Urinalysis Findings in Rhabdomyolysis
In rhabdomyolysis, urinalysis is characteristically ABNORMAL, not normal—it typically shows brown or tea-colored urine that is positive for blood on dipstick but has few or no red blood cells on microscopy, indicating myoglobinuria. 1
Key Urinalysis Abnormalities in Rhabdomyolysis
Classic Dipstick Findings
- Positive blood on dipstick WITHOUT red blood cells on microscopy is the hallmark finding that indicates myoglobinuria 1
- The urine appears brown, cloudy, or tea-colored due to myoglobin pigment 1
- This discrepancy occurs because the dipstick detects heme (from myoglobin) but microscopy reveals rare or absent RBCs 2
Microscopic Examination
- Rare or absent red blood cells despite positive blood on dipstick 2
- Myoglobin casts may be visible on microscopy in severe cases with acute kidney injury 2
- The presence of myoglobin casts on kidney biopsy confirms rhabdomyolysis-induced acute tubular injury 2
Important Clinical Caveats
When Urinalysis May Appear "Normal"
There is no truly normal urinalysis in active rhabdomyolysis. However, the urinalysis findings can be misleading in certain situations:
- Early presentation: Myoglobin has a short half-life and may be cleared rapidly, potentially resulting in a negative dipstick if the patient presents after the initial muscle injury 3
- Trace blood on dipstick: In some cases, only trace amounts of blood may be detected, which could be misinterpreted as insignificant 2
- Adequate hydration: With aggressive fluid resuscitation, myoglobin may be diluted, making detection more difficult 4
Critical Diagnostic Pitfall
Do not rely solely on urinalysis to diagnose or exclude rhabdomyolysis. A case report documented severe rhabdomyolysis with acute kidney injury where urinalysis showed only trace blood with rare RBCs, yet kidney biopsy revealed myoglobin casts and severe acute tubular injury 2. This demonstrates that CPK and clinical presentation must guide diagnosis, not urinalysis alone.
Diagnostic Approach
Primary Diagnostic Markers
- Creatine kinase (CK) elevation ≥5-10 times the upper limit of normal is the primary diagnostic criterion 4, 3
- Serial CK measurements should be obtained every 6-12 hours initially in severe cases 5
- Serum myoglobin can be detected earlier than CK but has a shorter half-life 5
Supporting Laboratory Tests
- Complete electrolyte panel to assess for hyperkalemia, hypocalcemia, and hyperphosphatemia 1, 5
- Renal function tests (BUN and creatinine) to monitor for acute kidney injury 5
- Arterial blood gas to assess for metabolic acidosis 1, 5
- ECG and cardiac troponin in severe cases to rule out cardiac involvement 1, 5