BLOOD UREA NITROGEN (BUN)
High BUN occurs in chronic glomerulonephritis, pyelonephritis and other causes of chronic renal disease; with acute renal failure, decreased renal perfusion (prerenal azotemia) as in shock. With urinary tract obstruction BUN increases (postrenal azotemia), for example as caused by neoplastic infiltration of the ureters, hyperplasia or carcinoma of the prostate. BUN is useful to follow hemodialysis and other therapy. “Uremia” was defined by Luke as an expression of a constellation of signs and symptoms in patients with severe azotemia secondary to acute or chronic renal failure.1 Causes of increased BUN include severe congestive heart failure, catabolism, tetracyclines with diuretic use, hyperalimentation, ketoacidosis, and dehydration as in diabetes mellitus, but even moderate dehydration can cause BUN to increase. Corticosteroids tend to increase BUN by causing protein catabolism. Bleeding from the gastrointestinal tract is an important cause of high urea nitrogen, commonly accompanied by elevation of BUN:creatinine ratio. Nephrotoxic drugs must be considered.
Borderline high values may occur after recent ingestion of high protein meal and muscle wasting may cause an elevation as well.
With creatinine, BUN is used to monitor patients on dialysis.
Low BUN occurs in normal pregnancy, decreased protein intake, with intravenous fluids, with some antibiotics, and in some but not all instances of liver disease.