A BUN test measures the level of urea nitrogen in the blood using a standard blood sample. Abnormal BUN levels may indicate problems with liver or kidney functioning.

BUN stands for blood urea nitrogen, which is a natural byproduct of the breakdown of protein.

According to the American Association for Clinical Chemistry, doctors perform BUN tests to assess kidney health. However, altered BUN counts can result from almost any disease, drug, or condition that causes changes in the kidneys or liver.

Due to the variety of conditions a BUN test can indicate, a doctor may recommend a BUN test as part of routine check-ups. BUN tests are part of standard blood tests, such as metabolic panels.

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BUN levels can provide an update on kidney and liver health.

Healthcare professionals use BUN tests for a variety of reasons, including monitoring kidney and liver function.

The liver and kidneys influence BUN levels profoundly, as they are responsible for breaking down protein and managing the waste.

The liver produces urea as part of the body’s process of breaking down protein. The liver releases urea into the blood, and it travels to the kidneys for removal in the urine. Urea contains nitrogen and is also the compound responsible for helping clear excess nitrogen from the body.

For this reason, people often refer to both “urea” and “urea nitrogen” as the same compound. This is not technically correct, but the two go hand-in-hand and measuring the level of this in the bloodstream helps to give a reliable indication of kidney and liver health.

A component of many other blood tests, a doctor can order a BUN test for any person, even if they have no apparent symptoms or underlying conditions. A doctor will most often request a BUN test along with other related lab tests if they suspect kidney or liver diseases.

Kidney and liver conditions are common in adults, but they are easy to overlook or mistake for other disorders.

The BUN test requires only a blood sample. A doctor may carry out a BUN test alongside other kidney tests, such as the serum creatinine test.

Symptoms of liver and kidney disease

According to the Centers for Disease Control and Prevention (CDC), an estimated 15 percent of adults in the United States have long-term kidney disease, known as chronic kidney disease. Many people are unaware that they have the condition.

Symptoms of kidney disease include:

  • frequent urination, especially at night
  • change in urine volume
  • decreased need to urinate
  • a burning sensation during urination
  • brown or red urine that appears foamy
  • pain, especially in the area near the kidneys (flank region)
  • muscle cramps
  • numbness in the hands and feet
  • interrupted sleep
  • tiredness and a lack of concentration or alertness
  • itchiness
  • poor appetite
  • nausea
  • swelling or puffiness, especially around the face, wrists, ankles, abdomen, and thighs
  • high blood pressure

Symptoms of liver disease include:

  • dark colored urine
  • pale-colored stool
  • skin and whites of the eyes that appear yellow
  • bruising easily
  • loss of appetite
  • nausea or vomiting
  • diarrhea
  • weight loss or gain
  • ongoing tiredness or weakness
  • abdominal pain and swelling
  • swelling in the legs and ankles
  • itchiness

Preparing for a BUN test is relatively straightforward.

Those having a BUN test should eat and drink as usual before the test. This helps ensure that results accurately reflect levels as they usually are.

Tell the doctor about any medications or supplements taken before or on the day of the test.

To perform the BUN test, a healthcare professional draws blood from a vein. They will often use a vein in front of the elbow or the back of the hand.

Laboratory technicians will then examine the blood sample and send the results to the individual’s doctor.

After the test, most people return to their usual routine. However, some people may also feel faint, hungry, or dehydrated.

If the area around the draw site becomes painful, inflamed, or starts to discharge pus or excess blood, the person should seek medical attention.

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BUN tests show how much nitrogen is the blood/

A BUN test that demonstrates a rise or fall in blood urea nitrogen levels might indicate a wide range of health problems.

This is because the kidneys and liver are involved in a vast scope of bodily functions, and problems elsewhere in the body that affect kidneys or liver function can also end up leading to an abnormality in the BUN level.

General reference ranges for a normal BUN level are as follows:

  • Adults up to 60 years of age: 6-20 mg/dL
  • Adults over 60 years of age: 8-23 mg/dL

However, these ranges will likely vary from lab to lab. If you are significantly outside the normal range at the lab your blood is sent to, it could be due to underlying kidney or liver disease.

The following factors can lead to elevated BUN levels:

Low BUN levels, which are not common, can result from:

  • liver failure
  • over-hydration
  • not consuming enough protein in the diet due to poor nutrition or a very low-protein diet

Typically, BUN levels increase with age. Changes in BUN levels also occur and during pregnancy.

The ideal range of BUN values might vary based on the person’s age and the reference range that the laboratory uses. People should speak to their doctor about what their results mean.

Certain over-the-counter (OTC) painkillers, in particular, non-steroidal anti-inflammatory drugs (NSAIDs), can all adversely affect the kidneys. This class of drugs includes ibuprofen, naproxen, and aspirin.

Natural diuretics, such as caffeine, and prescription diuretics can also affect the kidneys. Antibiotics can also impact BUN levels.

Many prescription drugs can affect the kidneys, some less frequently than others, resulting in abnormal BUN levels, including:

  • amphotericin B
  • carbamazepine
  • celecoxib
  • cephalexin
  • furosemide
  • methotrexate
  • penicillin
  • rifampin
  • spironolactone
  • hydrochlorothiazide
  • furosemide
  • gentamicin
  • tetracycline
  • vancomycin
  • sulfonamide antibiotics

According to the National Kidney Foundation, herbal supplements can also adversely affect the kidneys of people with kidney disease.

Generally, the U.S. Food and Drug Administration (FDA) does not regulate natural health products. This means that the effective dosage, source, and even the precise mixture of the supplement are uncertain.

This lack of regulation also means herbal supplements may contain compounds that damage the kidneys, such as heavy metals or aristolochic acid.

People with kidney or liver disease should also limit or monitor phosphorous and potassium intake.

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BUN tests can provide a picture of kidney health but not diagnose a disease.

BUN testing alone is not enough to diagnose any condition. It may merely highlight the need for further, more specific testing.

If BUN results come back higher or lower than usual, a healthcare professional will usually follow up with other tests. A serum creatinine test combined with the BUN level can help highlight particular conditions.

A doctor may also carry out testing to measure the blood levels of specific electrolytes, such as potassium, sodium, and calcium, as well as ordering urine tests.

Commonly, a serum chemistry panel is ordered, which typically measures the levels of BUN, creatinine, and electrolytes.

In some instances, doctors may also follow up abnormal BUN results by evaluating the BUN-to-creatinine ratio in the bloodstream.

Creatinine is a waste product that occurs after the muscles break down creatine. Creatine is a product of amino acid metabolism made after protein breakdown, and it is necessary for the body, helping to produce energy and move the muscles.

The ratio of BUN to creatinine is usually between 10:1 and 20:1.

The course of treatment for each person with abnormal BUN levels varies depending on the cause and severity. Less serious health issues that lead to abnormal BUN levels might require less intense therapy and long-term monitoring.

People with more severe conditions, such as end-stage kidney disease, often need intensive and on-going treatment, such as dialysis.

In the case of abnormal BUN levels caused by eating too much protein, people can limit the amount of protein-rich foods, such as meat, fish, beans, and dairy, and increase the amount of fruit and vegetables they consume. Staying well-hydrated also helps prevent the buildup of BUN.

According to a 2014 cross-sectional study, a person with high BUN levels is more likely to have high blood pressure.

A 2014 survey of over 1 million military veterans from the U.S. linked high BUN levels with an increased risk of diabetes mellitus.

BUN tests are easy to perform and may be a useful indicator of overall health and kidney and liver function.