Blood urea nitrogen (BUN)
Chemistry ·Also called: BUN, urea, urea nitrogen, blood urea
What it is
When protein is broken down, ammonia is produced and the liver converts it to urea, which travels to the kidneys and leaves in the urine. BUN measures how much is circulating at the moment of the blood draw.
Why your vet measures it
BUN rises when kidneys are not filtering well — but it also rises simply from dehydration, a high-protein meal, or blood in the digestive tract being digested as protein. Read together with creatinine, SDMA and the urine concentration, it becomes a reliable part of the kidney picture rather than a false alarm.
What a high result can mean
- Dehydration, which reduces blood flow through the kidneys
- Kidney disease, acute or chronic
- An obstruction to urine flow, such as a blocked bladder — a life-threatening emergency in male cats
- Bleeding into the stomach or intestine, digested as protein
- A recent high-protein meal, or a very high-protein diet
- Heart failure or shock reducing blood supply to the kidneys
What a low result can mean
- Liver failure or a portosystemic shunt, where the liver cannot make urea
- A low-protein diet
- Over-hydration from intravenous fluids
- Late pregnancy
Dogs and cats differ
A blocked bladder in a male cat sends BUN and potassium up together within hours and is immediately life-threatening. In dogs, a raised BUN with a normal creatinine more often turns out to be dehydration, diet, or gut bleeding.
Questions worth asking your vet
- Is the creatinine raised too, or is the BUN up on its own?
- What did the urine concentration show — was my pet's urine dilute?
- Could dehydration or a recent meal explain this?
Related terms
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