BUN (Blood Urea Nitrogen) Test

What is the BUN test?

The blood urea nitrogen (BUN) test is a test of how well your kidneys are working. It measures the amount of nitrogen in your blood. The nitrogen is present in a chemical called urea. Urea is a waste product produced as your body digests protein. Urea is carried by the blood to the kidneys, which filter the urea out of the blood and into the urine.

Why is this test done?

BUN is usually measured to see how well your kidneys are working. Kidney disease often makes it hard for the kidneys to filter as much urea as they should. This causes high levels of urea in the blood. This test is also done if you are having kidney dialysis to see how well the dialysis is working.

This test may be used with another test called the creatinine test to see if you are dehydrated.

Some medicines are processed by the kidneys or can cause kidney damage as a side effect. The BUN test may be done to be sure you have normal kidney function before you start taking these medicines.

How do I prepare for this test?

• You may need to avoid taking certain medicines before the test because they might affect the test result. Make sure your health care provider knows about any medicines, herbs, or supplements that you are taking. Don't stop any of your regular medicines without first consulting with your health care provider.

How is the test done?

A small amount of blood is taken from your arm with a needle. The blood is collected in tubes and sent to a lab.

Having this test will take just a few minutes of your time. There is no risk of getting AIDS, hepatitis, or any other blood-borne disease from this test.

How will I get the test result?

Ask your health care provider when and how you will get the result of your test.

What does the test result mean?

The normal range for BUN is 7 to 20 milligrams per deciliter (mg/dL). The normal range may vary slightly from lab to lab. Normal ranges are usually shown next to your results in the lab report.

Your BUN level may be higher than normal because:

• Your kidneys aren't working well.
• You have low levels of fluid in your body (you are dehydrated).
• You've been eating a high-protein diet.
• You are bleeding into your stomach (from an ulcer, for example).
• You have heart failure or you've had a heart attack.
• You are in shock.
• Your urinary tract is blocked (for instance, from an enlarged prostate in men or from a kidney stone or tumor).

Some medicines can cause BUN to be higher than normal, such as:

• allopurinol
• some antibiotics (aminoglycosides, cephalosporins, rifampin)
• furosemide
• guanethidine
• indomethacin
• methotrexate
• methyldopa
• spironolactone
• thiazide diuretics
• triamterene
• any other drug that damages the kidney.

Your BUN level may be lower than normal because:

• Your liver isn't working well.
• You've been eating a low-protein diet.
• You've been malnourished.
• You've been drinking a lot of fluids.

What if my test result is not normal?

Test results are only one part of a larger picture that takes into account your medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your heath care provider about your result and ask questions.

If your test result is not normal, ask your health care provider:

• if you need additional tests
• what you can do to work toward a normal value
• when you need to be tested again.