Blood Urea Nitrogen (BUN)

Alternative Names
  • Blood Urea Nitrogen
  • BUN
  • Urea
  • Urea Nitrogen
  • Urea Nitrogen Clearance

Test Code: 4505

CPT: 84520
Tests Included BUN
Use The blood urea nitrogen or BUN test is primarily used, along with the creatinine test, to evaluate kidney function, help diagnose kidney disease, and to monitor people with acute or chronic kidney dysfunction or failure. Measurements of urea nitrogen can also be used in the diagnosis and treatment of metabolic disorders. Measurement of creatinine allows for the urea nitrogen/creatinine ratio, which is often used for diagnosis. 2
Clinical Utility Urea is a waste product formed in the liver when protein is metabolized. Urea is released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and released into the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood. However, when the kidneys cannot filter wastes out of the blood due to disease or damage, then the level of urea in the blood will rise.

Low BUN levels are not common and are not usually a cause for concern. They may be seen in severe liver disease, malnutrition, and sometimes when a person is overhydrated (too much fluid volume), but the BUN test is not usually used to diagnose or monitor these conditions. It is also normal during pregnancy.

Increased BUN levels suggest impaired kidney function. This may be due to acute or chronic kidney disease, damage, or failure. It may also be due to a condition that results in decreased blood flow to the kidneys, such as congestive heart failure, shock, stress, recent heart attack, or severe burns, to conditions that cause obstruction of urine flow, or to dehydration.
Intended Patient Population 18+ and Older Adult Males & Females
Patient Preparation None Specified
Sample Serum, Plasma
Tube Red, Green, Tiger
Volume 4mL Whole Blood (1mL Serum/Plasma)
Min Sample Volume 0.1 mLs
Reference Ranges M & F ≥ 18 yrs old; 7-25 mg/dL
Analytical Measurement Range 5-150 mg/dL
Critical Values ≥65.0 mg/dL
Units mg/dL
Test Methodology Quantitative Spectrophotometry
Test Turnaround Time 1 Day
Limitations Interpretation requires clinical information and the other electrolytes.
Shipping Requirements Refrigerated
Specimen Stability 7 Days RT
7 Days RF
Reject Criteria Icterus
Laboratory Developed Test (LDT) Yes
CMS Guidance None
  1. Tietz Textbook of Clinical Chemistry. Edited by CA Burtis, ER Ashwood. Philadelphia, WB Saunders Company, 1999
  2. Luke RG. Uremia and the BUN. N Engl J Med. 1981; 305(20):1213-1215

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