A 28-year-old woman presents with a 3-month history of arthralgia, malar rash, and photosensitivity. Laboratory investigations show ANA positivity (1:320, homogeneous pattern). Which investigation is most appropriate to confirm the diagnosis of SLE?

Explanation

## Diagnosis of SLE: Role of Anti-dsDNA and Complement ### ANA Positivity — Necessary but Not Sufficient **Key Point:** ANA is highly sensitive (>95%) for SLE but not specific. Many healthy individuals and patients with other autoimmune diseases have positive ANA. Confirmation requires additional specific serology. ### Anti-dsDNA Antibodies — Gold Standard Confirmatory Test **High-Yield:** Anti-dsDNA (anti-double-stranded DNA) antibodies are: - Present in 60–70% of SLE patients - Highly specific (>95%) for SLE - Correlate with disease activity and lupus nephritis - Part of the ACR/EULAR 2019 classification criteria for SLE ### Complement Levels (C3, C4) — Markers of Disease Activity **Clinical Pearl:** Low C3 and C4 levels indicate active SLE and are associated with: - Lupus nephritis - Active vasculitis - Immune complex deposition - Consumption via classical complement pathway ### Investigation Algorithm for SLE Confirmation ```mermaid flowchart TD A[Clinical suspicion of SLE]:::outcome --> B[ANA screening]:::action B --> C{ANA positive?}:::decision C -->|Negative| D[SLE unlikely]:::outcome C -->|Positive| E[Order specific antibodies]:::action E --> F[Anti-dsDNA + Anti-Sm]:::action E --> G[Serum complement C3, C4]:::action F --> H{Anti-dsDNA or Anti-Sm positive?}:::decision H -->|Yes| I[SLE confirmed]:::outcome H -->|No| J[Consider other ANA-positive diseases]:::outcome G --> K{Low C3/C4?}:::decision K -->|Yes| L[Active disease/Nephritis risk]:::outcome ``` ### Why Anti-dsDNA + Complement is the Answer 1. **Specificity:** Anti-dsDNA is the most specific antibody for SLE (>95%) 2. **Classification criteria:** Meets ACR/EULAR 2019 criteria for SLE diagnosis 3. **Prognostic value:** Predicts renal involvement and disease severity 4. **Complement assessment:** Identifies active disease and organ involvement risk **Mnemonic:** **SLAM** = Specific Lupus Antibodies Matter - Anti-dsDNA (double-stranded) - Anti-Sm (Smith antigen) - Complement depletion [cite:Harrison 21e Ch 312]