The diagnosis of autoimmune hepatitis (AIH) leans on a characteristic biopsy pattern, although none of the individual findings is exclusive to the disease.
Interface hepatitis
The histological pattern required for the diagnosis is interface hepatitis, inflammation at the portal-parenchymal interface (PPI) — the region between the portal triad (the portal tract, which carries a branch of the portal vein, hepatic artery and bile duct) and the lobule of the liver, where the connective tissue of the portal tract meets the hepatocytes. Interface hepatitis is necessary for the diagnosis but is not specific to AIH; other forms of chronic hepatitis can produce it as well.
Alongside it, the typical picture includes two other features:
- hepatocyte rosettes, clusters of hepatocytes arranged around a small lumen and embedded in connective tissue
- lymphocyte emperipolesis, a lymphocyte lying within the cytoplasm of a hepatocyte rather than between the cells
Plasma cells
Plasma cells, the antibody-secreting cells, can be abundant at the interface or throughout the acinus, the functional unit of liver parenchyma. When plasma cells are present together with moderate-to-severe interface hepatitis, the combination has a specificity of 81% and a positive predictive value of 68% for AIH; the positive predictive value is the proportion of patients with this finding who truly have the disease.
Necrosis patterns
Necrosis, the death of hepatocytes, can follow several patterns. Centrilobular necrosis of zone 3, the zone around the central vein, is seen in about 29% of AIH patients and is a feature of acute presentations. Bridging necrosis, in which necrosis links one vascular structure to another, and multi-acinar necrosis indicate a severe inflammatory reaction. Pan-acinar hepatitis, inflammation across the whole acinus, can be found during an acute severe relapse after withdrawal of treatment.