Red-brown urine in a cat with aortic thromboembolism
10-year-old domestic shorthair cat with hypertrophic cardiomyopathy and aortic thromboembolism of 30 hours' duration; the hind limbs are rigid and painful. Red-brown urine drains from the urinary catheter. The centrifuged plasma is clear, with no pink tinge.
| Parameter | Result | Reference |
|---|---|---|
| Haematocrit | 38% | 30–45 |
| CK | 92,000 U/L | 50–450 |
| Potassium | 6.9 mmol/L | 3.5–5.1 |
| Urine specific gravity | 1.038 | — |
| Blood (dipstick) | 3+ | Negative |
| Red cells in sediment | 0–2/hpf | < 5/hpf |
What is the most likely cause of the urine colour?
chevron_right Show answer and explanation
A. Haemoglobinuria from intravascular haemolysis
For haemoglobin to reach the urine it must saturate haptoglobin, and the plasma then looks pink or red. With clear plasma and a normal haematocrit, haemolysis is very unlikely.
B. Haematuria with lysis of red cells within the bladder
Lysis of red cells in urine occurs mainly in very dilute or very alkaline urine; at a specific gravity of 1.038, red cells or ghosts would be seen in the sediment.
C. Bilirubinuria from cholestasis secondary to low cardiac output
Bilirubin colours urine yellow-orange and reacts on its own pad, not the blood pad. It does not explain a 3+ blood reaction without red cells.
D. Myoglobinuria from ischaemic and reperfusion rhabdomyolysis
Correct. The blood pad reacts with both haemoglobin and myoglobin. Myoglobin is small and barely protein-bound, so it is filtered rapidly and the plasma stays clear; the huge CK and hyperkalaemia confirm muscle necrosis.
lightbulb Key takeaway
Positive blood on the dipstick without red cells: look at the plasma. Pink plasma = haemoglobinuria; clear plasma with very high CK = myoglobinuria. In the cat with thromboembolism, reperfusion releases myoglobin and potassium: watch for hyperkalaemia and kidney injury.
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Updated: November 1, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.