BUN (Blood Urea Nitrogen): Normal Range, What High & Low Levels Mean

BUN stands for Blood Urea Nitrogen. The test measures the amount of urea nitrogen circulating in your blood. Urea is a waste product your liver makes when it breaks down protein from the food you eat. Once urea is made, your kidneys filter it out of the blood and send it into your urine. Because the test captures activity at both ends — the liver making urea and the kidneys clearing it — your BUN level gives doctors useful information about your kidney filtration function and your protein metabolism at the same time. Talk to your healthcare provider about what your specific BUN result means for you.

Reference ranges can vary slightly between laboratories, but according to Mayo Clinic Laboratories, the typical range for adults aged 18 to 60 is 7 to 20 mg/dL (milligrams per deciliter). For adults over 60, the range is 8 to 23 mg/dL, reflecting the natural, gradual decline in kidney filtration that comes with age. Keep in mind that a result near the edge of a range is not automatically a cause for concern — your doctor evaluates your number in the context of your overall health, diet, hydration, and other lab values. Always discuss your specific result with your healthcare provider.

A BUN result above the reference range can have several causes, and dehydration is one of the most common — when you are not drinking enough fluids, less blood reaches the kidneys, so urea becomes more concentrated. Other causes include chronic kidney disease, acute kidney injury, a high-protein diet, heart failure, gastrointestinal bleeding (because digested blood protein raises BUN), severe infection or fever, and certain medications such as corticosteroids or tetracyclines. A single elevated reading does not confirm a diagnosis on its own. Your healthcare provider is the right person to explain what a high BUN result means for your individual situation.

A BUN result that falls below the normal range is less common than a high one and usually points to reduced urea production rather than a kidney problem. Possible causes include a low-protein diet or malnutrition, liver disease (which impairs the liver's ability to produce urea), overhydration (extra fluid dilutes BUN in the blood), pregnancy (increased blood volume and higher kidney filtration lower BUN naturally), and celiac disease (where malabsorption reduces the amount of protein available to break down). A low BUN on its own is often not alarming, but it can be a useful clue. Discuss your result with your healthcare provider to understand what it means for you.

When BUN is mildly elevated, many people have no symptoms at all — the number shows up on a routine panel without any obvious warning signs. If BUN climbs higher, especially due to reduced kidney function, you might experience fatigue and weakness, nausea or vomiting, loss of appetite, swelling in the legs and ankles, changes in urination, or in severe cases, confusion and difficulty concentrating. Low BUN typically causes no direct symptoms on its own, though you might notice signs of malnutrition if inadequate protein intake is the root cause. Symptoms are not a reliable way to judge your BUN level, so contact your healthcare provider to discuss what your result means.

The VERIFIED_DATA for this marker does not include specific pre-test preparation instructions, so this content cannot confirm fasting requirements here. What is worth knowing from the verified data is that factors like how much water you have been drinking, your recent protein intake, and certain medications can all influence your BUN result. A high-protein meal or dehydration before the draw, for example, could push your number higher than your baseline. Before your blood draw, ask your healthcare provider or the ordering lab whether you need to fast, adjust your fluid intake, or temporarily pause any medications to get the most accurate reading.