What is the diagnostic approach for suspected systemic lupus erythematosus?

Diagnostic Approach for Systemic Lupus Erythematosus

Begin with ANA testing by indirect immunofluorescence on HEp-2 cells at a titer ≥1:80 as the mandatory entry criterion, then proceed with confirmatory autoantibody testing and organ-specific assessment only if ANA is positive. 1, 2

Initial Screening: ANA Testing

ANA by indirect immunofluorescence (IIF) on HEp-2 cells is the single required first-line test, with 95-97% sensitivity effectively ruling out SLE when negative. 1, 2

Critical Technical Requirements:

• Use IIF on HEp-2 cells as the reference standard method—automated platforms (ELISA, multiplex) have lower sensitivity and must not be used as the sole screening test. 2
• Screen at 1:80 dilution for suspected SLE; use 1:160 cutoff in unselected populations to improve specificity from 74.7% to higher levels. 3
• Report both titer and immunofluorescence pattern—homogeneous patterns associate with more severe disease activity. 2
• If automated platforms are used, clearly document the specific method and do not label simply as "ANA test." 2

Interpretation Algorithm:

• ANA negative at 1:160: SLE is highly unlikely (>95% negative predictive value); pursue alternative diagnoses. 2
• ANA positive at ≥1:160: Proceed immediately to confirmatory testing panel. 1, 2
• ANA negative but strong clinical suspicion with multisystem involvement: Consider rare ANA-negative SLE (occurs in ~18% of biopsy-proven cutaneous lupus patients meeting SLE criteria); repeat ANA in 3-6 months. 2, 4

Common pitfall: The EULAR/ACR 2019 criteria require positive ANA as an entry criterion, but this has only 74.7% specificity at 1:80, meaning ANA alone cannot diagnose SLE—it merely selects patients for further evaluation. 3

Confirmatory Autoantibody Panel (After Positive ANA)

Order a comprehensive autoantibody panel including anti-dsDNA, anti-Sm, anti-Ro/SSA, anti-La/SSB, anti-RNP, and antiphospholipid antibodies, plus complement levels (C3, C4). 2

Anti-dsDNA Testing Strategy:

Use a double-screening approach: First perform a solid-phase assay (ELISA/FEIA) for high sensitivity, then confirm all positives with Crithidia luciliae immunofluorescence test (CLIFT) for high specificity. 3, 2

• Both SPA and CLIFT positive: Very high likelihood of SLE; proceed with full organ assessment. 2
• SPA positive but CLIFT negative: Interpret cautiously in clinical context; repeat anti-dsDNA in ~6 months. 2
• Exception: If ANA is negative but lupus nephritis is strongly suspected clinically, anti-dsDNA testing may still be performed. 2

Highly Specific Confirmatory Antibodies:

• Anti-dsDNA and anti-Sm are highly specific for SLE and have strong confirmatory power even when pretest probability is low. 5, 6
• Anti-Ro/SSA and anti-La/SSB: Essential before pregnancy planning. 2
• Antiphospholipid antibodies (lupus anticoagulant, anticardiolipin, anti-β2-glycoprotein I): Test before pregnancy, surgery, transplantation, or estrogen-containing treatments. 2, 5

Critical caveat: Do not order anti-dsDNA reflexively when ANA is positive without clinical suspicion—many laboratories automatically add this test, leading to false positives in low-prevalence populations. 3

Baseline Laboratory Assessment

Obtain the following tests in all patients with positive ANA and clinical suspicion for SLE: 2

Hematologic and Inflammatory Markers:

• Complete blood count with differential: Screen for cytopenias; severe lymphopenia (<500 cells/mm³) or neutropenia (<500 cells/mm³) signals high infection risk. 2
• ESR and CRP: Baseline inflammatory markers. 2
• Complement C3 and C4: Low levels correlate with disease activity. 2, 5

Renal Function Assessment:

• Serum creatinine or eGFR 2
• Urinalysis with microscopy 2
• Urine protein-to-creatinine ratio or 24-hour proteinuria 2
• Serum albumin: Identifies hypoalbuminemia from disease activity or renal loss. 2

Metabolic Panel:

• Liver function tests 2

Organ-Specific Clinical Assessment

Mucocutaneous Evaluation:

Document all skin lesions using the Cutaneous Lupus Erythematosus Disease Area and Severity Index (CLASI), classifying as LE-specific (malar rash, discoid lesions, subacute cutaneous lupus), LE-nonspecific, mimickers, or drug-related. 2

Renal Assessment (If Abnormal Urinalysis or Elevated Creatinine):

• Repeat urine protein-to-creatinine ratio, urine microscopy, renal ultrasound 2
• Renal biopsy is advised when uncertainty exists about the nature or severity of renal involvement (Level 1b evidence, Grade B recommendation). 2
• Patients with established nephropathy require monitoring every 3 months for the first 2-3 years. 2

Neuropsychiatric Assessment:

Specifically query for seizures, headaches, stroke-like symptoms, peripheral neuropathy, cognitive dysfunction (attention, concentration, word-finding, memory difficulties), and mood disorders. 2

Ophthalmologic Assessment:

Baseline eye examination is recommended, especially for patients who will receive glucocorticoids or antimalarials. 2

Pre-Treatment Infection Screening

Before initiating high-dose glucocorticoids or immunosuppression, screen for: 2

• HIV, hepatitis C, hepatitis B (based on risk factors)
• Tuberculosis (interferon-γ release assay or tuberculin skin test with chest radiography per local guidelines)

Disease Activity Measurement

Evaluate disease activity using a validated index at each visit, such as SLEDAI (SLE Disease Activity Index) or BILAG (British Isles Lupus Assessment Group). 3, 2

Assess organ damage annually using the SLICC Damage Index. 2

Tests to Avoid

Do Not Repeat ANA After Initial Positive Result:

Repeating ANA testing is neither appropriate nor cost-effective for monitoring disease activity or progression—use quantitative anti-dsDNA and complement levels instead. 5

Do Not Routinely Order Direct Coombs Test:

The direct Coombs test is not recommended as a routine first-line test in the initial SLE workup; order only if peripheral blood smear shows spherocytes or schistocytes. 2

Do Not Order Anti-CCP Antibodies:

Anti-CCP is specific for rheumatoid arthritis, not SLE, and has no role in initial serologic evaluation. 1

Special Considerations

ANA-Negative SLE:

Research shows that 18% of patients with biopsy-proven cutaneous lupus and negative ANA still meet ACR/SLICC criteria for SLE, with 84.4% having ≥1 organ system involved beyond skin. 4 If multisystem involvement persists despite negative ANA at 1:160, repeat ANA in 3-6 months and consider rare ANA-negative SLE. 2

Rheumatoid Factor Positivity:

RF can be positive in 30-40% of SLE patients, particularly those with antiphospholipid antibodies; the presence of RF with positive ANA, anti-dsDNA, and low complement strongly suggests SLE rather than RA. 5