How do you convert Blood Urea Nitrogen (BUN) to Urea?

Converting BUN to Urea

To convert Blood Urea Nitrogen (BUN) to total urea, multiply the BUN value by 2.14, as nitrogen comprises approximately 46.7% of urea's molecular weight. 1

Conversion Formula

The mathematical relationship is straightforward:

• Urea (mg/dL) = BUN (mg/dL) × 2.14 1
• Conversely: BUN (mg/dL) = Urea (mg/dL) ÷ 2.14 1

This conversion factor of 2.14 derives from the molecular composition of urea (CH₄N₂O), where the two nitrogen atoms account for 46.7% of the total molecular weight. 1

Clinical Context

BUN specifically measures only the nitrogen component of urea in blood, which is the standard reporting method in United States clinical laboratories and is typically expressed in mg/dL. 1 Some international laboratories may report total urea concentration instead, but the underlying measurement principle remains identical. 1

Practical Application in Dialysis

When calculating dialysis adequacy using Kt/V formulas, the predialysis and postdialysis BUN values are used directly in the equations without conversion to total urea. 2 The formulas from the National Kidney Foundation K/DOQI guidelines utilize BUN concentrations (C₀ and C) expressed in mg/dL. 2

Important Caveats

Proper sampling technique is critical - BUN samples must avoid dilution with saline or heparin, which artificially lowers measured values. 1 In hemodialysis patients, the Stop Flow/Stop Pump technique is recommended to ensure accurate BUN measurements for kinetic modeling. 2

The BUN value reflects more than kidney function alone and is affected by protein intake, hydration status, liver function, and tissue catabolism. 1