A 28-year-old man is brought after being trapped under rubble for 14 hours. He has a swollen thigh, serum creatinine 5.8 mg/dL, potassium 6.9 mEq/L, calcium 7.0 mg/dL, phosphate 8.4 mg/dL, and CK of 68,000 U/L. Urine dipstick tests strongly positive for blood but microscopy shows no red cells. The dipstick finding is explained by which substance?
- A Myoglobin released from injured muscle ✓
- B Haemoglobin from intravascular haemolysis
- C Concentrated uroerythrin in oliguric urine
- D False positivity due to ascorbic acid deficiency
Correct answer: A. Myoglobin released from injured muscle
Explanation
The heme portion of myoglobin reacts with the dipstick orthotolidine reagent exactly like haemoglobin, giving a positive blood test with no red cells on microscopy, the classic disconnection of pigment nephropathy. Crush injury releases myoglobin, which precipitates in tubules and causes ATN with the typical early hyperkalaemia, hyperphosphataemia, and hypocalcaemia. Haemolysis would show pink plasma rather than the brown pigmented granular casts of rhabdomyolysis.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.