BUN

Blood Urea Nitrogen (BUN)

Indications for performing this test: This test is part of the basic serum chemistry panel, and is used with creatinine as a useful indicator of renal function. BUN is also used as an indirect indicator of hepatic function.

    Urea is a byproduct of protein catabolism and is filtered from the blood by the kidneys. The urea concentration in the glomerular filtrate is the same as is found in the blood. Urea is simply filtered from the blood in a process that does not require energy. In the normal animal, about 30 percent of the BUN is then reabsorbed as it travels through the renal tubules. When the filtration apparatus fails for any reason, we see elevated levels of blood urea nitrogen. Elevations in the blood of nitrogen containing products is known as azotemia.

    In addition to standard laboratory analysis of BUN, BUN may be measured using reagent strips that estimate BUN concentration through the use of urease. It may also be measured with a chromatographic strip which produces a color change upon the release of ammonia. This method is preferred as it is more quantitative.

Azotemia is categorized in the following ways:

Prerenal azotemia - Some causes include: increased protein catabolism (high protein diet), corticosteroids, necrosis, starvation, prolonged exercise, infection, fever, or decreased renal perfusion due to shock, dehydration or cardiovascular disease.

Renal azotemia - Occurs after about 3/4 of the nephrons are nonfunctional. Renal azotemia can occur in both acute renal failure (AFR) and chronic renal failure (CRF).

Postrenal azotemia - Usually due to obstruction or postrenal leakage. May be differentiated due to the accompanying clinical signs of oliguria or anuria, though these signs can also occur with renal azotemia.

Important Species Differences

In the horse, BUN levels are usually not markedly elevated in renal failure because of intestinal excretion of urea.

In the ruminant, BUN levels do not increase proportionally to creatinine because of rumen excretion of urea.

Significance of Low BUN

Hepatic insufficiency would result in decreased levels of urea being produced by the liver and introduced into general circulation.

Low protein diets, and decreased protein catabolism may result in decreased BUN.

Anabolic steroids may also cause decreased levels of BUN.

For more information . . .

Veterinary Laboratory Medicine, Duncan, Prasse, and Mahaffey page 177-179.
Veterinary Laboratory Medicine, Interpretation & Diagnosis. Meyer & Harvey, pages 331.