What laboratory tests are used to diagnose autoimmune diseases?

Laboratory Tests for Diagnosing Autoimmune Diseases

The diagnosis of autoimmune diseases requires specific laboratory tests including antinuclear antibodies (ANA) by indirect immunofluorescence assay (IIFA) as the initial screening test, followed by targeted antibody panels based on clinical presentation and organ involvement. 1

Initial Screening Tests

Antinuclear Antibody (ANA) Testing

• Primary screening test: ANA by IIFA using HEp-2 cells at 1:160 dilution (optimal for adults)
  • Provides higher specificity (86.2%) while maintaining sensitivity (95.8%) 1
  • For children (<16 years), a lower screening dilution of 1:40 may be appropriate 1
• Pattern recognition: Nuclear and cytoplasmic patterns provide valuable information about potential antibody specificity 1
• Limitations: Up to 25% of healthy individuals may have positive ANA at low titers 1

Inflammatory Markers

• Erythrocyte sedimentation rate (ESR)
• C-reactive protein (CRP)
• Complete blood count with differential
• Comprehensive metabolic panel

Disease-Specific Antibody Testing

After positive ANA screening, targeted testing should be performed based on clinical presentation:

For Systemic Autoimmune Diseases

• Systemic Lupus Erythematosus (SLE):

  • Anti-double-stranded DNA (anti-dsDNA)
  • Anti-Smith (anti-Sm)
  • Anti-SSA/Ro and anti-SSB/La
  • Complement levels (C3, C4) - typically decreased in active disease 1

• Sjögren's Syndrome:

  • Anti-SSA/Ro and anti-SSB/La antibodies

• Systemic Sclerosis:

  • Anti-centromere antibodies
  • Anti-Scl-70 (topoisomerase I)
  • Anti-RNA polymerase III

• Inflammatory Myopathies:

  • Anti-Jo-1 and other antisynthetase antibodies
  • Anti-Mi-2, anti-SRP, anti-MDA5

For Organ-Specific Autoimmune Diseases

• Autoimmune Hepatitis:

  • Anti-smooth muscle antibodies (SMA)
  • Anti-liver/kidney microsomal antibody type 1 (anti-LKM-1)
  • Anti-liver cytosol type 1 antibody (anti-LC1)
  • Anti-soluble liver antigen/liver pancreas (anti-SLA/LP) 1
  • Recommended starting dilution for testing: 1:40 for adults, 1:20 for children 1

• Autoimmune Encephalitis:

  • CSF analysis (cell count, protein, glucose, oligoclonal bands)
  • Neural autoantibody panels (NMDAR, LGI1, CASPR2, etc.)
  • Additional blood tests: antithyroid antibodies, vitamin levels, inflammatory markers 1

• Rheumatoid Arthritis:

  • Rheumatoid factor (RF)
  • Anti-citrullinated protein antibodies (ACPA)

• Vasculitis:

  • Anti-neutrophil cytoplasmic antibodies (ANCA)
  • Anti-proteinase 3 (PR3-ANCA)
  • Anti-myeloperoxidase (MPO-ANCA)

Testing Methodology Considerations

Immunofluorescence vs. Solid-Phase Assays

• IIFA (gold standard): Provides pattern information but requires technical expertise 1
• ELISA and other solid-phase assays: More automated but may miss certain antibodies
  • Not recommended as sole primary screening test for autoimmune hepatitis 1
  • Should demonstrate performance equivalent to IIFA if used for ANA screening 1

Important Testing Principles

1. Include clinical information with test requests to guide laboratory interpretation 1
2. Repeat testing may be necessary as autoantibodies can vary during disease course 1
3. Collect blood samples before treatment with immunoglobulins or plasmapheresis to avoid false results 1
4. Send samples to reference laboratories for comprehensive evaluation in cases of diagnostic uncertainty 1

Pitfalls and Caveats

• False negatives: May occur due to:

  • Suboptimal test sensitivity
  • Antibody absorption
  • Immunosuppressive therapy
  • Antigen exhaustion
  • Immunosenescence 2

• False positives: May occur in:

  • Healthy individuals (especially at low titers)
  • Certain infections
  • Malignancies
  • Drug-induced autoimmunity

• Seronegative autoimmune disease: Some patients may have clinical disease without detectable autoantibodies, requiring diagnosis based on clinical features, histopathology, and imaging 2

• Titer interpretation: Antibody titers correlate only roughly with disease activity in adults but may be more useful biomarkers in pediatric patients 1

Remember that laboratory tests should be interpreted in the context of clinical presentation, and no single test is diagnostic for most autoimmune diseases.