Urea Nitrogen, 24-Hour Urine
Code:7015|CPT:84540|LOINC:3095-7, 3096-5
| Includes | Urea Nitrogen, U Urea Nitrogen, 24hr |
|---|
Analysis details
Methodology
- Urease method
Expected Turnaround Time
1 day
Special Instructions
- Record the total 24-hour urine volume on the test requisition.
- Avoid alcohol for 24 hours before starting the collection.
- For 12 hours before collection, avoid spicy or salty foods and items that may change urine color (eg, beets, carrots).
- If medically permissible, do not take diuretics for 48 hours before collection.
- Limit physical exertion and emotional stress throughout the 24-hour collection period.
How to use
Urea Nitrogen, 24-Hour Urine—also known as urine urea nitrogen (UUN) and used as a nitrogen balance study—assesses ongoing protein breakdown and estimates protein needs in patients receiving enteral or parenteral nutrition. The result assists in calculating protein prescriptions and monitoring response to nutrition therapy. When correlated with serum urea, the measurement helps evaluate renal excretory function in the setting of elevated serum urea and provides clinical context for azotemia.
Limitations
Urea is generated in the liver via the urea cycle, which detoxifies ammonia by converting it to a water‑soluble, minimally toxic compound that is eliminated in urine. Urea is freely filtered at the glomerulus, delivered to the tubular lumen, and roughly one‑third is passively reabsorbed; this reabsorption increases when urine flow is reduced. Urinary urea excretion parallels dietary protein intake. In nitrogen equilibrium, excretion of approximately 500 mmol urea (about 14 g urea nitrogen) per day corresponds to ingestion of roughly 100 g protein, and urinary urea accounts for about 90% of nitrogenous waste. Higher urinary urea output indicates negative nitrogen balance and can be seen after surgery, in hyperthyroidism, or when blood proteins are absorbed from the intestine following upper gastrointestinal bleeding. Reduced urinary urea excretion reflects positive nitrogen balance and occurs in renal diseases, in some hepatic disorders with impaired urea synthesis, in congenital deficiencies of urea cycle enzymes, and in settings of increased passive tubular reabsorption due to decreased renal perfusion or volume depletion (eg, massive hemorrhage, shock, dehydration, burns, heart failure), including age‑related renal hypoperfusion from renal artery atherosclerosis. Measurement of 24‑hour urine urea nitrogen (UUN) is used to estimate ongoing protein catabolism, guide protein prescription in patients requiring nutrition support, and aid interpretation of elevated serum urea in relation to renal excretory function.
| Unit | mg/dL | ||||
|---|---|---|---|---|---|
| Reference interval |
| ||||
| Indications | Quantification of nitrogen balance in critically ill patients receiving enteral or parenteral nutrition, Estimation of protein requirements in intensive care or severe illness during nutrition support, Assessment of renal excretory function in azotemia and in kidney disorders such as pyelonephritis, glomerulonephritis, renal amyloidosis, renal tuberculosis, and acute or chronic kidney failure |
Possible Causes of Abnormal Results
Increased levels
- cortisone
- dexamethasone
- hydrocortisone
- prednisolone
- quinine
- salicylates
- thyroxine (high dose)
Decreased levels
- anabolic hormones
- growth hormone
- insulin
- nephrotoxic drugs
- testosterone
Specimen Requirements
| Specimen | Urine |
|---|---|
| Container | 24-Hour Urine Collection Container |
| Volume | 10 mL (min 1 mL) |
| Storage Instructions | Room temperature, Refrigerated, Frozen |
References
Landry DW, Bazari H. Approach to the patient with renal disease. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011: chap 116.