What initial laboratory tests should be performed to evaluate a patient with suspected systemic lupus erythematosus?

Initial Laboratory Testing for Suspected Systemic Lupus Erythematosus

Begin with ANA testing by indirect immunofluorescence on HEp-2 cells at a screening dilution of 1:80 to 1:160, as this provides >95% sensitivity and is the most effective first-line test to rule out SLE. 1

First-Line Screening: ANA Testing

• ANA by indirect immunofluorescence (IIFA) on HEp-2 cells is the reference-standard screening method with >95% sensitivity for SLE 1
• Screen at a dilution of 1:80 to 1:160; titers ≥1:160 are considered clinically significant 1
• Both the titer and immunofluorescence pattern must be reported; homogeneous patterns associate with more severe disease 1
• A negative ANA makes SLE highly unlikely (negative predictive value >95%) and should prompt consideration of alternative diagnoses 1, 2
• Never use automated ANA platforms (ELISA, multiplex) as the sole screening test because they have lower sensitivity and may miss relevant antibodies 1

Comprehensive Autoantibody Panel (If ANA Positive)

When ANA is positive at ≥1:160, immediately order the following complete panel 3, 1:

• Anti-dsDNA antibodies (use double-screening: solid-phase assay confirmed by Crithidia luciliae immunofluorescence test for highest specificity) 1
• Anti-Sm antibodies (highly specific for SLE) 3, 1
• Anti-Ro/SSA antibodies 3, 1
• Anti-La/SSB antibodies 3, 1
• Anti-RNP antibodies 3, 1
• Antiphospholipid antibodies (lupus anticoagulant, anticardiolipin IgG/IgM, anti-β2-glycoprotein I IgG/IgM) 3, 1
• Complement C3 and C4 levels 3, 1

Essential Baseline Laboratory Tests

Obtain these tests in all patients with suspected SLE 3, 1:

Hematologic Assessment

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

Inflammatory Markers

• Erythrocyte sedimentation rate (ESR) 3, 1
• C-reactive protein (CRP) 3, 1

Renal Function Assessment

• Serum creatinine or estimated glomerular filtration rate (eGFR) 3, 1
• Urinalysis with microscopy 3, 1
• Urine protein-to-creatinine ratio (or 24-hour proteinuria) 3, 1

Additional Baseline Tests

• Serum albumin 3, 1
• Liver function tests as part of baseline metabolic panel 1

Pre-Treatment Infection Screening

Before initiating immunosuppression or high-dose glucocorticoids, complete the following screens 1:

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

Critical Interpretation Algorithm

If ANA is Negative at 1:160

• SLE is highly unlikely; pursue alternative diagnoses 1, 2
• In rare cases with persistent multisystem involvement, consider ANA-negative SLE and repeat ANA in 3-6 months 1
• Anti-dsDNA may still be tested if lupus nephritis is strongly suspected clinically despite negative ANA 1

If ANA is Positive ≥1:160

• Proceed immediately with comprehensive autoantibody panel and baseline labs 1
• Both solid-phase anti-dsDNA and Crithidia luciliae positive = very high likelihood of SLE 1
• If solid-phase anti-dsDNA positive but Crithidia luciliae negative, interpret cautiously and repeat in 6 months 1

Common Pitfalls to Avoid

• Do not repeat ANA testing after initial positive result—it does not provide clinical benefit for monitoring disease activity 1
• Do not rely solely on ANA without specific antibody testing, as this leads to misdiagnosis 1
• Do not use automated ANA platforms alone; if employed, clearly document the specific method used 1
• Do not order ANA in patients without multisystem involvement (≥2 organ systems), as low disease prevalence in primary care yields poor predictive value 2, 4