Your serum blood urea nitrogen, or BUN, level reflects a balance between your body's protein catabolism and kidney function. A wide variety of conditions and diseases influence this balance, as does your state of hydration and many medications. The serum BUN test is used for basic health screening and to help assess your kidneys' ability to remove waste products from the blood.
Normal Protein Catabolism and BUN
Serum BUN tests measure the amount of urea nitrogen in your blood, a waste product your liver produces during the regular breakdown of proteins. Urea nitrogen enters your bloodstream from the liver and is filtered out by your kidneys for excretion in the urine. This ongoing balancing act between liver production and kidney removal leaves a small amount of circulating urea nitrogen in your blood at all times. The normal BUN range for a healthy adult may vary slightly from laboratory to laboratory, but generally runs between 7 and 24 mg/dL, according to Mayo Medical Laboratories.
Testing Your BUN
Although primarily used to evaluate kidney function, because so many different conditions can affect your BUN level the test is also commonly included in panels of blood tests performed to assess your overall health status. Changes in diet, protein metabolism, liver function, blood flow to the kidneys, kidney function and blockages in the lower urinary tract can all affect your serum BUN. A variety of medications can also affect BUN levels, including common drugs such as antibiotics, chemotherapy agents and high doses of aspirin. Your health care provider might order a BUN test as part of a routine check-up, if you have non-specific symptoms, are acutely ill, have symptoms or signs suggestive of kidney disease, or they need to monitor your kidney function because of illness or medications you're taking.
Elevated BUN Levels
An increased BUN level suggests your kidneys are not functioning properly, but your kidneys might not be the main problem. Kidney diseases such as glomerulonephritis and infection, urinary tract obstructions from stones or prostate enlargement, and dehydration can all lead to a buildup of BUN in your blood. Congestive heart failure, recent heart attack, shock and other causes of low blood pressure lead to increased BUN levels by slowing the blood flow to your kidneys. Large amounts of dietary protein, increased protein breakdown by your body or intestinal bleeding also raise your BUN by increasing the nitrogen load on your liver and kidneys. Normal BUN levels increase slightly as you age as well, particularly after about age 60.
Low BUN Levels
Low BUN levels are less common, but might still indicate a significant medical disorder. Severe liver disease with liver failure, very low protein diets, malnutrition and over-hydration can all decrease your BUN level, but other blood tests are usually better suited to diagnosing these conditions. Although most medications that have an effect raise your BUN, the antibiotics chloramphenicol and streptomycin can result in lower levels, according to MedlinePlus. Your health care provider should be able to explain why they tested your BUN level and the implications of any result based on your medical history.
