Overview
A blood urea nitrogen test, commonly called a BUN test, is a blood test that measures urea nitrogen in your blood. Urea is a waste product made when your body breaks down protein. The liver makes urea. The kidneys filter it from the blood so it can leave your body in urine.
A BUN test is used to help check kidney function. It often is done with other blood and urine tests, such as creatinine, estimated glomerular filtration rate (eGFR) and urine albumin tests. A BUN result alone is not enough to diagnose a kidney condition.
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Why it's done
You may need a blood urea nitrogen (BUN) test to:
- Check how well your kidneys are filtering waste from your blood.
- Help find or monitor kidney disease or kidney injury.
- Check kidney function if you have a higher risk of kidney disease. If you have diabetes, high blood pressure, heart disease or a family history of kidney failure, you are at higher risk.
- Help evaluate dialysis treatment if you receive hemodialysis or peritoneal dialysis.
- Check kidney function while you take medicines that can affect the kidneys.
- Provide information about whether you're eating the right amount of protein and drinking the right amount of fluids to keep your body hydrated.
If you're having the test done to check kidney function, it may be included in a group of tests called a basic metabolic panel. The metabolic panel includes a BUN test, along with a creatinine test, glomerular filtration rate (eGFR) and other tests.
A creatinine test measures another waste product that the kidneys filter from the blood. Creatinine results often are used to calculate eGFR. The eGFR estimates how well your kidneys are filtering blood. A urine albumin-to-creatinine ratio test may be used to look for early signs of kidney damage. When kidneys don't work well, blood levels of both BUN and creatinine go up, and eGFR drops.
Risks
For most people, the risks of a blood urea nitrogen (BUN) test are minor. You may feel a quick sting when the needle goes in. You also may have mild bleeding, bruising or tenderness where the blood was drawn. Some people feel lightheaded or faint during or after a blood draw. Infection at the needle site is rare.
Tell the person drawing your blood if you feel lightheaded. Contact your healthcare team if bleeding does not stop after the blood draw. Also contact them if pain, swelling or other symptoms at the needle site get worse.
How you prepare
If your blood sample is being tested only for blood urea nitrogen, you typically can eat and drink as usual before the test.
Tell your care team about all medicines, vitamins and supplements you take. Some medicines can affect BUN results. Do not stop taking a medicine unless a member of your healthcare team tells you to do so.
If the same blood sample will be used for other tests, you may be told not to eat or drink for a certain amount of time before the blood draw. Follow the instructions from your care team.
What you can expect
A member of your healthcare team takes a blood sample, usually from a vein in your arm. The person drawing your blood may put an elastic band around your upper arm and clean the skin. Then a needle is put into the vein to collect blood into a tube.
The blood draw usually takes a few minutes. The blood sample is sent to a lab for testing.
After the needle is removed, pressure and a bandage may be applied over the needle site. You typically can return to your regular activities right away.
You may have mild bruising or tenderness where the needle was placed. This usually goes away quickly.
Results
Blood urea nitrogen (BUN) test results are reported in milligrams per deciliter (mg/dL) in the United States and in millimoles per liter (mmol/L) internationally. Blood tests performed outside of the United States may check urea instead of blood urea nitrogen.
Your lab report shows your BUN result and the lab's reference range. Use the reference range on your own lab report. Ranges can vary by lab method, age, sex and health conditions. Infants have lower levels than other people do, and the range in children varies.
In general, around 7 to 20 mg/dL (2.5 to 7.1 mmol/L) is considered a healthy range.
A BUN result above the healthy range may mean that your kidneys are not filtering urea as well as expected. In that case, blood creatinine also is expected to be higher than usual. A BUN result alone is not enough to diagnose kidney disease or kidney failure. BUN can rise for reasons that are not kidney disease.
Possible reasons for a BUN result above the healthy range include:
- Kidney disease or reduced kidney function.
- A blockage in the urinary tract.
- Bleeding in the stomach or intestines.
- Dehydration, which can happen from not drinking enough fluids or from other causes.
- Shock.
- Severe burns.
- Some medicines, such as prednisone.
- A high-protein diet.
A BUN result below the standard range is less common. Possible reasons include liver disease, a low-protein diet, malnutrition or too much fluid in the body.
Your care team may compare your BUN result with other test results, such as creatinine, eGFR, electrolytes and urine albumin. You may need repeat testing or other tests based on your symptoms, health history, medicines and results.
If kidney disease or kidney injury is a concern, ask your care team what may be affecting your kidneys and what steps may help protect your kidney health.
Aug. 22, 2026