The BUN test is primarily used, along with the creatinine test, to evaluate kidney function under a wide range of circumstances and to monitor patients with acute or chronic kidney dysfunction or failure. It also may be used to evaluate a person’s general health status when ordered as part of a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP).

BUN is part of both the BMP and CMP, groups of tests that are widely used:

• when someone has non-specific complaints,
• as part of a routine testing panel,
• to check how the kidneys are functioning before starting to take certain drug therapies,
• when an acutely ill person comes to the emergency room and/or is admitted to the hospital, or
• during a hospital stay.

BUN is often ordered with creatinine:

• when kidney problems are suspected,
• at regular intervals to monitor kidney function in patients with chronic diseases such as diabetes, congestive heart failure, and myocardial infarction (heart attack),
• at regular intervals to monitor kidney function and treatment in patients with known kidney disease,
• prior to and during certain drug treatments to monitor kidney function, or
• at regular intervals to monitor the effectiveness of dialysis.

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.

BUN concentrations may be elevated when there is excessive protein catabolism (breakdown), significantly increased protein in the diet, or gastrointestinal bleeding (because of the proteins present in the blood).

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 patient is overhydrated (too much fluid volume), but the BUN test is not usually used to diagnose or monitor these conditions.

Both decreased and increased BUN concentrations may be seen during a normal pregnancy.

If one kidney is fully functional, BUN concentrations may be normal even when significant dysfunction is present in the other kidney.

NOTE: The result of your BUN test is measured by your doctor against a reference range for the test to determine whether the result is “normal” (it is within the range of numbers), high (it is above the high end of the range), or low (it is below the low end of the range). Because there can be many variables that affect the determination of the reference range, the reference range for this test is specific to the lab where your test sample is analyzed. For this reason, the lab is required to report your results with an accompanying reference range. Typically, your doctor will have sufficient familiarity with the lab and your medical history to interpret the results appropriately.

While there is no such thing as a “standard” reference range for BUN, most labs will report a similar, though maybe not exactly the same, set of numbers as that included in medical textbooks or found elsewhere online. For this reason, we recommend that you talk with your doctor about your lab results. For general guidance only, we are providing the reference range for this test from the classic medical text, Tietz Textbook of Clinical Chemistry and Molecular Diagnostics.

For more information on reference ranges, please read Reference Ranges and What They Mean.

BUN levels can increase with the amount of protein in your diet. High-protein diets may cause abnormally high BUN levels while very low-protein diets can cause an abnormally low BUN.