What investigations should be ordered for suspected Systemic Lupus Erythematosus (SLE)?

Diagnostic Workup for Suspected Systemic Lupus Erythematosus

Begin with antinuclear antibody (ANA) testing by immunofluorescence on HEp-2 cells as the mandatory first-line investigation, as ANA positivity is an absolute requirement for SLE diagnosis under current classification criteria. 1, 2

Initial Serological Testing

Primary Screening

• ANA by indirect immunofluorescence (IIF) on HEp-2 cells is the essential entry criterion—without positive ANA, SLE classification cannot proceed regardless of clinical manifestations 2
• Order ANA testing only when patients present with unexplained involvement of two or more organ systems, as the low disease prevalence in primary care yields poor predictive value otherwise 3
• A titer of 1:40 or greater is considered positive, though higher titers (≥1:160) are more specific for SLE 3

If ANA is Positive: Proceed with Confirmatory Antibody Testing

• Anti-dsDNA antibodies using a dual-screening strategy: 1

  • First: Solid-phase assay (SPA) such as FEIA, CLIA, or ELISA for sensitivity
  • Second: Crithidia luciliae immunofluorescence test (CLIFT) for confirmation—this is considered pathognomonic when positive 1
  • Both positive results indicate "SLE very likely"; discordant results require clinical correlation and repeat testing in 3-6 months 1

• Anti-ENA (extractable nuclear antigens) panel including: 1

  • Anti-Sm (Smith) antibody—highly specific for SLE 3, 4
  • Anti-Ro/SSA and anti-La/SSB antibodies
  • Anti-RNP antibody

• Antiphospholipid antibodies (critical for risk stratification): 1

  • Anticardiolipin IgG and IgM
  • Anti-β2-glycoprotein I IgG and IgM
  • Lupus anticoagulant testing

If ANA is Negative

• Consider alternative diagnoses and symptom remission 1
• If symptoms persist with high clinical suspicion, repeat ANA testing in 3-6 months 1
• Rare ANA-negative SLE exists but requires persistent characteristic multisystem involvement before pursuing this diagnosis 3

Organ-Specific Laboratory Investigations

Hematological Assessment

• Complete blood count with differential to detect: 1, 2
  • Leukopenia (particularly lymphopenia)
  • Thrombocytopenia
  • Hemolytic anemia
• Direct Coombs test if anemia is present 4

Renal Function Testing

• Urinalysis with microscopy for: 1, 2
  • Proteinuria (>0.5 g/24 hours or protein/creatinine ratio)
  • Cellular casts (red blood cell or granular casts)
  • Active urinary sediment
• Serum creatinine and estimated GFR 2
• 24-hour urine protein or spot urine protein/creatinine ratio 2

Complement Levels

• C3 and C4 complement levels—low levels suggest active disease and are part of newer classification criteria 4

Additional Baseline Testing

• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) 1
• Comprehensive metabolic panel 1
• Lipid profile and fasting glucose for cardiovascular risk assessment 1, 2

Clinical Pitfalls and Caveats

Critical warning: Do not order ANA testing indiscriminately—the test has poor positive predictive value in low-prevalence populations and should be reserved for patients with genuine multisystem involvement 3. Ordering ANA for nonspecific symptoms like isolated fatigue or arthralgias leads to false-positive results and diagnostic confusion.

Methodological considerations: Anti-dsDNA testing shows significant inter-method variability 1. The dual-screening approach (SPA followed by CLIFT confirmation) minimizes false positives while maintaining sensitivity 1. Report results from both methods separately with clinical interpretation 1.

Timing matters: If initial testing is negative but clinical suspicion remains high, repeat serological testing in 3-6 months, as autoantibodies may appear before full clinical manifestations develop 1.

Remember: Classification criteria are not formal diagnostic criteria, though widely used in practice 2, 5. Clinical judgment remains paramount, particularly in early disease when insufficient features may be present to meet classification thresholds 5.