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The 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 and 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.
BUN, which measures urea concentration, is marker. It has little clinical significance in of itself, as urea is non-toxic. This was demonstrated by Johnson et al. by adding large amounts of urea to the dialysate of hemodialysis patients for several months and finding no ill effects.[1]
Units
BUN is measured in mg/dL. To convert BUN to a urea concentration in mmol/L multiply by 0.357.
See also
Reference
- ^ Johnson WJ, Hagge WW, Wagoner RD, Dinapoli RP, Rosevear JW. Effects of urea loading in patients with far-advanced renal failure. Mayo Clin Proc. 1972 Jan;47(1):21-9. PMID 5008253
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