Urea nitrogen is a waste product your body creates when it breaks down protein. Your liver converts a toxic compound called ammonia into urea, which then travels through your bloodstream to your kidneys, where it’s filtered out in urine. A blood urea nitrogen (BUN) test measures how much of this waste is circulating in your blood, and the result is one of the most common ways to check how well your kidneys are working.
How Your Body Produces Urea Nitrogen
Every time your body breaks down protein, whether from food you eat or from your own muscle tissue during fasting, ammonia is released as a byproduct. Bacteria in your gut also naturally produce ammonia. The problem is that ammonia is toxic, especially to the brain. If it builds up, it can cause swelling in the brain, confusion, slurred speech, seizures, and blurred vision.
To prevent this, your liver runs a process called the urea cycle. This energy-intensive series of chemical reactions, which takes place inside liver cells, converts ammonia into urea, a much safer compound. Urea dissolves easily in blood and travels to the kidneys, where it’s filtered out. About 40% of the urea that gets filtered is actually reabsorbed back into the bloodstream (your body recycles some of it), and the rest leaves your body in urine.
Normal BUN Levels by Age and Sex
BUN is measured in milligrams per deciliter (mg/dL) in the United States. Normal ranges vary by age and sex:
- Infants and toddlers (0 to 2 years): 2 to 19 mg/dL
- Children (3 to 12 years): 5 to 17 mg/dL
- Teens (13 to 18 years): 7 to 18 mg/dL
- Adult females: 6 to 21 mg/dL
- Adult males: 8 to 24 mg/dL
Outside the U.S., urea is often reported in millimoles per liter (mmol/L). To convert, multiply your BUN in mg/dL by 0.357 to get urea in mmol/L, or multiply mmol/L by 2.8 to get mg/dL.
What High Urea Nitrogen Means
An elevated BUN doesn’t automatically mean your kidneys are failing. The causes fall into three broad categories depending on where the problem originates.
Reduced Blood Flow to the Kidneys
This is the most common reason for elevated BUN. Your kidneys need steady blood flow to filter waste. Anything that reduces that flow, including dehydration, significant blood loss, heart failure, liver failure, burns, or regular use of common painkillers like ibuprofen and aspirin, can cause urea to accumulate in your blood. The kidneys themselves may be perfectly healthy in these cases. Once blood flow is restored, BUN typically drops back to normal.
Direct Kidney Damage
Infections, sepsis, blood clots in the kidneys, certain medications (including some chemotherapy drugs and antibiotics like penicillin), and toxins from drugs or alcohol can injure the kidney tissue itself. Inflammation from conditions like vasculitis can also cause this type of damage. Recovery depends on how severe and prolonged the injury is.
Urinary Tract Blockages
When urine can’t drain properly, urea backs up into the bloodstream. Kidney stones, urinary tract infections, and certain cancers can block the tubes connecting your kidneys to your bladder. Relieving the blockage typically resolves the elevated BUN.
Your provider can often distinguish between these causes by looking at the ratio of BUN to creatinine (another waste product filtered by the kidneys). A ratio above 20:1 generally suggests the problem is reduced blood flow rather than direct kidney damage, which tends to produce a lower ratio.
What Low Urea Nitrogen Means
Low BUN is less common and usually less urgent, but it can signal a few things worth knowing about. Since the liver produces urea, severe liver disease can lower BUN because the organ can’t perform the conversion efficiently. Malnutrition or a very low protein diet reduces the raw material for urea production in the first place. Overhydration, where excess fluid in the body dilutes the blood, can also push BUN below normal ranges.
How Diet Affects Your Results
What you eat in the days before a BUN test can meaningfully shift your number. In a study of healthy young men, eating double the normal amount of protein (2.4 grams per kilogram of body weight per day, versus 1.2 grams) for just seven days produced significantly higher BUN levels. That’s roughly the difference between eating a moderate amount of chicken and fish each day versus loading up on protein shakes and steaks at every meal. This doesn’t indicate kidney problems. It simply reflects your liver processing more protein and generating more urea as a result.
This is why context matters when reading your BUN result. A single high number in someone who just finished a week of heavy protein intake means something very different from the same number in someone eating normally.
What to Expect From the Test
A BUN test is a simple blood draw, typically from a vein in your arm. No fasting is usually required for BUN alone, though if your provider has ordered additional blood tests alongside it, you may be asked to skip food and drinks for several hours beforehand. Results are often available within a day.
BUN is rarely interpreted in isolation. It’s almost always read alongside creatinine levels and sometimes a full metabolic panel. The combination gives a much clearer picture of kidney function than any single number. A BUN that looks slightly out of range but pairs with a normal creatinine and normal urine output is a very different clinical picture from one where all three markers are off.