Autoimmune hepatitis with negative antinuclear antibody and normal early-phase IgG: diagnosis supported by histology, IgG dynamics, and steroid responsiveness.

Autoimmune hepatitis (AIH) with negative anti-nuclear antibody (ANA) has been reported to frequently exhibit acute clinical and histological features. We herein report a case of asymptomatic AIH with negative ANA, normal early-phase immunoglobulin G (IgG) levels, and chronic histological features. A 49-year-old asymptomatic female was found to have elevated liver enzymes during a routine health check. Liver biopsy demonstrated portal-predominant lymphoplasmacytic infiltrates with interface hepatitis, compatible with chronic AIH. Despite compatible histology, the simplified criteria yielded "unlikely" (score 3), and the revised International Autoimmune Hepatitis Group score "probable" (score 14) due to absent serology. However, the patient met the Japanese guidelines for "atypical AIH" based on histology and exclusion of other causes. Consequently, corticosteroid therapy was initiated. Notably, a rapid biochemical response and a delayed IgG elevation at treatment onset confirmed "typical AIH" by Japanese criteria, whereas the international scores remained non-definitive. This case highlights that AIH should not be excluded solely because ANA is negative and IgG levels are initially normal. Histological assessment, exclusion of alternative etiologies, monitoring IgG dynamics, and therapeutic response remain essential for establishing the diagnosis.