A 19-year-old man after a crush injury passes dark brown urine. Urine dipstick tests positive for blood, but microscopy shows no erythrocytes. Serum creatinine is rising. The dipstick-microscopy discordance occurs because:
- A Myoglobin, like hemoglobin, is heme-containing and oxidizes the chromogen on the dipstick, but being freely filtered it appears without intact red cells ✓
- B Myoglobin is too small to be detected by the dipstick reagent
- C Rhabdomyolysis destroys the leukocyte esterase needed for the reaction
- D Myoglobin precipitates in tubules and masks the microscopic field
Correct answer: A. Myoglobin, like hemoglobin, is heme-containing and oxidizes the chromogen on the dipstick, but being freely filtered it appears without intact red cells
Explanation
The dipstick detects heme peroxidase activity, which both hemoglobin and myoglobin possess, so myoglobinuria gives a positive blood reading. Because myoglobin (17 kDa) is freely filtered and there are no erythrocytes in rhabdomyolysis, microscopy shows no red cells. Myoglobin is small enough to be readily filtered, ruling out option B. Leukocyte esterase is irrelevant, and precipitation would still leave free myoglobin in the urine to react.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.