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KIDNEY VALUES · DOGS

BUN in dogs: what blood urea nitrogen on your dog’s blood work means

BUN is one of the first kidney values on a dog’s chemistry panel, and one of the most sensitive to everyday factors like a meal or hydration. Here is how to read it, what makes it high or low and why it is always read with creatinine.

Written by the AI DiagAnimals editorial teamReviewed by the AI DiagAnimals TeamUpdated Our editorial policy

The essentials

BUN (blood urea nitrogen) measures urea, a waste product made by the liver from protein and removed by the kidneys. A high BUN can come from reduced kidney filtration, but also from dehydration, a recent meal or bleeding in the upper digestive tract. A low BUN can reflect a low-protein diet, youth or reduced liver function. It is always read with creatinine.

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What BUN measures

When the body breaks down protein, it produces ammonia. The liver turns this toxic compound into urea, which the kidneys filter into the urine. Part of the filtered urea is reabsorbed, more so when urine flows slowly through the kidneys, as in dehydration [1].

BUN therefore depends on three things: how much protein is broken down, how well the liver makes urea and how well the kidneys filter it [1]. That makes it useful, but less specific than creatinine or SDMA.

US laboratories report urea nitrogen (BUN) in mg/dL; European laboratories report urea in mmol/L or g/L. They describe the same measurement: BUN mg/dL × 0.357 = urea mmol/L [1]. In French, see our article on urea in cats and dogs.

How to read the BUN line

Here is an example line, followed by a converter for reports in other units.

An annotated line from a lab report · Illustrative example
AbbreviationResultUnitLaboratory intervalFlag
BUN Blood urea nitrogen38mg/dL8–28H
  1. Abbreviation BUN, UREA or urea nitrogen, depending on the laboratory.
  2. Result The concentration at the time of sampling. A recent meal or dehydration can change it [1, 2].
  3. Unit mg/dL here; 38 mg/dL ≈ 13.6 mmol/L of urea.
  4. Laboratory interval The example uses the dog interval of the Merck Veterinary Manual [3]. Your laboratory’s interval applies.
  5. Flag H: above the interval. Read it with creatinine before drawing any conclusion.

Unit converter

Equivalent values

  • Urea, mmol/L–
  • Urea, g/L–

Conversion only changes the unit. Compare the result with the interval printed by the laboratory that measured it.

Meals and BUN: what a recent study shows

In a 2023 study of 100 healthy dogs, food was withheld overnight and blood was sampled before and up to 8 hours after a normal meal. BUN changed by more than the analytical error allowed for the test in 66 dogs. By contrast, creatinine moved that much in only 2 dogs and SDMA in 5 [2].

The authors concluded that withholding food is generally not necessary for routine blood work in healthy dogs, but that a meal adds variability to BUN, phosphorus and triglycerides [2]. A BUN measured after eating should be interpreted with that in mind, and a recheck on an empty stomach can clarify a borderline result.

Why BUN can be high in a dog

Vets sort the causes into a few families, using the examination, the history, creatinine and the urinalysis [1].

  • Before the kidney (pre-renal). Dehydration or reduced blood flow to the kidneys lowers filtration.
  • The kidney itself. Acute or chronic kidney disease reduces filtration.
  • After the kidney (post-renal). A blocked or ruptured urinary tract prevents urine from leaving the body.
  • More urea produced. A protein-rich meal, bleeding in the stomach or upper intestine, fever or other causes of protein breakdown can raise BUN without kidney disease [1].

Why BUN can be low

  • A low-protein diet, such as some prescription diets [1].
  • Young age: puppies use protein to grow and have lower urea than adults [1, 4].
  • Increased urine production, which removes more urea [1].
  • Less often, reduced liver function, because the liver makes urea [1].

BUN and creatinine together: the ratio

Creatinine is also cleared by the kidneys but is barely affected by meals, and depends on muscle mass [5]. Comparing the two helps locate the cause [1]. Some reports print a BUN/creatinine ratio as a shortcut for this comparison; there is no universal normal ratio, and it is read with the clinical picture.

BUNCreatinineWhat it can point to [1]
↑NormalProtein-rich meal, upper digestive bleeding, early dehydration.
↑↑Reduced filtration: pre-renal, renal or post-renal.
Normal or ↓↑Very muscular dog, low-protein diet, reduced liver production of urea.

To stage chronic kidney disease, IRIS uses fasting creatinine and SDMA, not BUN. For dogs, stage 2 starts at a creatinine of 1.4 mg/dL (125 µmol/L) or an SDMA of 18 µg/dL in a stable, well-hydrated dog [6].

The values often read with BUN

SDMA

Rises earlier than creatinine and is little affected by meals [2].

Read the article

Creatinine

The classic filtration marker; see how it is read in cats, with the same principles.

Read the article

Urinalysis

Urine concentration tells whether a high BUN comes with kidneys that still concentrate urine.

See also creatinine in dogs, our guide to the CBC blood test for dogs and the blood work abbreviations.

Questions to ask your veterinarian

  1. Is the BUN high on its own, or with creatinine and SDMA?
  2. Had my dog eaten before the sample, and was it well hydrated?
  3. Does the urinalysis show that the kidneys still concentrate urine?
  4. Should we repeat the test on an empty stomach, and when?
  5. Is my dog’s current diet appropriate for these results?
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Frequently asked questions

What does a BUN blood test measure in dogs?

The amount of urea nitrogen in the blood. Urea is made by the liver from protein and removed by the kidneys, so BUN reflects protein intake and breakdown, liver function and kidney filtration together [1].

What BUN number means kidney failure in a dog?

There is no single BUN threshold. Kidney disease is assessed with creatinine, SDMA, the urinalysis and repeated tests; IRIS stages chronic kidney disease on creatinine and SDMA, not on BUN [6].

Which is worse, high BUN or high creatinine?

Neither is “worse” on its own. A high BUN with normal creatinine often has causes outside the kidneys, while both rising together suggest reduced filtration [1]. Your vet reads them together.

Can food raise BUN in dogs?

Yes. In a study of 100 healthy dogs, a normal meal changed BUN beyond the test’s allowable error in two dogs out of three, while creatinine was rarely affected [2].

How do you treat a high BUN in a dog?

Treatment targets the cause: fluids for dehydration, diet changes or treatment of kidney, digestive or urinary problems, depending on what your vet finds. The number itself is not treated.

Sources

  1. eClinPath, Cornell University College of Veterinary Medicine. Urea nitrogen. eclinpath.com/chemistry/kidney/urea-nitrogen
  2. Yi KC et al. Effect of withholding food versus feeding on creatinine, symmetric dimethylarginine, cholesterol, triglycerides, and other biochemical analytes in 100 healthy dogs. J Vet Intern Med. 2023;37(2):626-634. doi.org/10.1111/jvim.16630
  3. Merck Veterinary Manual. Serum biochemical analysis reference ranges. merckvetmanual.com/reference-values-and-conversion-tables/reference-guides/serum-biochemical-analysis-reference-ranges
  4. Montoya Navarrete AL et al. Effect of age, sex, and body size on the blood biochemistry and physiological constants of dogs from 4 wk to > 52 wk of age. BMC Vet Res. 2021;17:265. doi.org/10.1186/s12917-021-02976-w
  5. eClinPath, Cornell University College of Veterinary Medicine. Creatinine. eclinpath.com/chemistry/kidney/creatinine
  6. International Renal Interest Society (IRIS). IRIS Staging of CKD (modified 2026). iris-kidney.com/s/IRIS_staging_guidelines-2026.pdf

Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.

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