Blood Urea Nitrogen (BUN)
A metabolic waste product of dietary protein digestion and liver metabolism, excreted primarily by the kidneys.
💬 In Plain English
BUN (Blood Urea Nitrogen) is a waste product created when the liver breaks down dietary proteins into urea. The kidneys filter this urea out into the urine. If kidney filtration slows down, BUN accumulates in the blood. However, unlike creatinine, BUN is also influenced by other factors outside the kidneys: eating a high-protein meal, dehydration, or bleeding in the upper stomach (digesting blood proteins) will cause BUN to spike even when the kidneys are healthy.
🔬 Clinical & Technical Depth
For Vets & TechniciansFormal Clinical Definition
Nitrogenous waste product synthesized by the hepatic urea cycle from ammonia derived from amino acid catabolism. Filtered by renal glomeruli and passively reabsorbed in renal tubules proportionally to tubular fluid flow rates.
Why Veterinarians Use It
Veterinarians assess BUN in conjunction with creatinine to calculate the BUN:creatinine ratio and distinguish pre-renal azotemia (dehydration) from intrinsic renal failure.
Clinical Context Encountered
Found on all routine blood chemistry profiles alongside creatinine and electrolytes.
Clinical Reference Guide (Diagnostic Test)
- •Extremely high BUN (>100–150 mg/dL) leads to clinical uremia: painful oral ulcers, severe vomiting, hypothermia, and uremic breath.
❓ Questions to Ask Your Veterinarian
Take these actionable questions with you to your next clinic visit or appointment:
- 1Is the elevated BUN caused by dehydration (pre-renal) or true kidney dysfunction (renal)?
- 2Is creatinine elevated alongside BUN?
- 3Could a recent high-protein meal or upper gastrointestinal bleeding explain a high BUN?
Academic & Clinical References
- ASVCP Guidelines on Renal Biomarkers and Azotemia EvaluationOrganization: American Society for Veterinary Clinical Pathology (ASVCP) • Accessed: 2026-01-10https://www.asvcp.org