Laboratory Testing for Suspected Systemic Lupus Erythematosus
Begin with antinuclear antibody (ANA) testing by indirect immunofluorescence on HEp-2 cells as the first-line screening test, and if positive with clinical suspicion, proceed to anti-dsDNA antibodies and anti-ENA panel. 1
Initial Screening Approach
ANA Testing - The Gateway Test
- ANA by indirect immunofluorescence (IIFA) on HEp-2 cells is the reference screening method 1
- Order ANA only when there is clinical suspicion of SLE with unexplained involvement of two or more organ systems 2
- A negative ANA makes SLE highly unlikely (sensitivity nearly 100%) 3
- Screening dilution of 1:160 is generally appropriate for adult populations 1
- Report both the titer and the pattern when positive 1
Critical Pitfall: Do not order ANA as a general screening test for nonspecific symptoms like isolated fatigue or single joint pain - this leads to overdiagnosis and unnecessary follow-up 4. The positive predictive value in unselected populations is extremely low (2.1% for SLE even in rheumatology referral cohorts) 4.
Confirmatory Testing After Positive ANA
Anti-dsDNA Antibodies
When ANA is positive and clinical suspicion for SLE exists, order anti-dsDNA antibodies using a double-screening strategy 5:
- First: Solid phase assay (SPA) such as FEIA, ELISA, or CLIA - high sensitivity
- Second: Crithidia luciliae immunofluorescence test (CLIFT) - high specificity, pathognomonic for SLE
Interpretation algorithm 5:
- SPA negative + CLIFT negative: SLE diagnosis cannot be established at this time
- SPA positive + CLIFT positive: SLE very likely
- SPA positive + CLIFT negative: Evaluate in context of clinical characteristics; consider anti-nucleosome antibodies
- SPA negative + CLIFT positive: Repeat testing; if persistent, follow clinically
Anti-ENA (Extractable Nuclear Antigen) Panel
When ANA is positive, order anti-ENA antibodies for confirmatory testing 5, 1. The most relevant targets in SLE include:
- Anti-Smith (Sm): Highly specific for SLE
- Anti-Ro (SSA): Associated with photosensitivity, neonatal lupus
- Anti-La (SSB): Often accompanies anti-Ro
- Anti-U1-RNP: Associated with mixed connective tissue disease features
- Anti-ribosomal P protein: Associated with neuropsychiatric lupus
- Anti-chromatin antibodies
Additional Serologic Testing
Complement Levels
Always order complement C3 and C4 levels 5 - low levels indicate active disease and consumption, particularly with renal involvement.
Antiphospholipid Antibodies
Order antiphospholipid antibody panel when clinical features suggest SLE 5, as 30-40% of SLE patients are positive:
- Anticardiolipin antibodies (IgG and IgM)
- Anti-β2-glycoprotein I antibodies (IgG and IgM)
- Lupus anticoagulant
Specialized Testing for Specific Scenarios
Anti-nucleosome antibodies 5:
- Consider when SPA is positive but CLIFT is negative with persistent clinical suspicion
- Useful for monitoring lupus nephritis in anti-dsDNA negative patients
- Sensitivity 83.33%, specificity 96.67%
Anti-C1q antibodies 5:
- Consider in patients with suspected or confirmed lupus nephritis
- Found in almost 100% of patients with active lupus nephritis
- Excellent negative predictive value for renal flares
Supporting Laboratory Tests
Beyond autoantibodies, order:
- Complete blood count: Look for cytopenias (leukopenia, lymphopenia, thrombocytopenia, hemolytic anemia)
- Comprehensive metabolic panel: Assess renal function (creatinine, BUN)
- Urinalysis with microscopy: Screen for proteinuria, hematuria, cellular casts
- Urine protein-to-creatinine ratio: Quantify proteinuria if present
What NOT to Do
- Do not repeat ANA testing for monitoring disease activity - it is not cost-effective and not recommended 5
- Do not order "ANA reflex panels" automatically - interpret positive ANA in clinical context first 4
- Do not use tests based on restricted mixtures of antigens and call them "ANA tests" 1
- Do not order anti-dsDNA when ANA is negative unless there is exceptionally high clinical suspicion 5
Monitoring Established SLE
For patients with confirmed SLE: