Blood Urea Nitrogen (BUN) Test: About This Test

What is it?

A blood urea nitrogen (BUN) test measures the amount of nitrogen in your blood that comes from the waste product urea. Urea is made in the liver and passed out of your body in the urine. If your kidneys are not able to remove urea from the blood normally, your BUN level rises. Dehydration can also make your BUN level higher.

A BUN test may be done with a blood creatinine test. The level of creatinine in your blood also tells how well your kidneys are working. A high creatinine level may mean your kidneys are not working properly. BUN and creatinine tests can be used together to find the BUN-to-creatinine ratio.

Why is this test done?

A BUN test is done to:

• See if your kidneys are working normally.
• See if your kidney disease is getting worse.
• See if treatment of your kidney disease is working.
• Check for severe dehydration. Dehydration generally causes BUN levels to rise more than creatinine levels. This causes a high BUN-to-creatinine ratio. Kidney disease or blockage of the flow of urine from your kidney causes both BUN and creatinine levels to go up.

How do you prepare for the test?

Do not eat a lot of meat or other protein in the 24 hours before having a BUN test.

How is the test done?

A health professional uses a needle to take a blood sample, usually from the arm.

What happens after the test?

• You will probably be able to go home right away.
• You can go back to your usual activities right away.