BUN stands for Blood Urea Nitrogen. It is a marker of protein metabolism, kidney function, and hydration.
The BUN represents the circulating nitrogen from urea, a by-product of protein metabolism. Protein breakdown releases nitrogen as ammonia which the liver converts to urea and sends to the kidney for excretion in the urine. Measuring the BUN measures the nitrogen from urea but does not reflect the total urea in circulation which is a different biomarker.
The protein broken down into urea comes primarily from dietary protein but can also reflect protein from catabolic processes such as tissue breakdown, infection, and fever. An increasing BUN usually reflects the kidney's inability to clear the urea effectively though it may also reflect an acute catabolic process or dehydration which concentrates the urine.
A low BUN may be associated with liver failure, malnutrition, insufficient protein intake, overhydration, nephrotic syndrome, and the use of certain medications.
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