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Blood urea nitrogen: a measurement of nitrogen in the form of urea in the blood, usually reported as mg/dl, in common usage in the USA (each molecule of urea has two nitrogen atoms, each of molar mass 14 g/mol). Elsewhere, the concentration of urea in the serum is reported as mmol/L:serum urea(in mmol/L)=BUN(in mg/dl of nitrogen)/2.8

BUN or serum urea is used as a measure of kidney function but is less precise than the serum creatinine or estimates of glomerular filtration rate based on the serum creatinine (see eGFR). A disproportionate rise in blood urea nitrogen (or serum urea) compared with creatinine may be seen with volume depletion, cardiac failure, high protein diets, gastrointestinal bleeding, loss of muscle (the classic example is the bilateral amputee), and catabolic states associated with severe burns and fevers.

Subjects: Medicine and Health.

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