What are the basic tests for a young patient with suspected autoimmune disease?

Basic Laboratory Testing for Young Patients with Suspected Autoimmune Disease

Begin with antinuclear antibody (ANA) testing by indirect immunofluorescence assay (IIFA) on HEp-2 cells at a screening dilution of 1:160, as this is the reference standard for initial screening of systemic autoimmune rheumatic diseases. 1, 2

First-Line Screening Tests

Core Autoantibody Testing

• ANA by IIFA on HEp-2 cells is the mandatory first test, with both titer and pattern (nuclear, cytoplasmic, or mitotic) reported, as patterns provide critical diagnostic information 1, 2
• If automated methods (ELISA, multiplex) are used and negative but clinical suspicion remains high, IIFA must be performed due to superior sensitivity for SLE and systemic sclerosis 2
• Both nuclear and cytoplasmic staining patterns must be reported as they provide valuable diagnostic information 1

Essential Laboratory Panel

• Complete blood count with differential to detect cytopenias, anemia, or abnormal cells indicating disease activity 2, 3
• Comprehensive metabolic panel including serum creatinine (or eGFR), serum albumin, and liver function tests to assess organ involvement 1, 2, 3
• Inflammatory markers: erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) to evaluate acute phase response and disease activity 1, 2, 4
• Quantitative immunoglobulin levels (IgG, IgA, IgM) to identify immunodeficiency states 2
• Urinalysis with urine protein/creatinine ratio to detect renal involvement 2

Reflex Testing Based on Initial Results

If ANA is Positive

• Anti-dsDNA antibodies when SLE is clinically suspected, using Crithidia luciliae immunofluorescence test or Farr assay for high specificity 1, 2, 5
• Specific antibodies panel including anti-Ro/SSA, anti-La/SSB, anti-RNP, anti-Sm based on clinical presentation and ANA pattern 2, 5
• Complement levels (C3, C4) essential for SLE evaluation and monitoring 2, 4
• Anti-phospholipid antibodies (lupus anticoagulant, anticardiolipin, anti-β2-glycoprotein I) if thrombosis, recurrent pregnancy loss, or thrombocytopenia present 2

Disease-Specific Antibodies

• For Sjögren's syndrome: anti-SSA/Ro and anti-SSB/La antibodies 1, 5
• For systemic sclerosis: anti-Scl-70/topoisomerase-1 (diffuse disease) and anti-centromere (limited disease/CREST) 1, 5
• For inflammatory myopathies: myositis-specific antibodies including anti-Jo-1 and other antisynthetase antibodies 1, 5

Pre-Treatment Screening

Before initiating immunosuppressive therapy, perform:

• Infectious disease screening for HIV, hepatitis B and C based on risk factors 2
• Tuberculosis screening according to local guidelines 1, 2

Critical Pitfalls to Avoid

• A negative ANA does not exclude autoimmune disease, as sensitivity is not 100% even at 1:160 dilution 2
• ANA positivity occurs in up to 31.7% of healthy individuals at 1:40 dilution and 5% at 1:160, so clinical context is essential 2, 6
• Low-titer ANA can be clinically significant—titers above the screening threshold do not correlate with disease activity 2
• Do not use limited antigen panels (ELISA with restricted antigens) as initial ANA screening, as they miss important patterns and have lower sensitivity 2
• Autoantibody positivity alone does not make a diagnosis; absence of autoantibodies does not exclude diagnosis 6
• Thyroid function tests may be misleading if performed during acute illness due to euthyroid sick syndrome from hyperglycemia, ketosis, or weight loss 7

Special Considerations for Young Patients

• No consensus exists for screening dilution in children under 16 years—some laboratories use 1:40 2
• In young females, check anti-Ro and anti-La antibodies before pregnancy due to risk of congenital heart block 2
• For type 1 diabetes patients, screen for thyroid autoantibodies (antithyroid peroxidase and antithyroglobulin) and celiac disease (IgA tissue transglutaminase with total IgA levels) soon after diagnosis 7

Interpretation Strategy

The ESR and CRP combination provides the most useful information in chronic diseases, with CRP showing rapid changes in the first 48 hours useful for monitoring treatment efficacy 4. ANA testing is primarily intended for diagnostic purposes, not for monitoring disease progression 1. Autoantibody testing should be performed before initiating immunosuppressive therapy when possible, as treatment may affect results 1.