A 28-year-old woman presents with a 6-week history of arthralgia, photosensitive malar rash, and oral ulcers. She also reports recent-onset dyspnea and pleuritic chest pain. Physical examination reveals a butterfly rash, mild fever, and bilateral pleural effusions on auscultation. Serum ANA is positive at 1:640 with a homogeneous pattern. Which investigation is most appropriate to confirm the diagnosis of SLE and assess for lupus nephritis?

Explanation

## Diagnostic Confirmation in SLE ### Role of Anti-dsDNA and Complement Levels **Key Point:** Anti-dsDNA antibodies are highly specific (95–98%) for SLE and are part of the ACR/EULAR classification criteria. Serum complement (C3, C4) depletion is a hallmark of active SLE, particularly lupus nephritis. **High-Yield:** Anti-dsDNA positivity + low complement (especially C3) = strong evidence of active lupus nephritis, which this patient is at risk for given pleuritic symptoms and dyspnea. ### Why Anti-dsDNA + Complement? 1. **Specificity**: Anti-dsDNA is far more specific than ANA (which is positive in many autoimmune conditions) 2. **Disease activity marker**: Complement depletion correlates with glomerulonephritis and systemic disease activity 3. **Prognostic value**: Low C3/C4 predicts renal involvement and guides immunosuppressive therapy intensity 4. **Classification criteria**: These antibodies are required in ACR/EULAR 2019 SLE classification criteria ### Complementary Investigations (Not First-Line) | Investigation | Timing | Purpose | |---|---|---| | Urinalysis + 24h proteinuria | After serology confirmed | Assess for lupus nephritis | | Renal biopsy | If proteinuria >0.5 g/day or rising creatinine | Histologic classification of glomerulonephritis | | Skin biopsy | If diagnosis unclear | Confirms cutaneous lupus (LE band test) | | Imaging (CXR, CT) | For organ involvement | Documents serositis, ILD | **Clinical Pearl:** A patient with new-onset pleurisy + positive ANA + anti-dsDNA + low complement has lupus until proven otherwise. Immediate urinalysis and 24-hour proteinuria should follow serology confirmation. **Mnemonic: SLED** — Serology (anti-dsDNA), Lupus activity, Evaluation of organs, Determine complement status.