The range is wide because of normal variations due to protein intake, endogenous protein catabolism, state of hydration, hepatic urea synthesis, and renal urea excretion. The normal range of urea nitrogen in blood or serum is 5 to 20 mg/dl, or 1.8 to 7.1 mmol urea per liter.

The BUN, then, is roughly one-half (7/15 or 0.466) of the blood urea. In Europe, the whole urea molecule is assayed, whereas in the United States only the nitrogen component of urea (the blood or serum urea nitrogen, i.e., BUN or SUN) is measured. Both are relatively small molecules (60 and 113 daltons, respectively) that distribute throughout total body water. Creatinine is the product of muscle creatine catabolism. Urea is the primary metabolite derived from dietary protein and tissue protein turnover. Urea and creatinine are nitrogenous end products of metabolism. The principle behind this ratio is the fact that both urea (BUN) and creatinine are freely filtered by the glomerulus however, urea reabsorbed by the renal tubules can be regulated (increased or decreased) whereas creatinine reabsorption remains the same (minimal reabsorption).

In the United States, both quantities are given in mg/dL The ratio may be used to determine the cause of acute kidney injury or dehydration. BUN only reflects the nitrogen content of urea (MW 28) and urea measurement reflects the whole of the molecule (MW 60), urea is just over twice BUN (60/28 = 2.14). In medicine, the urea-to-creatinine ratio ( UCR ), known in the United States as BUN-to-creatinine ratio, is the ratio of the blood levels of urea ( BUN) (mmol/L) and creatinine (Cr) (μmol/L). Medical laboratory value urea-to-creatinine ratio

Normal range bun