A 42-year-old woman with systemic lupus erythematosus presents with acute kidney injury. Serum creatinine 2.8 mg/dL, urine protein 3.5 g/day, and active urinary sediment with RBC casts. Which finding most reliably distinguishes acute glomerulonephritis from acute tubular necrosis as the cause of her AKI?

## Distinguishing Acute Glomerulonephritis from ATN **Key Point:** The presence of **RBC casts and dysmorphic RBCs** is the pathognomonic urinary finding that distinguishes acute glomerulonephritis (AGN) from ATN. These indicate glomerular injury with red cell passage through damaged basement membrane. ### Urinary Findings Comparison | Feature | Acute Glomerulonephritis | ATN | |---------|-------------------------|-----| | **RBC casts** | Present (hallmark) | Absent | | **Dysmorphic RBCs** | Present | Absent | | **WBC casts** | May be present | Absent | | **Muddy brown casts** | Absent | Present (hallmark) | | **Proteinuria** | 1–3 g/day (nephritic) | <1 g/day | | **Hematuria** | Gross or microscopic | Absent | | **FENa** | Variable (often <1%) | >2% | **High-Yield:** RBC casts = glomerular disease until proven otherwise. They form when RBCs are trapped in Tamm-Horsfall protein within the tubular lumen—evidence of glomerular bleeding, not tubular necrosis. ### Mechanism of RBC Cast Formation 1. Glomerular basement membrane damage (from immune complex deposition, ANCA, anti-GBM antibodies, etc.) 2. RBCs leak into Bowman's space 3. RBCs are trapped in Tamm-Horsfall protein (uromodulin) in the tubular lumen 4. RBC casts form and are excreted in urine **Clinical Pearl:** In SLE-related acute glomerulonephritis (Class III or IV lupus nephritis), the combination of RBC casts, proteinuria 1–3 g/day, and active urinary sediment is diagnostic of glomerular disease. The presence of anti-dsDNA antibodies and low C3/C4 would further confirm lupus nephritis. **Mnemonic:** **GLOM** = **G**ranular (immune complexes), **L**ow complement, **O**liguria, **M**orphology (RBC casts). This triad of findings points to acute glomerulonephritis. **Warning:** Do not confuse dysmorphic RBCs (irregular, fragmented) with isomorphic RBCs (uniform, from lower urinary tract bleeding). Dysmorphic RBCs indicate glomerular origin.