BUN & Urea - What's the Difference?

A common question we get at ODX is, "What's the difference between BUN and Urea?" This post aims to answer that question!

The test for measuring urea in the blood is similar worldwide, but the results are reported in 2 different ways depending on the country doing the lab testing.

In the United States and a few other countries, the urea level in plasma or serum is measured as nitrogen and is called "Blood Urea Nitrogen" or BUN. The unit used for BUN is mg/dL.

In the rest of the world, the whole urea molecule is measured, not just the nitrogen, and is reported in Standard International units of mmol/L.

This measurement is about twice as high as the BUN measurement because BUN only measures the nitrogen part of the molecule (a Molecular Weight of 28), while urea measures the whole molecule (a Molecular weight of 60). Thus urea is approximately twice that of BUN (60/28 = 2.14)

So, a BUN result of 10 mg/dL is equal to a urea result of 21.4 mg/dL.

However, in order to provide a uniform way of looking at BUN and Urea from the same perspective, we need to be able to convert one into the other. 

Converting BUN (mg/dl) to Urea (mmol/L)

Multiply by 10 to convert from /dl to /L and then divide by 28 (the molecular weight of the nitrogen part of the BUN molecule) to convert from mg BUN to mmol urea i.e. 10/28 = 0.357

0.357 is our conversion factor. ODX uses this conversion factor in the software to automatically convert from BUN to Urea or Urea to BUN.

Here's how to do that:

BUN mg/dl multiplied by 0.357 = urea mmol/L

Urea mmol/l divided by 0.357 = BUN mg/dl

As we mentioned, we do this calculation in the software using 0.357 as the conversion. What this conversion factor does is create a situation, in the software and in the Functional Health Reporting where the result for BUN and the result for Urea are looking at the same thing.

Please note: The above conversion will not work if your lab reports Urea in mg/dl. The conversion works to move BUN mg/dl into Urea mmol/L and vis versa. Urea in mg/dl is NOT the same as BUN mg/dl. 

Clinical Implications of Urea vs BUN 

As mentioned above, the critical difference between Urea and BUN is how the measurement is expressed. The interpretation used in Optimal DX is the same. We use it to primarily help uncover renal dysfunction and to assess protein intake, liver function, and protein digestion.