The serum (or blood) urea nitrogen level and the serum creatinine levels are both indicators of kidney function. Both of these values tend to rise when the kidneys are not functioning efficiently.
The urea nitrogen level is less specific to kidney damage than the creatinine level. When kidney disease is present the BUN (Blood Urea Nitrogen) tends to rise faster than the creatinine. It is sometimes considered to be a more sensitive indicator of kidney function, as long as other causes for its rise are not present.
Creatinine is more specific for kidney disease, these levels rise when dogs are fed diets high in cooked meat (indicating damage to the kidneys). Over time, the rise in creatinine levels is a little better indicator for how glomerular damage is progressing than the BUN.
A chronic slow rise in the creatinine is an indication of ongoing damage.
For both creatinine and BUN, it is important to look at other indicators of how the body is doing. If the patient is normally hydrated but has large quantities of protein in the urine, then the glomerular disease might be present even if the BUN and creatinine are not very high. The BUN and creatinine are important indicators of kidney function, but they have to be considered based on the patient’s overall condition and potential for diseases other than a kidney disease.