Which additional laboratory tests should I order to rule out systemic lupus erythematosus?

Laboratory Testing to Rule Out Systemic Lupus Erythematosus

When ANA testing is positive or clinical suspicion for SLE exists, order a comprehensive autoantibody panel including anti-dsDNA, anti-Sm, anti-Ro/SSA, anti-La/SSB, anti-RNP, antiphospholipid antibodies (lupus anticoagulant, anticardiolipin, anti-β2-glycoprotein I), and complement levels (C3, C4) to establish or exclude the diagnosis. 1

Initial Screening Strategy

ANA Testing as First-Line

• ANA by indirect immunofluorescence on HEp-2 cells is the reference screening method, with >95% sensitivity for SLE, making it the most effective test to rule out the disease 2, 1
• Screen at a dilution of 1:80–1:160; titers ≥1:160 are clinically significant 1
• Report both the titer and immunofluorescence pattern; a homogeneous pattern associates with more severe disease activity 1
• A negative ANA by immunofluorescence makes SLE highly unlikely (negative predictive value >95%) and should prompt consideration of alternative diagnoses 1

Critical Caveat About Alternative ANA Methods

• Automated ANA platforms (ELISA, multiplex) must not be used as the sole screening test because they have lower sensitivity and may miss relevant antibodies 1
• If automated platforms are used, the specific method must be documented and should not be labeled simply as "ANA test" 2, 1

Comprehensive Autoantibody Panel (When ANA Positive)

Specific Autoantibodies

• Anti-dsDNA antibodies: Use a double-screening strategy with solid-phase assay followed by Crithidia luciliae immunofluorescence test (CLIFT) for higher specificity 2, 1
• Anti-Sm antibodies: Highly specific for SLE 3, 1
• Anti-Ro/SSA and anti-La/SSB antibodies: Important for neonatal lupus risk assessment before pregnancy 1
• Anti-RNP antibodies: Part of the comprehensive panel 1
• Antiphospholipid antibodies (lupus anticoagulant, anticardiolipin IgG/IgM, anti-β2-glycoprotein I IgG/IgM): Essential for thrombotic risk stratification 1

Complement Testing

• C3 and C4 levels should be assessed as part of initial diagnostic evaluation 1
• Low complement levels correlate with disease activity 3
• C1q levels are associated with SLE flare-ups 3

Essential Baseline Laboratory Tests

Hematologic Assessment

• Complete blood count with differential to screen for cytopenias; severe lymphopenia (<500 cells/mm³) or neutropenia (<500 cells/mm³) signals high infection risk 1

Renal Function Assessment

• Serum creatinine or estimated glomerular filtration rate (eGFR) 1
• Urinalysis with microscopy 1
• Quantitative protein assessment (urine protein/creatinine ratio or 24-hour proteinuria) to detect renal involvement 1

Inflammatory Markers

• Erythrocyte sedimentation rate (ESR) 1
• C-reactive protein (CRP) 1

Additional Baseline Tests

• Serum albumin to identify hypoalbuminemia related to disease activity or renal loss 1
• Liver function tests as part of the baseline metabolic panel 1

Infection Screening Before Immunosuppression

Complete these screens before initiating high-dose glucocorticoids or immunosuppressive therapy:

• HIV, hepatitis C, and hepatitis B screening based on individual risk factors 1
• Tuberculosis screening according to local guidelines (interferon-γ release assay or tuberculin skin test with chest radiography) 1

Diagnostic Algorithm Based on Test Results

If ANA Negative at 1:160 Dilution

• SLE is highly unlikely; consider alternative diagnoses 1
• If clinical suspicion remains very high with multisystem involvement, consider rare ANA-negative SLE and repeat testing in 3–6 months 2

If ANA Positive

• Proceed with comprehensive autoantibody panel (anti-dsDNA, anti-Sm, anti-Ro/SSA, anti-La/SSB, anti-RNP, antiphospholipid antibodies) 1
• Measure complement levels (C3, C4) 1
• For anti-dsDNA testing: Use solid-phase assay (SPA) followed by CLIFT confirmation 2
  • If both SPA and CLIFT positive: SLE very likely 2
  • If SPA positive but CLIFT negative: Evaluate in clinical context and consider repeat testing in 6 months 2

Interpretation Based on ANA Pattern

• Homogeneous pattern: Associated with more severe disease activity; 82% of anti-dsDNA-positive patients with this pattern have SLE 1, 4
• Speckled pattern: Also clinically significant for SLE diagnosis 4
• Combining two quantitative anti-dsDNA methods (e.g., RIA + CLIA) in patients with homogeneous or speckled ANA patterns maximizes specificity and positive predictive value 4

Common Pitfalls to Avoid

• Do not rely solely on ANA without further specific antibody testing, as this can lead to misdiagnosis 1
• Do not repeat ANA testing once positive; it is neither appropriate nor cost-effective for monitoring disease activity 3
• Do not order ANA in patients without unexplained involvement of two or more organ systems, as low disease prevalence in primary care yields low predictive value 5
• Be aware that RF can be positive in 30–40% of SLE patients, particularly those with antiphospholipid antibodies, and does not indicate rheumatoid arthritis in this context 3