Autoimmune Hepatitis Subtypes and Type Determination
Determine the AIH subtype by screening serum for ANA, SMA, and anti-LKM1 antibodies: Type 1 AIH is defined by the presence of ANA and/or SMA, while Type 2 AIH is defined by anti-LKM1 and/or anti-LC1 antibodies. 1
Classification System
AIH classification is fundamentally based on the autoantibody profile present in the patient's serum:
Type 1 AIH (Most Common)
- Defining antibodies: Antinuclear antibodies (ANA) and/or anti-smooth muscle antibodies (ASMA/SMA)
- Prevalence: Represents 85-95% of AIH cases
- Detection rates:
- Pattern: Usually homogeneous staining on immunofluorescence 1
Type 2 AIH (Less Common)
- Defining antibodies: Anti-liver-kidney-microsomal type 1 (anti-LKM1) and/or anti-liver cytosol type 1 (anti-LC1) antibodies
- Geographic variation: Found in only 2-4% of AIH patients in Northern Europe and North America, but more common in Southern Europe 1
- Detection rates:
- Target antigens: Cytochrome P450 2D6 for anti-LKM1; formiminotransferase cyclodeaminase for anti-LC1 1
Diagnostic Algorithm for Type Determination
Initial Screening Approach
First-line testing should screen for ANA, SMA, and anti-LKM1 simultaneously using indirect immunofluorescence on rat triple tissue (IFA-RTT) or HEp-2 cells, performed in parallel with anti-SLA/LP testing by solid phase assays. 2
Starting serum dilution:
If Initial Screening is Negative
- Retest at lower dilution (1:40 in adults, 1:10 in children) to avoid false negatives 2
- Test for anti-SLA/LP antibodies using antigen-specific assays (ELISA or immunoblot), as these cannot be detected by immunofluorescence 1
- Consider atypical ANCA (aANCA) testing when AIH is suspected but standard antibodies are negative 1
Important Clinical Caveats
Seronegative AIH
Approximately 15% of AIH patients have undetectable autoantibodies at presentation, though antibodies may appear during follow-up. These antibody-negative patients have similar clinical features and treatment responses compared to antibody-positive patients 1, so absence of antibodies does not exclude AIH if other features are present.
IgG Levels
While polyclonal hypergammaglobulinemia (especially IgG) is characteristic, 10% of AIH patients have normal serum IgG at presentation 1, and this rises to 25-39% in acute AIH presentations 1. Normal IgG does not exclude the diagnosis.
Additional Supportive Antibodies
- Anti-SLA/LP: Found in up to one-third of patients with either Type 1 or Type 2 AIH; highly specific for AIH and associated with longer time to response and higher relapse rates after treatment withdrawal 1
- Antimitochondrial antibody (AMA): Found in 4-9% of AIH patients, though strongly associated with primary biliary cholangitis 1
Geographic and Technical Considerations
Anti-LKM1 antibodies can also occur in hepatitis C infection 1, so viral hepatitis must be excluded. The high specificity of VG (vessels and glomeruli) or VGT (vessels, glomeruli, tubules) staining patterns for SMA supports AIH diagnosis 2.
Pro-zone Effect Warning
In patients with extremely elevated IgG levels, falsely negative immunofluorescence results can occur due to the pro-zone effect. Retest at higher serial dilutions if clinical suspicion remains high despite negative initial testing 2.
Practical Testing Sequence
- Order simultaneous testing: ANA, SMA, anti-LKM1 by immunofluorescence + anti-SLA/LP by ELISA
- Interpret results:
- ANA and/or SMA positive → Type 1 AIH
- Anti-LKM1 and/or anti-LC1 positive → Type 2 AIH
- All negative → Retest at lower dilution, add aANCA testing
- Confirm with liver biopsy showing interface hepatitis, lymphoplasmacytic infiltrate, and hepatocyte rosetting
- Exclude viral hepatitis, drug-induced liver injury, and metabolic liver diseases before finalizing diagnosis
The subtype classification primarily serves epidemiological and research purposes rather than dictating different treatment approaches, as both types respond to immunosuppressive therapy with corticosteroids and azathioprine or mycophenolate mofetil.