A 32-year-old man presents to the emergency department 8 hours after a motor vehicle accident with a closed fracture of the right femur. On examination, he is alert, blood pressure 110/70 mmHg, heart rate 102/min, respiratory rate 18/min. The right thigh is swollen and tender. Urine dipstick shows 3+ blood but no RBCs on microscopy. Serum creatinine is 1.2 mg/dL (baseline 0.9 mg/dL). What is the most likely cause of the hematuria in this patient?

Explanation

## Clinical Diagnosis: Rhabdomyolysis-Induced Myoglobinuria **Key Point:** Massive muscle injury from long bone fractures (especially femur) releases myoglobin into the circulation, causing dark/cola-colored urine (myoglobinuria). Dipstick detects heme-like activity but microscopy shows no RBCs — this dissociation is pathognomonic for myoglobin. **High-Yield:** Rhabdomyolysis is a major complication of crush injuries and severe long bone fractures. The released myoglobin precipitates in renal tubules, causing acute tubular necrosis (ATN) and acute kidney injury (AKI). Early recognition and aggressive IV hydration are critical to prevent progression to oliguric renal failure. **Clinical Pearl:** In this case: - Closed femur fracture with significant soft tissue trauma → massive myoglobin release - Positive dipstick (heme activity) but negative RBC microscopy → myoglobin, not hemoglobin - Mild creatinine elevation (1.2) and normal urine output suggest early-stage rhabdomyolysis before severe AKI - Heart rate 102 reflects pain and early hypovolemia from muscle edema (third-spacing) **Mnemonic: CRASH** — Crush injury, Rhabdomyolysis, Acute kidney injury, Serum CK elevation, Hyperkalemia (from cell lysis) ### Pathophysiology ```mermaid flowchart TD A[Severe muscle trauma<br/>femur fracture]:::outcome --> B[Myoglobin release<br/>into circulation]:::outcome B --> C[Filtered at glomerulus]:::outcome C --> D[Precipitation in<br/>renal tubules]:::outcome D --> E[Tubular obstruction<br/>+ direct toxicity]:::outcome E --> F[Acute Tubular Necrosis]:::outcome F --> G[Acute Kidney Injury]:::urgent A --> H[Hyperkalemia<br/>from cell lysis]:::urgent H --> I[Cardiac arrhythmias]:::urgent ``` ### Diagnostic Clues | Feature | Myoglobinuria | Hemoglobinuria | RBC Hematuria | |---------|---------------|----------------|---------------| | Dipstick | Positive | Positive | Positive | | Microscopy RBCs | Absent/few | Absent | Present | | Urine color | Dark brown/cola | Red-brown | Red | | Clinical context | Crush/rhabdo | Hemolysis | Urinary tract injury | **Management priorities:** 1. Aggressive IV hydration (target urine output 200–300 mL/h) to prevent tubular precipitation 2. Urine alkalinization (sodium bicarbonate) to increase myoglobin solubility 3. Monitor serum CK (typically >5000 IU/L in significant rhabdo), potassium, and renal function 4. Early orthopedic fixation to limit ongoing muscle damage [cite:Rockwood & Green's Fractures in Adults Ch 1]