Converting Urea to BUN
To convert urea concentration from mmol/L to BUN in mg/dL, divide the urea value by 2.8 (or multiply by 0.357).
Conversion Formula
The standard conversion factor is clearly stated in the clinical practice guidelines:
- Urea (mmol/L) ÷ 2.8 = BUN (mg/dL)
- Alternatively: BUN (mg/dL) × 2.8 = Urea (mmol/L)
This conversion factor appears consistently across multiple K/DOQI guidelines 1, 2 and is the universally accepted standard in nephrology practice.
Understanding the Relationship
Blood Urea Nitrogen (BUN) measures only the nitrogen component of urea, not the entire urea molecule. Since urea has a molecular weight of 60 (with 28 from two nitrogen atoms), BUN represents approximately 46.7% of the total urea mass. The factor of 2.8 accounts for this molecular weight difference when converting between measurement systems.
Practical Application
When you see formulas in clinical practice:
- MDRD equations use BUN in mg/dL 1
- International laboratories often report urea in mmol/L
- U.S. laboratories typically report BUN in mg/dL
For example, if a laboratory reports urea as 14 mmol/L:
- BUN = 14 ÷ 2.8 = 5 mg/dL
Conversely, if BUN is 20 mg/dL:
- Urea = 20 × 2.8 = 56 mmol/L
Clinical Context
This conversion is essential when:
- Using the MDRD7 formula for GFR estimation, which requires BUN in mg/dL 1
- Calculating urea kinetic modeling parameters in dialysis patients 2
- Interpreting laboratory values from different countries or systems
- Applying clinical guidelines that specify BUN thresholds
The 2.8 conversion factor is the only accepted standard and should be used consistently to avoid dosing errors in nephrotoxic medications or dialysis prescription calculations.