A 28-year-old man trapped under collapsed masonry for 6 hours presents with a crushed right lower limb, dark brown urine, serum potassium 6.2 mEq/L, and creatinine 3.1 mg/dL. Urine dipstick tests strongly positive for blood, but microscopy shows fewer than 5 red blood cells per high power field. What explains the dipstick result?
- A Myoglobin excreted in urine reacts with the haemoglobin pad ✓
- B Haematuria from traumatic bladder injury
- C Concentrated urobilinogen giving a false colour reaction
- D Ascorbic acid interference producing a false positive
Correct answer: A. Myoglobin excreted in urine reacts with the haemoglobin pad
Explanation
The dipstick haemoglobin pad uses a pseudoperoxidase reaction that detects the haem moiety, so both haemoglobin and myoglobin give a positive result. In rhabdomyolysis, plasma myoglobin filtered at the glomerulus colours the urine while microscopy shows no red cells, a combination highly suggestive of pigment nephropathy. Ascorbic acid causes false negatives, not false positives, which eliminates that distractor.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.