BUN stands for blood urea nitrogen. Urea nitrogen is what forms when protein breaks down. This test is performed to measure how much urea nitrogen is evident in the blood. The BUN test is often done to check kidney function. Higher than normal levels would be due to is: congestive heart failure, excessive protein levels, gastrointestinal bleeding, hypovolemia, heart attack, kidney disease, including glomerulonephritis, pyelonephritis, and acute tubular necrosis, kidney failure, shock, and urinary tract obstruction. Lower than normal levels could be due to liver failure, low protein diet, malnutrition, or over-hydration.