UUN (Urinary Urea Nitrogen) Test
UUN (Urinary Urea Nitrogen) Test: Purpose, Method, Reference Range, and Clinical Interpretation
UUN (Urinary Urea Nitrogen) is an essential laboratory test used to evaluate protein metabolism, nitrogen balance, and nutritional status, especially in critically ill patients.
It is widely used in clinical nutrition, metabolic monitoring, renal assessment, and TPN/EN management.
This article summarizes the purpose, test principle (Urease–GLDH method), reference range, interpretation, and key clinical considerations.
✅ 1. What Is the Purpose of the UUN Test?
1) Assessment of Protein Catabolism
When the body breaks down proteins, ammonia is produced and converted to urea in the liver.
UUN measures the nitrogen component of urinary urea.
→ A direct indicator of protein catabolic rate.
2) Evaluation of Protein Intake
Measuring UUN in 24-hour urine helps estimate:
- Whether protein intake is appropriate
- Whether a patient is underfed or malnourished
3) Monitoring of Nutritional Therapy (TPN/EN)
In patients receiving enteral or parenteral nutrition:
→ UUN helps determine whether protein provision is adequate.
→ Particularly valuable in ICU settings.
4) Nitrogen Balance Assessment in Critically Ill Patients
Patients with severe infection, major surgery, trauma, or burns often enter a catabolic state.
→ UUN helps calculate nitrogen balance and tailor protein requirements.
5) Supplementary Renal Function Assessment
Changes in urea excretion may indirectly reflect renal function, especially when interpreted with BUN/creatinine.
✅ 2. Specimen
- 24-hour urine collection (recommended for accurate nitrogen balance)
- Random urine can be used but is less accurate.
✅ 3. Test Method: Urease–GLDH Colorimetric Assay
The standard laboratory method involves two enzymatic reactions:
Step 1. Urease Reaction
Urea in urine → hydrolyzed by urease → ammonium ions (NH₄⁺)
Step 2. GLDH Reaction
Ammonium enters the Glutamate Dehydrogenase (GLDH) reaction:
NADH → NAD⁺
Measurement
A decrease in absorbance at 340 nm is proportional to:
→ Ammonia produced
→ Urea nitrogen concentration
This method is highly specific, quantitative, and automation-friendly.
✅ 4. Reference Range (24-hour UUN)
(Values may vary by laboratory)
6,500 – 13,000 mg/day
Interpretation:
- High protein intake → Increased UUN
- Low intake or malnutrition → Decreased UUN
✅ 5. Conditions Associated with High UUN
🔺 High protein diet
🔺 Increased protein breakdown (catabolic state)
- Sepsis
- Postoperative state
- Trauma
- Burns
- Cancer cachexia
- Hyperthyroidism
🔺 High-protein nutritional therapy (TPN/EN)
✅ 6. Conditions Associated with Low UUN
🔻 Protein malnutrition
🔻 Severe liver dysfunction
- Impaired urea cycle → reduced urea synthesis
- UUN may appear falsely low
🔻 Renal failure
- Reduced urea excretion ↓
✅ 7. Key Points When Interpreting UUN
✔ 1) 24-hour urine collection is strongly recommended
Random urine is insufficient for accurate nitrogen balance.
✔ 2) Compare UUN with actual protein intake
UUN alone cannot describe protein metabolism without dietary context.
✔ 3) Consider Non-Urea Nitrogen (NUUN)
About 15–20% of urinary nitrogen is not urea
(e.g., creatinine, ammonia, uric acid).
→ When needed, use Total Urinary Nitrogen (TUN) for full assessment.
✔ 4) In severe liver failure, UUN may be falsely low
Because urea production is impaired despite ongoing muscle breakdown.
✅ 8. Summary Table
| Category | Details |
|---|---|
| Test Purpose | Assess protein metabolism, nitrogen balance, nutritional adequacy |
| Specimen | 24-hour urine |
| Method | Urease–GLDH colorimetric method |
| Increased UUN | High protein intake, catabolic state, TPN |
| Decreased UUN | Malnutrition, liver failure, renal dysfunction |
| Important Notes | Use 24h urine; compare with protein intake; consider NUUN |
📚 References
- Henry’s Clinical Diagnosis and Management by Laboratory Methods, 24th ed.
- Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, 6th ed.
- Laboratory Test Handbook, Lexi-Comp.
- McClave SA et al. ASPEN Guidelines for Nutrition Support in Adult Acute Care Patients.
- Kraut JA, Madias NE. Protein Metabolism and Nitrogen Balance in Critical Illness.