urea nitrogen in 24 hour urine
Reference interval
- 12-20 grams/24 hours
Principle
Urea is the major end product of protein & amino acid catabolism and is generated in the liver through the urea cycle. From the liver, urea enters the blood to be distributed to all intracellular & extracellular fluids, since urea is freely diffusible across most cell membranes. Most of the urea is ultimately excreted by the kidneys, but minimal amounts are also excreted in sweat & degraded by bacteria in the intestines.
In the ACA methodology, urease specifically hydrolyzes urea to form ammonia & carbon dioxide. The ammonia is utilized by the enzyme of glutamate dehydrogenase (GLDH) to reductively aminate a-keto glutamate (a-KG) with simultaneous oxidation of reduced nicotinamide-adenine dinucleotide (NADH).
Clinical significance
The clearance of urea is less used as an estimate of glomerular filtration rate than creatinine clearance, although the measurement of urea is accurate & precise. It parallels the true glomerular filtration rate. The excretion of urea is influenced by tubular reabsorption as well as filtration, the latter being influenced by urinary flow. In addition, urea production is affected by ingestion, catabolism, &/or loss of proteins into the gastrointestinal tract.
Because of the variability in metabolism & renal handling, elevations of circulating urea concentrations do not always correlate well with renal parenchymal function. Accordingly, a rise in urea can be attributed to pre-renal causes (excess urea production, diminished renal blood flow, or both), to post-renal causes (obstruction along the genitourinary tract), or to kidney perturbations (parenchymal kidney damage). If excessive production is the only factor elevating urea, the BUN rarely exceeds 40 mg/dL. Values beyond this usually indicate renal damage, urinary tract obstruction, & diminished renal blood flow. Because of the many factors influencing urea clearance, this test has been replaced by the creatinine clearance, but it may have some value when interpreted with the creatinine clearance in severe renal failure.
Increases
- increased dietary protein
- hyperthyroidism
- surgery, postoperative
Decreases
- infants & growing children
- pregnancy
- low protein diet
- liver disease
- renal insufficiency
Specimen
2 mL aliquot of a 24-hour urine collection. Proper 24 hour urine collection procedure should be followed, & collection container should be refrigerated at 2-6 C during collection. Upon receipt in the work area, it should be well-mixed & measured in a graduated cylinder. The total volume should be recorded.
More general terms
References
- ↑ Kaplan, Lawrence A. & Pesce, Amadeo J., Clinical Chemistry: Theory, Analysis, & Correlation, 2nd Edition, The C.V. Mosby Company, St. Louis, MO, 1989, pp. 354, 1021-1024.
- ↑ Henry, John Bernard, M.D., Clinical Diagnosis & Management by Laboratory Methods, 18th Edition, W.B. Saunders Company, Philadelphia, PA, 1991, pp. 121-125, 140-142.
- ↑ ACA IV Discrete Clinical Chemistry Analyzer Instrument Manual, Volume 3A, Chapter 6: Test Methodology, BUN 3
- ↑ Clinical Guide to Laboratory Tests, 4th edition, HB Wu ed, WB Saunders, Philadelphia, 2006
- ↑ Panel of 7 tests Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0020480.jsp
- ↑ Panel of 5 tests Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0000000.jsp