BUN (Blood Urea Nitrogen): Guide
Learn what BUN is, its normal ranges (7–20 mg/dL), why it may be high or low, and what to do next.
Updated: 9/21/2026
Reference ranges (typical adult values)
| Who | Range | Unit |
|---|---|---|
| Adults | 7 – 20 | mg/dL (mmol/L) |
| Adults(optimal) | 10 – 16 | mg/dL (mmol/L) |
Ranges differ between laboratories — the range printed on your own report always applies first.
What is BUN
BUN (Blood Urea Nitrogen) measures the amount of urea nitrogen in your blood. Urea is produced in the liver as a final product of protein metabolism: when your body breaks down proteins, ammonia is formed, which the liver converts to urea, and the kidneys then remove it through urine. The BUN test evaluates the kidneys' ability to filter urea from the blood and is one of the most important kidney function markers alongside creatinine and eGFR.
Reference Ranges
The normal BUN range for adults is typically 7–20 mg/dL (or 2.5–7.1 mmol/L in SI units). The optimal range used in preventive medicine is narrower: 10–16 mg/dL. Older adults may have slightly higher normal values. Always follow the reference ranges provided by your own laboratory.
Why BUN May Be High
Elevated BUN most commonly indicates reduced kidney function — the kidneys are unable to efficiently remove urea. Dehydration is a very common cause: when the body lacks fluids, blood becomes more concentrated and BUN rises. High-protein diets, intense physical exercise, or increased muscle breakdown also raise BUN levels. Heart failure can reduce blood flow through the kidneys and increase BUN. Gastrointestinal bleeding raises BUN because blood proteins are broken down in the liver. Certain medications (diuretics, NSAIDs) may also have an impact.
Why BUN May Be Low
Low BUN may indicate insufficient protein intake in the diet or severe liver disease, when the liver cannot effectively produce urea. During pregnancy, BUN physiologically decreases due to increased blood volume. Excessive fluid intake can also artificially lower the BUN value. In rare cases, low BUN may indicate malabsorption conditions.
What to Do Next
If your BUN is elevated, first consider the simplest causes: whether you are drinking enough water, whether there was intense exercise before the test, or whether you consume a lot of protein-rich foods. Your doctor typically evaluates BUN together with creatinine and calculates the BUN-to-creatinine ratio, which helps distinguish kidney-related from non-kidney causes. If consistently elevated BUN recurs across multiple tests, a more thorough kidney function evaluation may be warranted.
Tracking Over Time
BUN trends are more valuable than a single measurement. A gradual increase in BUN over several tests may indicate slowly declining kidney function. Fluctuations related to fluid intake or diet are normal. Compare BUN with creatinine and eGFR — if all three markers point in the same direction, the information is more reliable. Testing at least once a year is recommended, more frequently if kidney disease risk factors are present.
Questions for Your Doctor
- Could my elevated BUN be related to dehydration or diet?
- What is my BUN-to-creatinine ratio and what does it indicate?
- Do I need additional kidney function tests?
- How often should I repeat this test?
*This is not medical advice. Discuss health decisions with your doctor.*