Blood Urea Nitrogen (BUN)
A blood test measuring the amount of urea nitrogen — a waste product of protein metabolism — in the bloodstream. Elevated BUN can indicate kidney dysfunction, dehydration, gastrointestinal bleeding, or high-protein diet, and is routinely used alongside creatinine and SDMA to assess renal function in dogs.
Blood urea nitrogen (BUN) measures the concentration of urea nitrogen in serum. Urea is generated in the liver as an end product of protein catabolism: amino acids are deaminated, the resulting ammonia is converted to urea via the urea cycle, and urea is transported by the bloodstream to the kidneys for filtration and excretion.
Normal Ranges and Interpretation
Normal canine BUN typically ranges from 7 to 27 mg/dL, though reference ranges vary by laboratory. BUN is influenced by many factors beyond kidney function, making it less specific than creatinine or SDMA as a standalone renal marker:
- Prerenal causes of elevated BUN: dehydration, heart failure reducing renal perfusion, high-protein diet, gastrointestinal bleeding (digested blood is a protein load), fever, burns, or any catabolic state
- Renal causes: acute kidney injury, chronic kidney disease, pyelonephritis, nephrotoxin exposure
- Postrenal causes: urinary obstruction, bladder rupture
- Low BUN: can occur with severe liver disease (reduced urea synthesis), very low protein diets, or overhydration
BUN:Creatinine Ratio
The ratio of BUN to creatinine helps differentiate causes of azotemia:
- Normal ratio (10:1 to 20:1): Suggests proportional elevation — typically renal in origin
- Elevated ratio (>20:1): Points to prerenal causes (dehydration, GI bleeding, high-protein intake) or early kidney disease where tubular urea reabsorption is disproportionately increased
- Low ratio (<10:1): May indicate liver dysfunction or low-protein diet
Role in Kidney Disease Monitoring
BUN is included in every standard serum chemistry panel and is part of routine monitoring for dogs with known kidney disease. Serial trending provides more clinical value than isolated measurements. A steadily rising BUN, even within reference range, signals declining renal function and warrants further investigation with urinalysis (urine specific gravity, protein:creatinine ratio) and SDMA.
Relevance to Longevity
Kidney disease is among the leading causes of morbidity in senior dogs. BUN, while less sensitive than SDMA for early detection, remains a practical and widely available marker. Its inclusion in routine screening protocols allows clinicians to detect renal trends over time. Combining BUN, creatinine, SDMA, and urinalysis in annual or biannual wellness panels maximizes the chance of catching kidney decline before clinical signs appear.