Urea Nitrogen

Urea Nitrogen, Blood (BUN)—also known as blood urea or carbamide—is used alongside creatinine to assess renal function and nitrogen balance. It supports the diagnosis and monitoring of acute kidney injury and chronic kidney disease, helps distinguish prerenal azotemia from intrinsic renal disease and postrenal obstruction, and is used to track the effectiveness of hemodialysis or other renal replacement therapy. Elevated BUN can reflect decreased renal perfusion (eg, shock), urinary tract obstruction, catabolic states, dehydration, gastrointestinal bleeding, high protein intake, or medication effects; an increase out of proportion to creatinine supports a prerenal etiology. Low BUN may be seen with overhydration, normal pregnancy, reduced protein intake, or impaired hepatic urea synthesis.

Urea is synthesized in the liver from ammonia released during protein catabolism, enters the circulation, and is delivered to the kidneys, where it is freely filtered at the glomerulus and excreted in urine. Accordingly, the blood urea nitrogen concentration reflects the balance between hepatic production and renal elimination. Dietary protein intake, the degree of catabolism, hydration status, and kidney function all influence BUN levels. Reduced hepatic capacity to produce urea lowers BUN and can be accompanied by hyperammonemia and hepatic encephalopathy. Evaluation of uremia is most informative when blood urea nitrogen is interpreted together with creatinine; in prerenal and postrenal azotemia, BUN often increases more than creatinine. In progressive chronic kidney disease, substantial nephron loss typically occurs before overt azotemia is apparent.