BUN vs Urea: Key Differences and Clinical Application
BUN (blood urea nitrogen) measures only the nitrogen component of urea and equals approximately 46.7% of total urea concentration, requiring multiplication of BUN by 2.14 to obtain total urea levels. 1
Fundamental Distinction
Molecular Relationship:
- BUN specifically quantifies the nitrogen portion of the urea molecule, typically reported in mg/dL in the United States 1
- The conversion formula is: urea nitrogen = total urea ÷ 2.14, since nitrogen comprises 46.7% of urea's molecular weight 1
- To convert BUN to total urea: multiply BUN × 2.14
- To convert total urea to BUN: divide total urea ÷ 2.14
Terminology by Region:
- BUN is the standard term in North American medical systems 1
- Some international laboratories report total urea concentration instead, but the underlying measurement principle remains identical 1
Normal Reference Ranges
BUN (United States standard):
- Normal range: 7-20 mg/dL (varies slightly by laboratory)
- Normal BUN:creatinine ratio: 10-15:1 2
- Disproportionate elevation defined as BUN:Cr ratio >20:1 2
Total Urea (when reported):
- Normal range: approximately 15-43 mg/dL (calculated by multiplying BUN × 2.14)
Critical Clinical Pitfall: Why the Distinction Matters
In dialysis patients, using BUN rather than total urea is mandatory because BUN is required to compute protein catabolic rate (PCR) and dialysis adequacy (Kt/V); substituting the wrong measurement produces inaccurate PCR and Kt/V results. 1
Specific dialysis calculations requiring BUN:
- Kt/V calculation (urea clearance) requires predialysis and postdialysis BUN samples 1
- Protein catabolic rate (PCR) calculation for nutritional assessment 1, 3
- Confusing total urea with BUN causes laboratory errors and erroneous dialysis dosing calculations 1
Factors Affecting BUN Beyond Kidney Function
BUN reflects more than just renal function and is influenced by: 4
- Protein intake (higher intake raises BUN)
- Hydration status (dehydration concentrates BUN)
- Liver function (urea synthesis occurs in liver)
- Tissue catabolism (increased breakdown raises BUN)
Disproportionate BUN elevation (BUN:Cr >20:1) occurs with: 2
- Hypovolemia and prerenal azotemia
- Congestive heart failure
- Septic or hypovolemic shock
- High protein intake (>100 g/day)
- Gastrointestinal bleeding
- High-dose corticosteroids
- Severe malnutrition (albumin <2.5 g/dL)
Proper Sampling Technique
Hemodialysis facilities should adopt a single standardized BUN sampling method, with the sampling method remaining consistent for each patient. 5
Pre-dialysis BUN sampling: 6
- For arteriovenous fistula/graft: obtain blood from arterial needle before connecting arterial blood tubing
- For venous catheter: withdraw heparin/saline from arterial port following protocol
Post-dialysis BUN sampling: 5
- Use slow flow/stop pump technique for accuracy 1
- Avoid dilution with saline or heparin, which artificially lowers values 1, 6
- Saline reinfusion at treatment end (150-500 mL) can dilute plasma and lower postdialysis BUN, particularly in small patients 5
Laboratory processing: 5
- Process predialysis and postdialysis BUN samples from the same patient in the same batch analysis to minimize interassay variability (approximately 3%)
Clinical Interpretation
Prognostic significance:
- Elevated BUN independently predicts mortality in acute coronary syndromes, even with normal to mildly reduced GFR 7
- Higher BUN levels independently associate with adverse renal outcomes in CKD stages 3-5, independent of eGFR 8
- BUN ≥100 mg/dL with uremic symptoms typically indicates urgent hemodialysis 6
In dialysis patients: