A 26-year-old man is rescued from under collapsed debris with crush injury to both lower limbs. He passes dark brown urine. Urine dipstick tests strongly positive for blood, but urine microscopy shows fewer than 3 red cells per high power field. The dipstick reagent strip reacts with which substance?

• A Haemoglobin released by intravascular haemolysis
• B Myoglobin liberated from damaged muscle ✓
• C Conjugated bilirubin excreted in urine
• D Porphobilinogen from acute intermittent porphyria

Explanation

The dipstick heme reaction detects any peroxidase-active heme pigment, including free myoglobin. In rhabdomyolysis, massive myoglobin release produces tea-coloured urine that is heme positive with an inactive sediment, because myoglobin is small enough to pass the glomerulus without forming cells. The same dissociation occurs in haemoglobinuria, but the crush injury context makes myoglobin the correct source here.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.