Blood Urea Nitrogen (BUN) Test

A blood urea nitrogen (BUN) test measures the amount of urea nitrogen in your blood. Urea nitrogen is a waste product that develops when your body breaks down protein in the foods you eat. It forms in your liver and then travels through your blood to your kidneys. Your kidneys filter it out of your blood, and it leaves your body in your pee.

BUN level is one marker that allows healthcare providers to see how well your kidneys are working. A small amount of urea nitrogen in your blood is normal. But if you have too much, that means you may have a condition that prevents your kidneys from filtering it properly.

Why is it done?

Your healthcare provider may recommend a BUN test during a regular checkup. It’s usually a part of a comprehensive metabolic panel (CMP) or basic metabolic panel (BMP).

They may also recommend it if you’re at risk of chronic kidney disease (CKD). The early stages of CKD usually don’t have any symptoms. But an early diagnosis and treatment can help you maintain kidney function for as long as possible.

If you have late-stage CKD, your provider may recommend BUN tests as part of your routine testing.

How does a BUN test work?

It’s a simple blood draw. Your healthcare provider takes a small blood sample, usually from a vein in your arm. Your provider then sends the sample to a lab, where a specialist will examine it for high levels of urea nitrogen.

How do I prepare for it?

Before a BUN test, your healthcare provider may ask if you have any bleeding risks. These include having a bleeding disorder or taking any blood-thinning medications.

Your provider may recommend that you have other blood tests at the same appointment. These may require you to avoid eating or drinking (fast) for eight to 12 hours before the test. It’s a good idea to ask your provider if you need to fast before a BUN test or any other type of blood test.

Let your provider know if you have a fear of needles (trypanophobia). They can recommend some strategies to help you relax during the blood draw.

What should I expect during the test?

The blood draw for a BUN test only takes a few minutes. Your healthcare provider will:

1. Disinfect the skin around a vein in your arm with a skin cleaner
2. Use a thin needle to withdraw blood from the vein and collect it in small vials
3. Place a small bandage on your arm
4. Send your blood sample to a lab for analysis

The needle might cause a slight pinch and some discomfort for a few moments.

What are the risks?

A BUN test is a safe procedure. There is little risk of side effects.

The affected area may bruise afterward. But any marks from the needle should go away quickly.

You may feel lightheaded immediately after a blood draw. This usually doesn’t last long. If you feel lightheaded, your provider may help you lie down and get something for you to eat or drink.

What to expect after the BUN test

You should get your results within one to three days. A healthcare provider will contact you to explain your results and answer any questions. If they suspect you have chronic kidney disease, they may recommend additional tests. These may include imaging tests, like a kidney ultrasound, or other tests that are specific to your situation.

What is a normal level?

A normal BUN level varies according to your age and sex. Providers measure it in milligrams per deciliter (mg/dL):

• For children between the ages of 1 and 17, a normal BUN level is between 7 and 20 mg/dL.
• For adult females, a normal BUN level is between 6 and 21 mg/dL.
• For adult males, a normal BUN level is between 8 and 24 mg/dL.

What do high levels mean?

High BUN levels may suggest that your kidneys aren’t working as well as they should. But even if your kidneys are working properly, you may still have elevated BUN levels. Possible causes include:

• Eating a lot of protein
• Dehydration
• Aging (infants and children have lower BUN levels)
• Certain medications, including carbamazepine, methotrexate and tetracycline
• Burns
• A blockage in your urinary system that prevents you from peeing
• Stress
• Heart attack
• Gastrointestinal (GI) bleeding

What do low levels mean?

Low BUN levels aren’t common. Possible causes include:

• Not eating enough protein
• Having a small body type
• Having too much water in your body (overhydration)
• Liver disease

What level indicates kidney failure?

Healthcare providers don’t use BUN to define kidney failure. But if your BUN levels are higher than expected and your creatinine is also high, you likely have kidney failure. Creatinine is a waste product from muscle tissue breakdown.

When should I call my doctor?

Contact your healthcare provider if it's been several days since your BUN test and you haven't heard from them about the results.

You should also reach out if your BUN and creatine tests indicate kidney damage and you develop any symptoms of late-stage CKD. These may include:

• Feeling more tired than usual
• Lack of appetite
• Peeing more than usual
• Having trouble concentrating and sleeping
• Swelling, especially around your feet and ankles
• Muscle cramps
• Itchy skin

During your appointment, you may want to ask questions like:

• What do my test results mean?
• Do you think I need other tests?
• What tests do you recommend?
• Is a health condition causing my abnormal BUN levels?
• How can I lower my BUN levels?

How do you fix high BUN levels?

Healthcare providers don’t “fix” high BUN levels. They’ll find out why your kidneys aren’t working as expected and treat the cause. But the easiest way to reduce high BUN levels is to make changes to what you eat.

If you don’t have kidney disease and aren’t on dialysis, you should make sure to drink more fluids. Your BUN levels may rise while your serum creatinine is low if you don’t drink enough water.

Eating more low-protein foods can also bring down high BUN levels. Healthy low-protein foods include:

• Grains: Oats, rice and pasta
• Fruits: Apples, berries and bananas
• Vegetables: Leafy greens, peppers and broccoli
• Healthy fats: Avocados, nuts and coconut oil

What foods should I avoid if my levels are high?

If your BUN levels are high, limit or avoid high-protein foods. These include:

• Meats: Beef, chicken, pork and salmon
• Dairy: Milk, cheese and yogurt

Most people should consume between 40 and 60 grams of protein each day. But you may need to reduce your intake by as much as half if your BUN levels are high.