What Is BUN (Blood Urea Nitrogen)?

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 and then reabsorb part of it. How much they reabsorb depends on how fast urine is flowing. This reabsorption explains much of why BUN is useful and much of why it can be hard to interpret. 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 trend over time and the matching creatinine matter far more than one isolated number.

What Affects BUN

Several everyday factors move BUN in predictable directions, so a single value needs context before you can interpret it:

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, BUN read alone would give a misleading picture.

High and Low BUN

A high BUN (above about 20 mg/dL) most often reflects dehydration or a high protein load. 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 one 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.

To estimate the filtration rate itself, a blood level is not used alone. Creatinine, not BUN, is the input to estimates such as creatinine clearance and eGFR, which translate a blood level into an actual filtration rate. BUN is best read 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 works as a broad screening test rather than a precise measurement of kidney function.

How to Prepare and What Affects the Result

A BUN test usually needs no special fasting, 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, so the result is always read against the whole clinical picture rather than on its own.

Why the Kidney Reabsorbs Urea but Not Creatinine

BUN and creatinine behave differently because of 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 why dehydration raises BUN out of proportion to creatinine and pushes the BUN/creatinine ratio upward. The kidney conserves 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 and leave a normal-looking number that hides a real problem. This is one more reason BUN 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.