Unit Converter
Urea
(Major Nitrogenous Waste Product - Key Marker of Renal Function, Hydration Status & Protein Catabolism)
Synonyms
- Blood urea
- Urea nitrogen
- BUN (Blood Urea Nitrogen - U.S. terminology)
- Carbamide
- Serum urea
Units of Measurement
- mmol/L
- µmol/L
- mg/dL
- mg/100 mL
- mg%
- mg/L
- µg/mL
Unit Conversions
Molecular Weight of Urea = 60.06 g/mol
mmol/L ↔ mg/dL
1 mmol/L=6.006 mg/dL1\ \text{mmol/L} = 6.006\ \text{mg/dL}1 mmol/L=6.006 mg/dL 1 mg/dL=0.166 mmol/L1\ \text{mg/dL} = 0.166\ \text{mmol/L}1 mg/dL=0.166 mmol/L
µmol/L ↔ mg/L
1 µmol/L=0.060 mg/L1\ \text{µmol/L} = 0.060\ \text{mg/L}1 µmol/L=0.060 mg/L 1 mg/L=16.6 µmol/L1\ \text{mg/L} = 16.6\ \text{µmol/L}1 mg/L=16.6 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
Urea is the principal end-product of protein and amino acid metabolism, synthesized in the liver via the urea cycle and excreted primarily by the kidneys.
Urea concentration reflects the balance of:
- Renal excretion
- Protein intake & catabolism
- Hepatic urea production
Because it is less specific than creatinine but responsive to fluid status and protein load, urea participates in overall renal and metabolic assessment.
Physiological Role
- Removal of nitrogenous waste
- Contributes to medullary osmotic gradient for urine concentration
- Reflects hepatic urea-cycle efficiency
- Sensitive to hydration, diet, and GI bleeding
Clinical Significance
ELEVATED UREA (Azotemia / Uremia)
1. Pre-Renal Azotemia (most common)
Due to reduced renal perfusion:
- Dehydration
- Hypovolemia
- Heart failure
- Shock
- GI hemorrhage (↑ nitrogen absorption)
- High-protein diet
Pattern:
Urea rises more than creatinine → high urea:creatinine ratio.
2. Renal (Intrinsic) Azotemia
Due to kidney parenchymal disease:
- Acute tubular necrosis
- Glomerulonephritis
- Interstitial nephritis
- Chronic kidney disease
Pattern:
Urea and creatinine rise proportionally.
3. Post-Renal Obstruction
- BPH
- Stones
- Tumors
- Strictures
4. Increased Protein Catabolism
- Burns
- Fever
- Hyperthyroidism
- Severe infections
- Corticosteroids
- Tetracyclines
LOW UREA
Less common.
Causes
- Liver failure (reduced urea synthesis)
- Low protein intake / malnutrition
- Pregnancy (increased plasma volume)
- SIADH
- Overhydration (dilutional)
Reference Intervals
(Tietz 8E + KDIGO + Mayo + ARUP)
Serum Urea
- 2.5 – 7.5 mmol/L
(= 15 – 45 mg/dL)
Serum Urea Nitrogen
(Used in the U.S.)
BUN (mg/dL)=Urea (mg/dL)2.14\text{BUN (mg/dL)} = \frac{\text{Urea (mg/dL)}}{2.14}BUN (mg/dL)=2.14Urea (mg/dL)
Critical Levels
- >30 mmol/L (>180 mg/dL) → severe renal failure / uremia
- >50 mmol/L (>300 mg/dL) → urgent dialysis consideration
Diagnostic Uses
1. Evaluation of Kidney Function
Part of standard renal panel (with creatinine).
2. Assess Hydration Status
- High urea → dehydration
- Low urea → overhydration/SIADH
3. Monitor Renal Failure (AKI/CKD)
4. Diagnosis of GI Bleeding
Urea rises due to digestion of blood.
5. Nutritional Assessment
Low urea can reflect low protein intake.
6. Liver Function Assessment
Low urea in hepatic failure.
7. ICU / Critical Illness Monitoring
Reflects catabolic state; used in scoring systems.
Analytical Notes
- Enzymatic assays using urease are standard.
- Hemolysis minimally affects urea; lipemia may interfere.
- Fasting sample preferred for consistency.
- Urea rises rapidly in dehydration; interpretation must consider volume status.
Clinical Pearls
- High urea + normal creatinine = dehydration until proven otherwise.
- High urea + high creatinine = intrinsic renal disease.
- High urea with very high ratio (>20:1) → upper GI bleed.
- Liver failure → low urea despite renal injury (misleadingly low).
- Creatinine is a better GFR marker; urea is a better indicator of protein catabolism & fluid status.
Interesting Fact
The urea cycle is one of the earliest described metabolic cycles (Krebs–Henseleit cycle) and is essential for survival-without it, ammonia would rise to fatal levels.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Renal Function Tests
- KDIGO Acute Kidney Injury & CKD Guidelines
- Mayo Clinic Laboratories - Urea / BUN
- ARUP Consult - Renal Panel
- NIH / MedlinePlus - Urea Test
- Standard nephrology and laboratory references.