A blood urea nitrogen (BUN) test measures the amount of urea nitrogen in your blood, which is a waste product made when your liver breaks down protein. A BUN test is often used to determine how well your kidneys are working, as healthy kidneys filter urea and other waste products from your blood. A high BUN level may indicate that your kidneys are not working properly, but it can also be due to dehydration or other factors. Your doctor will compare your BUN results with other test results, such as creatinine and estimated glomerular filtration rate (eGFR), to determine the next steps. In general, a normal BUN level is about 7-20 mg/dL, but this can vary depending on your age and other health conditions. If you have symptoms of high or low urea nitrogen in your blood, you should talk to a healthcare provider who can order a BUN test.