Most of the urea produced by the body is excreted through the kidneys. Reduced glomerular filtration rate increases serum urea (blood urea nitrogen or BUN), but BUN is also affected by many non-renal factors. Increased dietary protein, upper gastrointestinal bleeding, and increased protein catabolism may increase BUN, while low protein intake and reduced hepatic function may reduce it.  BUN is also affected by tubular flow rate, as such hydration status is relevant to interpretation.  Patients may experience significant reduction in functional nephron number and GFR (>75%) before serum urea is significantly increased. Reductions in GFR from prerenal, renal, or postrenal causes cannot be distinguished as all may increase serum urea.

Major differentials for increased BUN include reduced renal perfusion (dehydration, shock, cardiovascular disease, hypoadrenocorticism), renal  disease, and non-renal causes including urinary tract obstruction or rupture, excess dietary protein, gastrointestinal haemorrhage, or a catabolic state. Major differentials for decrease BUN include hepatic dysfunction/shunt, protein malnutrition/malabsorption, fluid therapy, and polydipsia.


All species (but less useful in birds/reptiles)


Plasma or Serum


Gel,  plain or lithium heparin tube

Collection protocol:

Fasted sample preferred.


Reference(s): Stockham, S.L and Scott, M.A. Fundamentals of Veterinary Clinical Pathology 2nd Edition 2008. Thrall M.A. Veterinary Hematology and Clinical Chemistry 2006