LSD diagnostics: VUS often need targeted biochemical confirmation; specialized labs improve interpretation | (pqac-00000028, pqac-00000031) | https://doi.org/10.51847/isa3beaurx (2024-01); https://doi.org/10.3390/jcm13051465 (2024-03-01) |


*Table: This table summarizes the core diagnostic workflow and major differential-diagnostic issues for congenital sialidosis type II. It highlights the complementary roles of urine oligosaccharide screening, fibroblast neuraminidase assays, beta-galactosidase testing, and molecular confirmation, including key pitfalls such as VUS interpretation and pseudodeficiency.*