When the body digests protein ammonia is produced as a waste product. If you can imagine the ammonia you buy at the store, it is a very caustic substance that definitely should not be coursing around the body. So, the liver quickly absorbs it and converts it to urea, a less toxic waste product. The urea is released into the blood and then filtered out by the kidney and concentrated to make urine. BUN, then, is often used to check kidney function.
Increased BUN may be caused by:
-Kidney disease: BUN will build up in the blood if the kidneys are not filtering it out properly.
-Decreased GFR: or glomerular filtration rate. This means the
kidney is filtering the blood more slowly than usual. This allows
substances such as BUN to be re-absorbed at a greater rate than they
would normally be, resulting in higher than usual concentrations of BUN
in the blood.
-High protein diets: more protein equals more waste. Because this test can be influenced by an animal's diet it isn't reliable on its own. This is why BUN and Creatinine are often looked at together.
-Tissue damage: when the body's own proteins are damaged or destroyed by fever or necrosis it digests them. Large amounts of damage may increase the BUN.
Decreased BUN may be caused by:
-Low protein diet: less protein means less BUN produced. Because this test can be influenced by an animal's diet it isn't
reliable on its own. This is why BUN and Creatinine are often looked at
together.
-Liver disease: if the liver is not functioning it won't produce the urea that results in BUN. Of course, if this is suspected ammonia should be looked at as well. If it isn't being converted to BUN, than it should be building up in the blood.
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