Equis ISSN 2398-2977

Blood: biochemistry - urea

Synonym(s): Blood urea nitrogen, BUN

Contributor(s): Annalisa Barrelet, Kathleen P Freeman, Frank Taylor


  • Produced in liver from ammonia derived from tissue/dietary protein and excreted by kidneys.
  • Important marker of kidney function and protein metabolism.
  • Quantity of urea in blood may be expressed as entire urea molecule (urea) or nitrogen portion (BUN - most common in USA).


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  • Urease (enzymatic method) most specific. No interference from free blood ammonia.
  • Typically measured as (total) blood urea nitrogen (BUN).
  • Blood strips can be used to get a rapid result. Do not give precise reading but will show if levels are significantly elevated.


  • Widely available.



Increases in BUN do not occur until the majority of nephrons (about 75%) are non-functional. Complete loss of one kidney (provided that the other is fully functional) will not produce azotemia. Therefore, not very useful in evaluating early or minor changes in GFR


  • Relatively non-specific. Alterations may occur with pre-renal, renal or post-renal conditions or diseases affecting other organs or systems.

Technique (intrinsic) limitations

  • Only significant in conjunction with other laboratory and clinical findings.
    Most useful to assess in conjunction with a urine specific gravity   Urine: specific gravity 

Result Data

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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Ogilvie G K, Engelking L R & Anwer M S (1985) Effects of plasma sample storage on blood ammonia, bilirubin and urea nitrogen concentrations - cats and horses. Am J Vet Res 46 (12), 2619-2622.

Other sources of information

  • Kaneko J J, Harvey J W & Bruss M L (1997) Eds. Clinical Biochemistry of Domestic Animals. 5th edn. Academic Press, USA.


  • Veterinary commercial laboratories, State diagnostic laboratories or Veterinary College laboratories.