What urinalysis findings are indicative of rhabdomyolysis?

Urinalysis Findings Indicative of Rhabdomyolysis

The most important urinalysis finding indicative of rhabdomyolysis is red-brown or tea-colored urine that tests positive for blood on dipstick but shows few or no red blood cells under microscopic examination, indicating myoglobinuria.

Key Urinalysis Findings

Primary Indicators

• Urine Color: Red to brown or tea-colored appearance
• Dipstick Results:
  • Positive for blood/hemoglobin without corresponding RBCs on microscopy
  • Positive orthotolidine reaction (blood reagent strip) without visible RBCs
• Microscopic Examination:
  • Few or absent red blood cells despite positive dipstick for blood
  • Absence of RBC casts

Secondary Findings

• Urine Myoglobin: Directly detectable in specialized tests
• Urine pH: Often acidic
• Specific Gravity: May be elevated
• Protein: May show mild to moderate proteinuria
• Sediment: May contain granular casts

Clinical Context and Interpretation

The discrepancy between a positive dipstick test for blood and the absence of RBCs on microscopy is the hallmark laboratory finding of myoglobinuria 1, 2. This occurs because standard dipstick tests cannot differentiate between hemoglobin and myoglobin, both of which contain heme groups that catalyze the peroxidase reaction on the dipstick 3.

When interpreting urinalysis results for suspected rhabdomyolysis:

1. Confirm the discrepancy: Positive dipstick for blood + minimal/absent RBCs on microscopy
2. Assess urine color: Red-brown or tea-colored urine strongly suggests myoglobinuria
3. Consider clinical context: Symptoms of muscle pain, weakness, or history of potential causes (trauma, excessive exercise, medications)

Supporting Laboratory Tests

While not part of the urinalysis itself, these additional tests should be ordered when rhabdomyolysis is suspected:

• Serum Creatine Kinase (CK): Markedly elevated (typically >5-10 times upper limit of normal)
• Serum Electrolytes: Check for hyperkalemia, hypocalcemia, hyperphosphatemia
• Kidney Function Tests: BUN and creatinine to assess for acute kidney injury
• Serum Myoglobin: May be elevated but clears more rapidly than CK

Common Pitfalls in Diagnosis

1. False Negatives: Myoglobin may clear rapidly from urine, especially with aggressive hydration
2. Misinterpretation: Confusing myoglobinuria with hematuria or hemoglobinuria
3. Delayed Testing: Urine samples collected after significant hydration may show negative results despite ongoing muscle damage
4. Relying solely on visual assessment: Tea-colored urine should always be tested with dipstick and microscopy

Clinical Applications

The presence of myoglobinuria on urinalysis should prompt immediate action to prevent acute kidney injury:

1. Aggressive IV fluid resuscitation: The cornerstone of treatment
2. Monitor kidney function: Serial BUN and creatinine measurements
3. Electrolyte monitoring: Particularly potassium levels due to risk of life-threatening hyperkalemia
4. Consider compartment syndrome: In traumatic cases

While the utility of urine myoglobin testing for predicting acute renal failure has been questioned due to limited evidence 4, the combination of typical urinalysis findings with elevated serum CK remains the standard approach to diagnosis.

Remember that dark-colored urine should always raise suspicion for rhabdomyolysis, especially in patients with risk factors such as trauma, excessive exercise, or certain medications 2.