Blood urea nitrogen (bun) test

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

A BUN test may be done with a blood creatinine test. The level of creatinine in your child's blood also tells how well your child's kidneys are working. A high creatinine level may mean that your child's kidneys aren't working as they should. BUN and creatinine tests can be used together to find the BUN-to-creatinine ratio.

A BUN test is done to:

• See if your child's kidneys are working normally.
• See if your child's kidney disease is getting worse.
• See if treatment of your child's 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.

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

Don't let your child eat a lot of meat or other protein in the 24 hours before the BUN test.

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

Low values

• A low BUN value may be caused by a diet very low in protein, by malnutrition, or by severe liver damage.
• Drinking too much liquid may cause overhydration and cause a low BUN value.
• Women and children may have lower BUN levels than men because of how their bodies break down protein.
• A low BUN-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).