Blood Urea Nitrogen (BUN) is a test that measures the amount of urea nitrogen found in the blood. This test is used to determine how well your kidneys are working. Urea nitrogen is a waste product, produced in the liver due to breakdown of proteins. Normally kidneys filter it and is excrete in the urine. If the level of BUN are high in blood it may be a sign of a kidney or a liver problems. The test is performed if there is a suspect of kidney disease with symptoms of urinary changes, swelling in arms or legs, muscle cramps, or frequent episodes of fatigue. The test may also be done if the doctor suspects kidney damage, to evaluate kidney function, to determine the effectiveness of dialysis or to help in diagnosis of other conditions like liver damage, urinary tract obstruction, congestive heart failure or gastrointestinal bleeding.