Urea nitrogen, BUN, serum BUN
A blood urea nitrogen (BUN) test measures the amount of urea nitrogen found in your blood. BUN levels help your doctor see how well your kidneys are working. The test may be used along with other measurements to help diagnose a kidney disorder or find out how well your treatment for kidney disease is working.
If your health care provider suspects that you have kidney problems, he or she may order a BUN test to help diagnose the condition. Symptoms that can mean kidney problems include:
Needing to urinate more or less often
Joint or bone pain
Restless legs while you're trying to sleep
Swelling in the arms or legs
Many people with kidney problems may not have symptoms right away.
Your doctor might order a BUN test for other reasons as well. Depending on your overall health, it may be part of a routine health check to find out how your kidneys are working. If you need dialysis or medication for kidney function, your doctor might order the test to check the health of your kidneys before and/or after the procedure. BUN tests are also routine during hospital stays for certain conditions.
Your doctor may also check the creatinine levels in your blood. On its own, the BUN level does little to tell your doctor about your kidney, or renal, function. But when it is measured along with creatinine levels, your doctor can look at how these two levels compare to help find out your kidney function. If this ratio is higher or lower than normal, you may have some type of kidney problem.
If your doctor suspects you may have a kidney problem, he or she may also order other blood and urine tests, as well as check your blood pressure, to help find out your kidney function.
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
The normal BUN level is between 7 and 21 milligrams per deciliter (mg/dL). Unless this level is greater than 60 mg/dL, though, it doesn't help your doctor measure your kidney health.
A better measure is the ratio of BUN to creatinine found in your blood. Typically, the ratio of BUN to creatinine should be between 10:1 and 20:1. If it's lower or higher than that, it may mean you have a problem with your kidneys.
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
A high-protein diet, steroid use, dehydration, burn injuries, and advancing age are some of the many reasons why your BUN levels might rise even if your kidneys are working as they should. This is why the ratio of BUN levels to creatinine levels is a more reliable measure of kidney health.
You don't need to prepare for this test.