What Is BUN (Blood Urea Nitrogen)?
The blood test for the nitrogen waste your kidneys remove.
Medically reviewed by Dr. Rishi Kumar Kafle, MBBS, MD, FASN · Last reviewed June 2026
Blood urea nitrogen (BUN) measures the nitrogen carried by urea in the blood. Urea is a waste product the liver makes from protein, and the kidneys remove it. The normal adult range is about 7–20 mg/dL. Because the value rises and falls with hydration, diet, and kidney function at the same time, BUN is almost always read alongside creatinine rather than on its own.
Where BUN Comes From
When the body breaks down protein, the liver converts the leftover nitrogen into urea, a small, water-soluble molecule that travels in the blood to the kidneys for disposal. The kidneys filter urea at the glomerulus, then reabsorb a portion of it back into the body — and how much they reabsorb depends on how fast urine is flowing. That single fact explains most of what makes BUN useful and most of what makes it tricky: when the body is holding on to water, it holds on to more urea, and the blood level climbs even though the kidneys themselves may be healthy.
A BUN result is reported in milligrams per deciliter (mg/dL) in the United States. Outside the U.S. the same analyte is often reported as blood urea in mmol/L, which is a different number for the same information. The two are not interchangeable without converting, so always check the units and the lab's own reference range printed beside the result.
What a Normal BUN Looks Like
For most adults a BUN of 7–20 mg/dL is considered normal, though the exact cutoffs vary slightly between laboratories. Values tend to run a little higher in older adults and a little lower in children and during pregnancy, when plasma volume expands. A result just outside the range is rarely meaningful by itself; the pattern over time and the matching creatinine carry far more weight than one isolated number.
What Affects BUN
Several everyday factors move BUN in predictable directions, which is why a single value needs context before it means anything:
| Factor | Effect on BUN | Why |
|---|---|---|
| Dehydration | Raises BUN | Slower urine flow lets the kidney reabsorb more urea |
| High-protein diet | Raises BUN | More protein breakdown makes more urea |
| Gastrointestinal bleeding | Raises BUN | Digested blood is a protein load |
| Reduced kidney function | Raises BUN | Less urea is filtered out |
| Liver disease | Lowers BUN | A failing liver makes less urea |
| Low-protein diet or overhydration | Lowers BUN | Less urea made, or more diluted |
Consider three quick examples. A hiker who has not had water all day may show a BUN of 28 mg/dL purely from dehydration, with a normal creatinine. A person on a very high-protein diet might sit at 22 mg/dL with healthy kidneys. And someone with advanced liver disease can have a BUN of just 5 mg/dL despite reduced kidney function, because the liver is no longer producing much urea at all. In each case the BUN alone would mislead.
High and Low BUN
A high BUN (above about 20 mg/dL) most often reflects dehydration or a high protein load, but it can also signal reduced kidney filtration, heart failure that lowers blood flow to the kidneys, or bleeding in the gut. A low BUN (below about 7 mg/dL) usually points to liver disease, a low-protein diet, or overhydration. Neither direction is diagnostic on its own — each is a prompt to look at the creatinine, the clinical picture, and the trend.
BUN vs Creatinine
Because BUN responds strongly to hydration and protein, it is less specific to the kidneys than creatinine, which the body produces at a far steadier rate from muscle. Creatinine is the more dependable marker of filtration, while BUN adds information about volume status and protein turnover. Reading the two together as the BUN/creatinine ratio helps separate dehydration and other pre-renal causes from a true kidney problem. A ratio above 20:1 leans toward a pre-renal cause such as dehydration; a ratio inside the usual 10:1–20:1 band fits a problem within the kidney itself.
For the bigger picture of filtration, neither BUN nor creatinine is used alone — they feed estimates such as creatinine clearance and eGFR, which translate a blood level into an actual filtration rate. BUN is best thought of as a supporting clue rather than a stand-alone verdict on kidney health.
When BUN Is Measured
BUN is rarely ordered alone. It is part of the basic metabolic panel and comprehensive metabolic panel — routine blood tests that screen kidney and overall metabolic health. Clinicians look at it when assessing hydration, before starting drugs that the kidneys clear, when monitoring known kidney disease, and during illnesses where fluid balance shifts quickly, such as vomiting, diarrhea, or heart failure. Because the value reflects several systems at once, it earns its place as a broad-strokes screen rather than a precise kidney measurement.
How to Prepare and What Affects the Result
No special fasting is usually required for a BUN test, though a panel ordered with it may call for fasting. Two things commonly distort a reading. First, a high-protein meal or supplement the day before can raise BUN temporarily. Second, hydration status on the morning of the draw matters: arriving dehydrated nudges the value up. Several medications also move BUN, including corticosteroids and some diuretics, which is why the result is always read in the context of the whole picture rather than as a stand-alone verdict.
Why the Kidney Reabsorbs Urea but Not Creatinine
The central reason BUN and creatinine behave differently comes down to how the kidney handles each molecule after filtering it. Both are filtered freely at the glomerulus, but urea is small and follows water: when the tubules reabsorb water to conserve fluid, a share of urea is dragged back into the blood with it. Creatinine is not reabsorbed in any meaningful amount, so its blood level tracks filtration far more faithfully. This is exactly why dehydration raises BUN out of proportion to creatinine and pushes the BUN/creatinine ratio upward — the kidney is conserving water and urea together while creatinine keeps leaving at its usual pace.
The same mechanism explains why BUN can look reassuringly normal in someone with genuine kidney disease who is also overhydrated or eating little protein. Two opposing influences — reduced filtration pushing BUN up, dilution and low protein pulling it down — can cancel out, leaving a normal-looking number that hides a real problem. It is one more reason the value is never read alone, and why eGFR built from creatinine remains the more reliable measure of filtration.
Calculate the Ratio
To turn your BUN and creatinine into the ratio that separates pre-renal from kidney causes, use the calculator below.