Test Overview

A blood urea nitrogen (BUN) or urea test measures the amount of nitrogen in your blood that comes from the waste product urea. Urea is made when protein is broken down in your body. Urea is made in the liver and passed out of your body in the urine.

A urea test is done to see how well your kidneys are working. If your kidneys are not able to remove urea from the blood normally, your urea level rises. Heart failure, dehydration, or a diet high in protein can also make your urea level higher. Liver disease or damage can lower your urea level. A low urea level can occur normally in the second or third trimester of pregnancy.

Urea to creatinine ratio (urea:creatinine)

A urea 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. Urea and creatinine tests can be used together to find the urea-to-creatinine ratio (urea:creatinine). A urea-to-creatinine ratio can help your doctor check for problems, such as dehydration, that may cause abnormal urea and creatinine levels.

Why It Is Done

A urea 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 urea levels to rise more than creatinine levels. This causes a high urea-to-creatinine ratio. Kidney disease or blockage of the flow of urine from your kidney causes both urea and creatinine levels to go up.

How To Prepare

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

How It Is Done

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

Watch

How It Feels

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Risks

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Results

Normal

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

High values

• A high urea value can mean kidney injury or disease is present. Kidney damage can be caused by diabetes or high blood pressure that directly affects the kidneys. High urea levels can also be caused by low blood flow to the kidneys caused by dehydration or heart failure.
• Many medicines may cause a high urea level. Be sure to tell your doctor about all the non-prescription and prescription medicines you take.
• A high urea value may be caused by a high-protein diet, Addison's disease, tissue damage (such as from severe burns), or from bleeding in the gastrointestinal tract.
• High urea-to-creatinine ratios occur with sudden (acute) kidney problems, which may be caused by shock or severe dehydration. A very high urea-to-creatinine ratio may be caused by bleeding in the digestive tract or respiratory tract.

Low values

• A low urea value may be caused by a diet very low in protein, malnutrition, or severe liver damage.
• Drinking excessive amounts of liquid may cause overhydration and cause a low urea value.
• Women and children may have lower urea levels than men because of how their bodies break down protein.
• A low urea-to-creatinine ratio may be caused by a diet low in protein, a severe muscle injury called rhabdomyolysis, pregnancy, cirrhosis, or syndrome of inappropriate antidiuretic hormone secretion (SIADH).