Blood urea nitrogen
Urea production and excretion do not occur at a constant rate. While renal dysfunction can cause increased BUN concentration,
nonrenal causes also often result in increased BUN concentration. In the liver, BUN is a byproduct of the urea cycle and protein
catabolism. Urea production and excretion increase after a high-protein meal, so an eight- to 12-hour fast is recommended
before measuring BUN concentration to avoid the effect of feeding on urea production. Clinical conditions characterized by
increased catabolism (e.g. starvation, infection, fever) also can increase BUN concentrations.
Gastrointestinal (GI) hemorrhage may also increase BUN concentrations because blood is an endogenous protein source. In 52
dogs with hematemesis, melena, or both, BUN concentrations and BUN:creatinine ratios were significantly higher than in age-matched
control dogs, suggesting that GI hemorrhage contributes to increased BUN concentrations in dogs as a consequence of increased
GI absorption of nitrogenous compounds.9
BUN concentrations may be increased by prerenal factors such as dehydration, which increases urine volume, and some drugs
that increase tissue catabolism (e.g. glucocorticoids, azathioprine) or decrease protein synthesis (e.g. tetracyclines), but these effects are usually minimal.9
In contrast, BUN concentrations may decline in patients with portosystemic shunts or hepatic failure and those receiving low-protein
diets. Reduced BUN concentrations may be an undesirable finding in patients with CKD because this may indicate protein calorie
malnutrition from inadequate protein intake as a consequence of improperly formulated diets or patients not consuming adequate
amounts of food. Because many extrarenal factors influence the BUN concentration, creatinine concentrations are often used
as a more reliable measure of GFR in patients with CKD.
BUN:creatinine ratio
BUN concentrations should be interpreted with knowledge of simultaneously obtained serum creatinine concentrations, particularly
in patients consuming reduced-protein diets. The ratio of BUN to serum creatinine concentration should decline to the lower
end of the reference range when dietary protein intake is reduced (usually to around 10 to 15; reference range = 7 to 37).9 In patients consuming reduced-protein diets, an increased ratio may suggest poor dietary compliance, enhanced protein catabolism,
GI hemorrhage, dehydration, anorexia, or declining muscle mass.2
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