The BUN medical abbreviation means blood urea nitrogen, a measure of the nitrogen carried in urea, a waste product made when the body breaks down protein. The liver makes urea and the kidneys filter it into urine, so the BUN level gives clues about both kidney function and fluid balance.
BUN is part of the routine chemistry panels, the BMP and CMP, and is almost always read next to creatinine. You will see it in admission labs, kidney notes and portal results. This page covers what the number means, typical ranges, the BUN to creatinine ratio and the common reasons it rises or falls.
At a glance
Stands for
Blood urea nitrogenNormal range
Often about 7 to 20 mg/dL in adults; labs varyRead with
Creatinine (Cr) and GFRFound in
Units
mg/dL in the US; mmol/L where urea is reported directlyDo not confuse with
Serum urea, which some countries report instead of BUN
Understanding the BUN medical abbreviation
When you eat protein, or when the body breaks down its own tissue, nitrogen is left over as ammonia. The liver turns that ammonia into urea, which is less harmful, and releases it into the blood. Healthy kidneys filter urea out, and it leaves in urine. BUN measures how much urea nitrogen is in the blood at that moment.
A BUN level reflects a balance: how much urea is being made and how well it is being removed. That is why it changes with diet, hydration, bleeding in the gut and liver health, not just kidney function. Clinicians rarely judge it alone.
BUN and creatinine together
Creatinine is a waste product from muscle that the kidneys also clear, and it is a steadier marker of filtering than BUN. Comparing the two helps sort out why a value is off.
| Pattern | A common interpretation |
|---|---|
| Ratio about 10 to 20 | Typical balance |
| Ratio above about 20 | Often low blood flow to the kidneys, such as dehydration, or bleeding in the upper gut |
| Both raised with a ratio near normal | Can suggest the kidneys themselves are filtering less |
These are rules of thumb used in teaching, not firm cutoffs. Your clinician uses them alongside your history, exam and urine tests.
Sample lines with BUN
In an emergency department note: "BUN 38, Cr 1.2, ratio greater than 20: likely prerenal, give IV fluids and recheck." Prerenal means the problem is before the kidney, usually not enough fluid reaching it. The plan is fluids and a repeat test.
Lab results may also show BUN inside a fishbone shaped chemistry diagram, where sodium, chloride and BUN sit on the top row and potassium, bicarbonate and creatinine sit below, with glucose off to the side. Students learn this layout early.
Common reasons BUN goes up or down
- Higher: dehydration, reduced kidney function as in AKI or CKD, bleeding in the stomach or upper intestine, heart failure, a very high protein diet, and some medicines such as steroids
- Lower: severe liver disease, low protein intake or malnutrition, and extra fluid in the body; it can also be lower in pregnancy
A single high or low result often gets repeated. Trend over days tends to tell more than one number.
Where BUN fits among kidney abbreviations
Kidney work ups pull in a cluster of shorthand. Cr is creatinine. GFR or eGFR is the estimated filtering rate. UA is urinalysis. AKI is acute kidney injury. You may also hear azotemia, an older word for a raised BUN and creatinine. Dozens more sit in our roundup of test shorthand.
One kidney number rarely tells the whole story. Your care team can put your BUN in context with the rest of your results and say whether anything needs to happen next.
Frequently asked questions
What does BUN stand for in a blood test?
BUN stands for blood urea nitrogen. It measures urea, a waste product of protein breakdown that the kidneys filter out.
What is a normal BUN level?
Many labs list about 7 to 20 mg/dL in adults; the exact span differs between labs and age groups. Check the range printed next to your result.
What does a high BUN mean?
It can reflect dehydration, reduced kidney function, bleeding in the upper gut, a high protein diet or some medicines. Your clinician reads it with creatinine and other tests.
Is BUN the same as creatinine?
No. Both are waste products cleared by the kidneys, but BUN comes from protein breakdown and creatinine comes from muscle. Creatinine is the steadier marker of kidney filtering.
Why do doctors look at the BUN to creatinine ratio?
The ratio helps suggest whether a problem comes from low fluid or blood flow to the kidneys, from the kidneys themselves, or from something like gut bleeding. It is a guide, not a diagnosis.