What does the urine dipstick test look like in rhabdomyolysis (muscle breakdown disease)?

Urine Dipstick Findings in Rhabdomyolysis

In rhabdomyolysis, urine dipstick typically shows positive blood (hemoglobin) with absent red blood cells on microscopy, which indicates myoglobinuria rather than true hematuria. However, this classic finding is present in only 41-55% of cases, making it an inadequate screening tool by itself 1.

Key Urinalysis Findings in Rhabdomyolysis

• Blood/Hemoglobin: Positive on dipstick in approximately 86% of rhabdomyolysis cases due to cross-reactivity with myoglobin 1
• Microscopy: Absence or minimal presence of RBCs despite positive dipstick for blood (the classic finding) 1
• Color: Dark, tea-colored or cola-colored urine due to myoglobinuria 2
• Protein: May show proteinuria due to myoglobin excretion 3
• pH: Often acidic; monitoring urine pH is important as maintaining pH ≥6.5 helps prevent myoglobin precipitation in renal tubules 3

Diagnostic Performance

• The sensitivity of the classic finding (positive dipstick for blood with negative microscopy for RBCs) is only 41% (95% CI, 35%-47%) for all rhabdomyolysis cases 1
• Even in severe rhabdomyolysis (CPK ≥10,000 IU/L), sensitivity only increases to 55% (95% CI, 43%-67%) 1
• Broadening the definition of negative microscopy from 0-3 RBCs to <10 RBCs increases sensitivity to 79% (95% CI, 73%-83%) 1

Additional Laboratory Findings

• Myoglobin: Quantitative urine myoglobin testing is more specific but less readily available than dipstick testing 4
• Creatine Kinase (CPK): Serum CPK is the gold standard for diagnosis of rhabdomyolysis, with levels typically >1,000 IU/L and often >5,000 IU/L 5
• Ketones: May be positive in some cases of rhabdomyolysis associated with metabolic disorders 3, 6

Clinical Implications

• A negative dipstick for blood does not rule out rhabdomyolysis (false negative rate ~14%) 1
• The absence of the classic finding (positive dipstick with negative microscopy) should not be used to exclude the diagnosis 1
• Urinalysis results can predict the absence of significant myoglobinuria when completely negative for blood 4
• Monitoring urine output and pH is crucial for preventing acute kidney injury in rhabdomyolysis patients 3

Prevention of Complications

• Bladder catheterization for monitoring hourly urine output is recommended in severe cases 3
• Maintaining urine pH ≥6.5 helps prevent myoglobin precipitation in renal tubules 3
• Early aggressive fluid resuscitation (3-6L/day for moderate cases, >6L for severe cases with CPK >15,000 IU/L) is essential to prevent acute kidney injury 3
• Testing for blood in urine can help detect evidence of rhabdomyolysis and potential for renal injury 3

Important Caveats

• Urine dipstick alone is inadequate for screening for rhabdomyolysis due to limited sensitivity 1
• Diagnosis should be confirmed with serum CPK levels, which are more reliable 5
• False-positive results for blood on dipstick can occur with highly colored urine 3
• Myoglobinuria may be intermittent, especially if tested late in the course of illness 2