Unit Converter
Urea nitrogen (BUN)
(Blood Urea Nitrogen - Key Marker of Kidney Function, Hydration, Catabolism & GI Bleeding)
Synonyms
- Blood Urea Nitrogen
- BUN
- Urea-N
- Serum urea nitrogen
- Urea (nitrogen fraction)
- Urea-N mg/dL
Units of Measurement
- mmol/L
- µmol/L
- mg/dL
- mg/100 mL
- mg%
- mg/L
- µg/mL
Unit Conversions
Definition
BUN measures the nitrogen component of urea.
Urea molecular weight = 60.06 g/mol
Nitrogen portion = 28 g/mol
Hence:
1 mg/dL BUN = 2.14 mg/dL Urea\textbf{1 mg/dL BUN = 2.14 mg/dL Urea}1 mg/dL BUN = 2.14 mg/dL Urea Urea (mg/dL) = BUN × 2.14\textbf{Urea (mg/dL) = BUN × 2.14}Urea (mg/dL) = BUN × 2.14
BUN (mg/dL) ↔ mmol/L
1 mmol/L BUN=2.8 mg/dL1\ \text{mmol/L BUN} = 2.8\ \text{mg/dL}1 mmol/L BUN=2.8 mg/dL 1 mg/dL=0.357 mmol/L1\ \text{mg/dL} = 0.357\ \text{mmol/L}1 mg/dL=0.357 mmol/L
µmol/L ↔ mg/L
1 µmol/L=0.028 mg/L1\ \text{µmol/L} = 0.028\ \text{mg/L}1 µmol/L=0.028 mg/L 1 mg/L=35.7 µmol/L1\ \text{mg/L} = 35.7\ \text{µmol/L}1 mg/L=35.7 µmol/L
mg/100 mL = mg% = mg/dL
µg/mL ↔ mg/L
1 µg/mL=1 mg/L1\ \text{µg/mL} = 1\ \text{mg/L}1 µg/mL=1 mg/L
Description
BUN reflects the nitrogen content of urea, the main metabolic waste product of protein metabolism produced in the liver and excreted via the kidneys.
It is influenced by:
- Renal function
- Hydration status
- Protein intake
- GI bleeding
- Catabolic state
BUN is more sensitive to pre-renal and post-renal factors than creatinine.
Physiological Role
Urea nitrogen is:
- A measure of body nitrogen balance
- An indicator of glomerular filtration
- A reflection of hepatic urea cycle function
- A marker of intravascular volume status
Clinical Significance
HIGH BUN
1. Pre-Renal Azotemia
Most common cause:
- Dehydration
- Shock
- Heart failure
- Severe hypotension
- Burns
- GI hemorrhage (digested blood ↑ nitrogen)
- High-protein diet
Pattern:
BUN rises much more than creatinine (BUN:Cr ratio > 20:1)
2. Renal (Intrinsic) Azotemia
- Acute tubular necrosis
- Glomerulonephritis
- Interstitial nephritis
- CKD
Pattern:
BUN and creatinine rise proportionally (BUN:Cr ratio ~10–15:1)
3. Post-Renal Obstruction
- Stones
- BPH
- Urethral strictures
- Tumors
4. Increased Protein Breakdown
- Sepsis
- Trauma
- Fever
- Steroids
- Tetracyclines
- Hyperthyroidism
LOW BUN
Causes
- Liver failure (reduced urea cycle activity)
- Severe malnutrition
- Low-protein diet
- Pregnancy (hemodilution)
- SIADH
- Overhydration / excess IV fluids
Reference Intervals
(Tietz 8E + KDIGO + Mayo + ARUP)
Serum BUN
- 7 – 20 mg/dL
(= 2.5 – 7.1 mmol/L)
Critical Values
- > 40 mmol/L (>112 mg/dL) → severe azotemia
- > 60 mmol/L (>168 mg/dL) → urgent evaluation / dialysis consideration
Diagnostic Uses
1. Assess Renal Function
Along with creatinine and GFR trends.
2. Evaluate Volume Status
- High BUN → dehydration
- Low BUN → overhydration/SIADH
3. Detect Upper GI Bleeding
BUN rises from digestion of blood proteins.
4. Monitor Kidney Injury in ICU
5. Assess Protein Metabolism
- Catabolic states
- Nutritional status
6. Liver Function Evaluation
Low BUN → hepatic failure.
Analytical Notes
- Enzymatic urease–glutamate dehydrogenase method is standard.
- Hemolysis does not significantly alter BUN.
- Gross lipemia may interfere.
- Fasting sample preferred but not essential.
Clinical Pearls
- High BUN + normal creatinine = dehydration.
- High BUN + low sodium = SIADH? Not typically; SIADH → low BUN.
- High BUN + high creatinine = renal failure (evaluate ratio).
- In GI bleed: BUN rises disproportionately (classic diagnostic clue).
- Liver failure → low BUN even in renal dysfunction.
- BUN fluctuates faster than creatinine in acute volume changes.
Interesting Fact
In upper GI bleeding, every 500 mL of digested blood increases BUN by ~10 mg/dL, making BUN a surprisingly useful bedside clue.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Renal Function Tests
- KDIGO AKI & CKD Guidelines
- Mayo Clinic Laboratories - BUN
- ARUP Consult - Renal Test Interpretation
- NIH / MedlinePlus - Blood Urea Nitrogen
- Standard nephrology & biochemistry sources.