A normal BUN (blood urea nitrogen) level for adults falls between 6 and 24 mg/dL, though the exact range depends on your sex and age. This is one of the most common markers on a routine blood panel, and it reflects how well your kidneys are filtering waste from your blood.
Normal Ranges by Age and Sex
BUN measures the amount of urea nitrogen in your blood. Urea is a waste product your liver creates when it breaks down protein, and your kidneys are responsible for clearing it out. When everything is working normally, BUN stays within a predictable range.
For adults 18 and older, the normal ranges are:
- Women: 6 to 21 mg/dL
- Men: 8 to 24 mg/dL
Children ages 1 through 17 typically fall between 7 and 20 mg/dL regardless of sex. Reliable reference ranges haven’t been established for infants under 12 months, so results in that age group require more individualized interpretation from a pediatrician.
Older adults tend to run slightly higher because kidney filtration naturally slows with age. A BUN of 22 or 23 in a healthy 75-year-old may not carry the same significance as the same number in a 30-year-old.
What BUN Actually Tells You
Your body constantly breaks down protein from food and from its own tissues. The liver converts the nitrogen released during that process into urea, which travels through the bloodstream to the kidneys. The kidneys filter it out and send it into the urine. BUN is essentially a snapshot of that whole chain: protein breakdown, liver processing, and kidney clearance.
Because so many steps are involved, an abnormal BUN level doesn’t automatically point to one specific problem. It’s a signal that something in the chain is off, and your doctor will look at it alongside other markers, especially creatinine, to narrow down the cause. The ratio of BUN to creatinine is often more informative than either number alone.
Why BUN Can Run High
An elevated BUN level doesn’t always mean kidney disease. Some of the most common causes are surprisingly mundane. Dehydration is a frequent culprit: when your body is low on fluid, the kidneys concentrate urea more, and BUN rises. A high-protein diet can push levels up for the same reason, since more protein means more urea to process. Even a recent steak dinner can temporarily bump the number.
Burns, a recent heart attack, and conditions that reduce blood flow to the kidneys can also raise BUN. Heart failure, for instance, slows blood delivery to the kidneys, making them less efficient at clearing waste.
Several common medications raise BUN as well. Diuretics (water pills) are among the most frequent offenders because they reduce fluid volume. Certain antibiotics, some seizure medications, and steroids can also push levels higher without necessarily indicating kidney damage. If you’re on any of these and see an elevated result, that context matters.
When BUN is genuinely elevated because of kidney problems, it usually shows up alongside a high creatinine level and possibly other signs like changes in urination, swelling, or fatigue.
Why BUN Can Run Low
Low BUN gets less attention but still has meaningful causes. The most common are low protein intake and liver disease. Since the liver is responsible for creating urea in the first place, a liver that isn’t functioning well produces less of it, and BUN drops. Malnutrition or a very low-protein diet has the same effect simply because there’s less protein being broken down.
Overhydration, whether from drinking excessive fluids or from IV fluids in a hospital setting, dilutes the blood and lowers the concentration of urea.
BUN Levels During Pregnancy
If you’re pregnant and notice your BUN looks unusually low, that’s expected. Blood volume expands by 40 to 50 percent during pregnancy, which dilutes waste products in the bloodstream. On top of that, your kidneys ramp up their filtration rate significantly, clearing urea faster than usual.
Normal pregnancy ranges are noticeably lower than the standard adult range:
- First trimester: 7 to 12 mg/dL
- Second trimester: 3 to 13 mg/dL
- Third trimester: 3 to 11 mg/dL
A BUN of 5 mg/dL that would prompt questions in a non-pregnant adult is perfectly normal at 30 weeks.
What Can Affect Your Results
Several everyday factors can shift your BUN reading without reflecting any real health change. Dehydration is the big one. If you gave blood on a hot day after skipping water, your BUN may read higher than your true baseline. A high-protein meal the night before can do the same. Pregnancy, aging, and steroid use all influence results as well.
Fasting isn’t strictly required before a BUN test in most cases, but if it’s being drawn as part of a larger metabolic panel, you may be asked to fast for 8 to 12 hours for the sake of other tests on the same panel. Staying well-hydrated with water before your blood draw gives the most representative result.
If your BUN comes back outside the normal range, a single reading in isolation rarely tells the whole story. The number becomes much more useful when your doctor compares it with creatinine, looks at trends over multiple tests, and considers what medications you’re taking and how hydrated you were on the day of the draw.