A 28-year-old woman of Indian origin presents with a 6-month history of photosensitive malar rash, oral ulcers, and polyarthralgia affecting the hands and knees. She reports morning stiffness lasting 2 hours. Laboratory investigations show ANA 1:640 (homogeneous pattern), anti-dsDNA antibodies positive (120 IU/mL), anti-Smith antibodies positive, ESR 78 mm/hr, and normal complement levels (C3 and C4). Urinalysis shows 2+ proteinuria and occasional RBC casts. What is the most likely diagnosis and which single investigation would best assess her current renal involvement?

Explanation

## Diagnosis: Systemic Lupus Erythematosus **Key Point:** This patient meets ≥4 of the revised ACR/EULAR 2019 criteria for SLE: photosensitive malar rash, oral ulcers, polyarthritis (non-erosive), ANA positivity, anti-dsDNA and anti-Smith positivity, and lupus nephritis (proteinuria + RBC casts on urinalysis). **High-Yield:** Anti-Smith (anti-Sm) antibodies are **pathognomonic** for SLE and appear in ~30% of cases; anti-dsDNA is highly specific (~95%) and correlates with nephritis activity. The combination of both is virtually diagnostic. ### Assessment of Renal Involvement **Clinical Pearl:** Lupus nephritis is present in this patient (evidenced by proteinuria and RBC casts). The gold standard for quantifying proteinuria and assessing severity is **24-hour urine protein estimation**. | Investigation | Purpose | Interpretation | | --- | --- | --- | | 24-hour urine protein | Quantifies proteinuria; baseline for monitoring | >0.5 g/day suggests active nephritis | | Serum creatinine/eGFR | Assesses glomerular filtration rate | Reflects renal function, not activity | | Kidney biopsy | Defines histological class (I–VI) | Reserved for atypical presentations or treatment decisions | | Urinalysis | Screens for hematuria, casts | Does not quantify proteinuria | **Mnemonic: ANCA** — Anti-Neutrophil Cytoplasmic Antibodies (NOT relevant here; this is SLE, not vasculitis). **Warning:** Normal C3/C4 levels do NOT exclude lupus nephritis; low complement (especially C3 and C4) suggests active disease but is not required for diagnosis. This patient's normal complement may reflect stable disease despite proteinuria. ## Why 24-Hour Urine Protein? 1. **Quantifies proteinuria** — essential for grading severity and monitoring response to immunosuppression. 2. **Baseline for follow-up** — allows serial comparison to detect progression or remission. 3. **Prognostic value** — >3 g/day at presentation predicts worse renal outcomes. [cite:Harrison 21e Ch 319]