A 24-year-old man is brought to the emergency department after being extricated from under a collapsed wall. He has crush injuries to both lower limbs. Urine is dark brown. Urine dipstick tests strongly positive for blood, but microscopy shows no red blood cells. Creatinine is rising rapidly. Which finding explains the dipstick-microscopy discrepancy?
- A Myoglobin released from injured muscle is detected by the haem portion of the dipstick ✓
- B Haemoglobin filtered freely because glomerular basement membrane damage
- C False positive reaction caused by concentrated uroerythrin in oliguric urine
- D Red cell lysis in dilute alkaline urine releasing free haemoglobin
Explanation
The urine dipstick detects the peroxidase activity of the haem moiety, so it cannot distinguish haemoglobin from myoglobin. In rhabdomyolysis, myoglobin filtered by the glomerulus gives a strongly positive dipstick while microscopy shows no red cells, because the pigment is dissolved rather than cellular. This pigment nephropathy precipitates in tubules and causes ATN. Haemoglobin from intravascular haemolysis would give the same picture, but the clinical context of crush injury points to muscle injury, and free haemoglobin would also appear without red cells only after intravascular lysis.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.