Urea Nitrogen, 24-Hour Urine without Creatinine Test

This quantitative urea nitrogen test, performed with a 24-hour urine specimen, may help estimate nitrogen balance and determine protein need in patients in critical conditions or receiving intravenous administration of nutrition.

Urea is the main nitrogen-containing product of protein breakdown and makes up over 75% of total nonprotein nitrogen excreted. Approximately 90% of the urea excretion is through the kidneys, and the rest is through the gastrointestinal tract and skin. Because urea secretion is closely related to protein catabolism, urinary urea, commonly expressed by the concentration of urinary urea nitrogen, may be used to assess nitrogen balance and guide protein intake [1,2]. Blood and urinary urea levels were previously used as kidney function markers, but are only considered useful in certain clinical scenarios; they have been generally replaced by creatine levels [1].

Urinary urea nitrogen level may be increased in individuals with hyperthyroidism or excess protein intake or breakdown [3]. It may be decreased in individuals with malnutrition, kidney damage or insufficiency, low-protein and high-carbohydrate diet, or liver disease. Pregnant persons and healthy children may also have low urinary nitrogen levels [3].

The results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings.

References

1. Lamb EJ, et al. Kidney function tests. Rifai N, et al. eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elservier Inc; 2022
2. Oh MS, et al. Evaluation of renal function, water, electrolytes, and acid-base balance. In: McPherson RA, et al, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021
3. Rao LV, et al. Laboratory tests. In: Rao LV, eds. Wallach's Interpretation of Diagnostic Tests. Pathways to Arriving at a Clinical Diagnosis. 11th ed. Wolters Kluwer; 2020.