What initial laboratory tests should be ordered for 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 1:80–1:160, as this provides >95% sensitivity for SLE and serves as the most effective screening test to rule out the disease. 1

First-Line Screening Test

• ANA by indirect immunofluorescence (IIFA) on HEp-2 cells is the reference-standard screening method, providing >95% sensitivity for SLE and therefore the most effective test to exclude the diagnosis 1
• The screening dilution should be 1:80–1:160; titers ≥1:160 are considered clinically significant with 86.2% specificity and 95.8% sensitivity for systemic autoimmune rheumatic diseases 1
• Both the ANA titer and immunofluorescence pattern must be reported, as the pattern guides subsequent specific antibody testing 1
• A negative ANA by IIFA makes SLE highly unlikely (negative predictive value >95%) and should prompt consideration of alternative diagnoses 1

Critical Pitfall to Avoid

• Automated ANA platforms (ELISA, multiplex) must not be used as the sole screening test because they have lower sensitivity and may miss relevant antibodies; if an automated platform is used, the specific method must be clearly documented 1

Comprehensive Autoantibody Panel (If ANA Positive)

When ANA is positive at ≥1:160, immediately order the following specific autoantibodies:

• Anti-dsDNA antibodies using a double-screening strategy: solid-phase assay (ELISA/FEIA) first for sensitivity, followed by Crithidia luciliae immunofluorescence test (CLIFT) for confirmation and higher specificity 2, 1
• Anti-Sm antibodies – highly specific for SLE with strong confirmatory power 1, 3
• Anti-Ro/SSA and anti-La/SSB antibodies – identify subgroups at risk for specific manifestations including neonatal lupus and congenital heart block 1, 3
• Anti-RNP antibodies – useful for identifying overlap syndromes 1, 3
• Antiphospholipid antibodies: lupus anticoagulant, anticardiolipin IgG/IgM, and anti-β2-glycoprotein I IgG/IgM 1, 3

Complement and Inflammatory Markers

• Complement C3 and C4 levels should be measured as part of the initial evaluation, as low levels indicate active disease and correlate with disease activity 1
• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) should be obtained as baseline inflammatory markers 1

Hematologic and Renal Assessment

• Complete blood count (CBC) with differential to screen for cytopenias (leukopenia, lymphopenia, thrombocytopenia, or hemolytic anemia) 1
• Serum creatinine or estimated glomerular filtration rate (eGFR) to assess baseline renal function 1
• Urinalysis with microscopy to detect hematuria, pyuria, or cellular casts 1
• Urine protein-to-creatinine ratio (or 24-hour urine protein) to quantify proteinuria 1
• Serum albumin to identify hypoalbuminemia related to disease activity or renal protein loss 1

Special Considerations for ANA-Negative Cases

• If ANA is negative at 1:160 but clinical suspicion remains high with multisystem involvement, consider rare ANA-negative SLE and repeat ANA testing in 3–6 months 1
• In high clinical suspicion cases, anti-dsDNA testing may be performed even when ANA is negative, particularly if there is strong clinical evidence of lupus nephritis 2

Infection Screening Before Immunosuppression

• Screen for HIV, hepatitis B, hepatitis C, and tuberculosis according to local guidelines and individual risk factors before initiating high-dose glucocorticoids or immunosuppressive therapy 1

Algorithmic Interpretation

If ANA negative at 1:160: SLE is highly unlikely; pursue alternative diagnoses 1

If ANA positive 1:40–1:80: Exercise caution—13.3% of healthy individuals are positive at 1:80; order specific antibodies only if multisystem involvement is present 1

If ANA positive ≥1:160: High specificity for systemic autoimmune disease; immediately order the complete autoantibody panel, complement levels, CBC, renal function tests, and urinalysis 1

If anti-dsDNA positive by solid-phase assay only (CLIFT negative): Interpret in clinical context and repeat anti-dsDNA testing in approximately 6 months 2, 1

If both solid-phase assay and CLIFT positive for anti-dsDNA: SLE is very likely; proceed with full organ-specific assessment 2, 1