Is the urea level equivalent to blood urea nitrogen (BUN)?

Is Urea Level the Same as BUN?

No, urea level and BUN (blood urea nitrogen) are not identical measurements, though they measure the same substance—BUN specifically quantifies only the nitrogen component of urea, which comprises approximately 46.7% of urea's molecular weight. 1

The Mathematical Relationship

• The conversion formula is: urea nitrogen = total urea ÷ 2.14, as established by the National Kidney Foundation guidelines. 1
• This means that if you measure total urea concentration, you must divide by 2.14 to obtain the BUN value. 1
• The factor 2.14 reflects that nitrogen accounts for 46.7% of urea's molecular weight (molecular weight of urea = 60 g/mol; two nitrogen atoms = 28 g/mol). 1

Clinical Terminology and Reporting

• BUN is the standard term used in clinical practice globally, particularly in North American medical systems, and is typically reported in mg/dL in the United States. 1
• Some international laboratories may report total urea concentration instead of BUN, but the underlying measurement principle remains the same—they are measuring the same molecule, just reporting different components of it. 1
• When measuring dialysis adequacy, clinicians obtain predialysis BUN and postdialysis BUN samples to calculate urea clearance (Kt/V), demonstrating that BUN is the clinically utilized parameter. 1

Why This Distinction Matters Clinically

• In dialysis patients, proper terminology is critical because BUN measurements are used to calculate protein catabolic rate (PCR) and dialysis adequacy (Kt/V), and using the wrong value would lead to incorrect calculations. 2
• Laboratory errors can occur if clinicians confuse total urea with BUN—for example, dilution of predialysis BUN blood samples with saline reduces the true predialysis BUN concentration and results in incorrect calculations. 2
• The timing and technique for BUN sampling is critical to avoid dilution with saline or heparin, which artificially lowers values, and this precision matters because we are measuring the nitrogen component specifically. 1, 3

Practical Clinical Application

• When ordering laboratory tests, specify "BUN" rather than "urea" to ensure you receive the nitrogen component measurement that is used in all clinical formulas and decision-making algorithms. 1
• If you receive a "urea" result from an international laboratory, divide by 2.14 to convert to BUN for use in clinical calculations and comparison with North American reference ranges. 1
• Both measurements assess the same physiological process—urea production in the liver and filtration by the kidneys—but BUN is the standardized clinical parameter. 1, 3