A common blood test, the blood urea nitrogen (BUN) test reveals important information about how well your kidneys and liver are working. A BUN test measures the amount of urea nitrogen that's in your blood.
Here's how your body typically forms and gets rid of urea nitrogen:
A BUN test can reveal whether your urea nitrogen levels are higher than normal, suggesting that your kidneys or liver may not be working properly.
You may need a blood urea nitrogen test:
If kidney problems are the main concern, the creatinine levels in your blood will likely also be measured when your blood is tested for urea nitrogen levels. Creatinine is another waste product that healthy kidneys filter out of your body through urine. High levels of creatinine in your blood may be a sign of kidney damage.
Your doctor may also test how well your kidneys are removing waste from the blood. To do this, you may have a blood sample taken to calculate your estimated glomerular filtration rate (GFR). The GFR estimates the percentage of kidney function you have left.
If your blood sample is being tested only for blood urea nitrogen, you can eat and drink normally before the test. If your blood sample will be used for additional tests, you may need to fast for a certain amount of time before the test. Your doctor will give you specific instructions.
During the blood urea nitrogen test, a member of your health care team takes a sample of blood by inserting a needle into a vein in your arm. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately.
Results of the blood urea nitrogen test are measured in milligrams per deciliter (mg/dL) in the United States and in millimoles per liter (mmol/L) internationally. In general, around 7 to 20 mg/dL (2.5 to 7.1 mmol/L) is considered normal.
But normal ranges may vary, depending on the reference range used by the lab, and your age. Ask your doctor to explain your results.
Urea nitrogen levels tend to increase with age. Infants have lower levels than other people do, and the range in children varies.
Generally, a high blood urea nitrogen level means your kidneys aren't working well. But elevated blood urea nitrogen can also be due to:
If kidney damage is a concern, ask your doctor what factors may be contributing to the damage and what steps you can take to try to control them.