Serum Urea or BUN what is the difference?

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Urea is the principal nitrogenous waste product of metabolism and is generated from protein breakdown.

It is eliminated from the body almost exclusively by the kidneys in urine, and measurement of its concentration, first in urine and later in blood, has had clinical application in the assessment of kidney (renal) function for over 150 years.

Around the world, essentially the same method of urea analysis is used, but the result is expressed in two different ways . In the US and a few other countries, plasma or serum urea concentration is expressed as the amount of urea nitrogen. Although plasma or serum is used for the analysis, the test is still, somewhat confusingly, commonly referred to as blood urea nitrogen (BUN), and the unit of BUN concentration is mg/dL.

In all other parts of the world, urea is expressed as the whole molecule (not just the nitrogen part of the molecule) in SI units (mmol/L).

Since BUN reflects only the nitrogen content of urea (Molecular weight 28) and urea measurement reflects the whole of the molecule (Molecular weight 60), urea is approximately twice (60/28 = 2.14) that of BUN.

Thus BUN 10 mg/dL is equivalent to urea 21.4 mg/dL.

To convert BUN (mg/dL) to urea (mmol/L):

multiply by 10 to convert from /dL to /L and divide by 28 to convert from mg BUN to mmol urea, i.e. 10/28 = 0.357

So the conversion factor is 0.357

BUN mg/dL multiplied by 0.357 = urea (mmol/L)

Urea (mmol/L) divided by 0.357 = BUN (mg/dL)

Approximate reference (normal) range:

Serum/plasma urea 2.5-7.8 mmol/L

Serum/plasma BUN 7.0-22 mg/dL.