A 42-year-old woman is admitted with acute kidney injury following rhabdomyolysis from a crush injury. Serum myoglobin is elevated, and urinalysis shows dark brown urine with positive dipstick for blood but no RBCs on microscopy. Which investigation is most appropriate to confirm myoglobin-induced acute tubular necrosis and assess severity?

Explanation

## Diagnosis of Myoglobin-Induced ATN ### Urine Myoglobin: The Specific Marker **Key Point:** Urine myoglobin level and 24-hour urine myoglobin excretion are the most appropriate investigations to confirm myoglobin-induced ATN and quantify the burden of myoglobinuria. ### Pathophysiology of Myoglobin-Induced ATN ```mermaid flowchart TD A[Rhabdomyolysis]:::outcome --> B[Myoglobin release]:::outcome B --> C[Filtered at glomerulus]:::action C --> D[Myoglobin precipitates in tubules]:::action D --> E[Tubular obstruction + oxidative injury]:::action E --> F[Acute Tubular Necrosis]:::outcome G[Acidic urine pH < 5.6]:::decision --> H[Worsens precipitation]:::urgent H --> I[Increased ATN severity]:::urgent ``` ### Diagnostic Features of Myoglobinuria | Feature | Finding | Interpretation | | --- | --- | --- | | Urine color | Dark brown/cola-colored | Myoglobin pigment | | Dipstick blood | Positive | Heme moiety of myoglobin | | RBC microscopy | Absent/few | Confirms myoglobinuria, not hematuria | | Urine myoglobin | >100 mg/L (positive) | Diagnostic for myoglobinuria | | 24-hour excretion | >5 g/24 h | Indicates severe rhabdomyolysis | | Serum myoglobin | >1000 ng/mL | Correlates with ATN risk | **High-Yield:** The classic triad for myoglobin-induced ATN: 1. **Positive urine dipstick** (blood) + **negative RBC microscopy** = myoglobinuria 2. **Dark brown urine** (cola-colored) 3. **Elevated serum CK** (>1000 IU/L, often >5000 IU/L) ### Why Urine Myoglobin is Definitive **Clinical Pearl:** Urine myoglobin measurement: - **Confirms** the presence and magnitude of myoglobinuria - **Quantifies** 24-hour excretion to assess severity and ATN risk - **Guides** aggressive hydration strategy (target urine output 200–300 mL/h to prevent precipitation) - **Predicts** need for dialysis and recovery timeline ### Specimen Collection Urine myoglobin should be measured: - From the first morning void or random spot urine (rapid assay) - As 24-hour collection for quantification - Early in presentation (myoglobin clears within 24–48 hours) **Mnemonic:** **MYOGLOBIN** = **M**yoglobinuria **Y**ields **O**ptimal **G**uidance for **L**iquid **O**verload **B**ased on **I**njury **N**eed