A patient with AKI due to rhabdomyolysis has a creatinine of 4.8 mg/dL on day 3. Urine dipstick is strongly positive for blood but microscopy shows no red blood cells. The mechanism responsible for this discordant finding is:

• A Haematuria with immediate lysis of red blood cells in the tubules
• B Haemoglobinuria from concomitant intravascular haemolysis
• C Dipstick detects only free myoglobin, not haemoglobin
• D Myoglobin cross-reacts with the orthotoluidine-based peroxidase dipstick reagent ✓

Explanation

The urinary dipstick detects haem using an orthotoluidine peroxidase-based reaction that cannot distinguish between haemoglobin and myoglobin. In rhabdomyolysis, myoglobin released from damaged muscle is freely filtered by the glomerulus and appears in urine, producing a strongly positive dipstick for blood despite the absence of red blood cells on microscopy. This pseudohaematuria pattern (dipstick positive, microscopy negative) is the hallmark of myoglobinuria/haemoglobinuria and is clinically important for distinguishing rhabdomyolysis from nephritis.

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

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