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Blood Urea NitrogenThe blood urea nitrogen (BUN) test is a measure of the amount of nitrogen in the blood that comes from urea. Urea is a substance secreted by the liver, and removed from the blood by the kidneys. Methodology The test as originally carried out was by flame photometry; now chemical colorimetric tests are more widely used. Physiology The liver produces urea in the urea cycle as a waste product of the digestion of protein. Normal human adult blood should contain between 7-25 mg of urea nitrogen per 100 ml of blood. Individual laboratories may have different "usual clinical ranges", and this is because the procedure may vary. Interpretation The most common cause of an elevated BUN, azotemia, is due to renal failure. This can be due to a temporary condition such as dehydration or shock, or could even be a result of too much protein in the diet. Another, less common cause is a gastrointestinal hemorrhage; blood proteins are reabsorbed by the gut and modified, increasing turnover in the urea cycle. Heart attacks also raise BUN. A greatly elevated BUN (>100 mg/dl) usually indicates renal failure. A low BUN usually has little significance, but its causes include liver problems, poor nutrition (insufficient protein or some vitamins), or excessive alcohol consumption. Overhydration, possibly from intravenous fluids, can also result in a low BUN. Pregnancy also lowers BUN. A serum creatinine level is a somewhat more specific measure of kidney function.
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