A 28-year-old man is brought to the emergency department after being trapped under a collapsed wall for 6 hours. He has a swollen thigh. Urine output is minimal. Urinalysis shows 3+ reaction for blood on dipstick but microscopy reveals no red cells. Serum creatinine is 4.8 mg/dL, potassium 6.1 mEq/L, calcium 7.2 mg/dL, phosphate 7.9 mg/dL, and creatine kinase 48,000 IU/L. Which finding best explains the dipstick result?

• A The dipstick cross-reacts with the high concentration of serum creatine kinase
• B Lysed tubular epithelial cells release intracellular haemoglobin into the tubular lumen
• C Myoglobin released from injured muscle is filtered and reacts with the heme portion of the dipstick ✓
• D Hyperkalaemia alters the pH buffer of the dipstick producing a false positive

Explanation

In rhabdomyolysis, myoglobin released from necrotic muscle is freely filtered and its haem moiety gives a positive orthotolidine reaction on the urine dipstick, identical to haemoglobin and red cells. Because there is no bleeding in the urinary tract, microscopy shows no red cells, giving the classic dipstick positive, sediment negative picture. Haemoglobinuria requires intravascular haemolysis, which would show a fall in serum free haptoglobin and raised LDH, absent here.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.