A 28-year-old man trapped under collapsed masonry for six hours presents with tea-coloured urine. Serum creatinine is 3.4 mg/dL, potassium 6.2 mEq/L, calcium 7.6 mg/dL, phosphorus 7.8 mg/dL, and creatine kinase 46,000 U/L. Urine dipstick tests strongly positive for blood, but urine microscopy shows fewer than 5 red cells per high power field. The dipstick positivity is explained by:
- A Myoglobinuria ✓
- B Haemoglobinuria from intravascular haemolysis
- C Contamination with menstrual blood
- D False positive reaction due to concentrated urine
Correct answer: A. Myoglobinuria
Explanation
The heme portion of myoglobin reacts with the dipstick orthotolidine reagent just like hemoglobin, so crushed muscle releasing myoglobin produces a strongly blood-positive dipstick with few or no red cells on microscopy. This dissociation between dipstick and microscopy is the classic bedside clue to pigment nephropathy from rhabdomyolysis. Haemoglobinuria from hemolysis also gives this pattern, but the crush injury and markedly elevated creatine kinase point to myoglobin.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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