BUN stands for blood urea nitrogen, a waste product your kidneys filter out of your blood. A normal BUN level for most adults falls between 7 and 20 mg/dL, though this range shifts with age and sex. It’s one of the most common markers on routine blood work, and it gives a quick snapshot of how well your kidneys are clearing waste.
How Your Body Produces Urea
When you eat protein, your digestive system breaks it down into amino acids. More than 90% of those amino acids travel to the liver, where they’re processed for energy and other uses. The leftover nitrogen from that processing gets converted into urea, a relatively harmless compound, through a series of chemical reactions called the urea cycle. Over 99% of urea production happens in the liver.
Your kidneys then filter urea out of the blood and send it into your urine. On an average day, your body excretes about 10 grams of urea this way. At normal urine flow rates, roughly 40% of the filtered urea gets reabsorbed back into the blood, which is why some urea is always circulating. When you’re dehydrated and urine flow drops, reabsorption can climb to 60%, which is one reason dehydration alone can push your BUN higher.
What Normal BUN Looks Like
Most labs in the United States report BUN in milligrams per deciliter (mg/dL), with a general reference range of about 7 to 20 mg/dL for adults. Outside the U.S., labs often measure urea in millimoles per liter (mmol/L). To convert between the two, multiply BUN in mg/dL by 0.357 to get mmol/L, or multiply mmol/L by 2.8 to get mg/dL.
Your expected range depends on your age and sex. BUN levels gradually rise as you get older, largely because kidney filtration naturally slows with age. Men tend to run slightly higher than women at every age. For example, men in their 20s and 30s typically center around 4.4 to 4.5 mmol/L (about 12 to 13 mg/dL), while women in the same age range center closer to 3.8 to 3.9 mmol/L (about 11 mg/dL). By age 60 to 69, the midpoint for men rises to about 5.4 mmol/L (15 mg/dL) and for women to about 5.0 mmol/L (14 mg/dL).
A single BUN reading slightly outside the reference range isn’t necessarily alarming. Hydration status, what you ate the day before, and even the time of day can all nudge the number. Restricting water while fasting for a blood draw, for instance, can artificially raise urea levels. Making sure you’re adequately hydrated before the test gives the most accurate result.
What a High BUN Level Means
An elevated BUN most commonly signals that your kidneys aren’t filtering waste as efficiently as they should. But it doesn’t always mean kidney disease. Several non-kidney factors can push BUN up:
- Dehydration: Less fluid moving through the kidneys means more urea gets reabsorbed back into the blood.
- High-protein diet: More protein in means more urea produced by the liver.
- Burns or severe tissue injury: Damaged tissue releases extra protein into the bloodstream.
- Certain medications: Some drugs affect kidney blood flow or urea production.
- Heart problems: A recent heart attack or heart failure can reduce blood flow to the kidneys, slowing filtration.
This is why doctors rarely interpret BUN alone. They almost always look at it alongside creatinine, another kidney waste marker. The ratio between BUN and creatinine helps distinguish what’s going on. A BUN-to-creatinine ratio above 20 often suggests the kidneys themselves are fine but aren’t getting enough blood flow, a situation called prerenal azotemia. This happens with dehydration, heart failure, or heavy bleeding. A ratio between 15 and 20 is generally considered the healthiest range. Ratios above 40 are associated with significantly higher risk of serious complications.
What a Low BUN Level Means
Low BUN is less common and usually points away from the kidneys. Since the liver produces nearly all urea, severe liver disease can lower BUN because the organ can no longer run the urea cycle efficiently. Other causes include malnutrition or a very low-protein diet (less raw material for urea production) and overhydration, where excess fluid in the body dilutes the concentration of urea in the blood. Pregnancy can also lower BUN because blood volume expands significantly, diluting urea, and the kidneys filter at a faster rate.
Symptoms When BUN Gets Very High
Mildly elevated BUN often produces no symptoms at all, which is why it’s usually caught on routine lab work. But when kidney function deteriorates enough that urea and other waste products build up significantly, a condition called uremia, symptoms become noticeable and can affect nearly every system in the body.
Early signs tend to be vague: fatigue, nausea, loss of appetite, and a bad or metallic taste in the mouth. Muscle cramps, restless legs, and itchy skin are also common. As waste accumulates further, neurological symptoms can develop gradually. These include difficulty concentrating, forgetfulness, confusion, drowsiness, and emotional instability. Without treatment, severe uremia can progress to seizures, stupor, or coma.
In advanced cases, physical signs become more dramatic. Fluid can accumulate in the lungs or legs, causing swelling and shortness of breath. The breath may take on a urine-like odor, sometimes called uremic fetor. In rare, extreme situations, urea crystals can deposit on the skin as a whitish residue known as uremic frost. These severe signs indicate a medical emergency.
How BUN Fits Into Your Blood Work
BUN is typically part of a basic metabolic panel or a comprehensive metabolic panel, two of the most commonly ordered blood tests. Your doctor may order it to monitor existing kidney disease, check on how well a treatment is working, or simply screen for kidney problems during a routine checkup.
The test itself requires a standard blood draw. Fasting is sometimes requested depending on what other tests are being run at the same time, but BUN itself doesn’t strictly require fasting. The key preparation step is staying well hydrated beforehand. Skipping water unnecessarily while fasting can raise urea levels and make results look worse than they actually are.
If your BUN comes back elevated, the next step is usually looking at the full picture: your creatinine level, the BUN-to-creatinine ratio, your hydration status, medications you’re taking, and your recent diet. A single high reading after a steak dinner and an early-morning blood draw on an empty stomach tells a very different story than a persistently elevated BUN across multiple tests. Context turns a number on a page into useful information.