A 28-year-old man trapped under rubble for 6 hours after a building collapse develops dark brown urine. Urine dipstick tests strongly positive for blood, but microscopy shows fewer than 3 red cells per high power field. Serum creatinine is 3.4 mg/dL and creatine kinase is 42,000 U/L. The dipstick positivity despite absent red cells is best explained by which mechanism?

• A Haemoglobin released from intravascular haemolysis reacts with the dipstick reagent
• B Acute interstitial inflammation causes leakage of intact haem into the tubules
• C Myoglobin released from injured muscle is filtered and oxidises the chromogen on the dipstick ✓
• D Concentrated urine produces a false positive reaction independent of any haem protein

Explanation

The urine dipstick uses a peroxidase-like reaction in which any haem-containing protein oxidises the chromogen. In rhabdomyolysis, myoglobin released from necrotic muscle is freely filtered, so the dipstick reads positive for blood while microscopy shows few or no red cells. This distinguishes myoglobinuria from true haematuria. Haemoglobinuria from intravascular haemolysis gives the same pattern, but the markedly elevated creatine kinase and crush injury here confirm rhabdomyolysis as the source.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.