Critical Analysis of Metallothionein Immunohistochemistry for Diagnosis of Pediatric Wilson Disease: Histologic Scoring Facilitates Distinction from Clinical Mimics.

Background/Objectives: Wilson disease (WD) is difficult to diagnose in children because it can mimic other pediatric liver diseases. It lacks specific routine histologic features. Recent studies suggest that metallothionein (MT) immunohistochemistry (IHC) may aid diagnosis, but pediatric data remain limited. Methods: This is a retrospective study of 121 pediatric liver biopsies, including WD (n = 63), primary sclerosing cholangitis (n = 23), metabolic dysfunction-associated steatotic liver disease (n = 19), autoimmune hepatitis (n = 13), and multidrug resistance protein 3 deficiency (n = 3). MT IHC was assessed for extent, intensity, pattern, and distribution. Results: MT positivity was identified in 96.8% of WD cases compared with 73.9% of PSC, 38.5% of AIH, and 10.5% of MASLD cases (p < 0.001). In WD, MT IHC characteristically demonstrated diffuse non-zonal cytoplasmic staining involving ≥25% of hepatocytes, most frequently >50%, with moderate-to-strong intensity. A threshold of ≥25% positive hepatocytes achieved the highest overall diagnostic accuracy (87.6%), with 80.9% sensitivity and 94.8% specificity. Diffuse non-zonal staining demonstrated 100% specificity in this cohort. Conclusions: MT IHC is a sensitive adjunctive marker for pediatric WD. Interpretation of staining extent, intensity, and distribution improves diagnostic specificity and assists in distinguishing WD from histologic mimics and cholestatic disorders.