BUN (blood urea nitrogen) test: what it means
What it measures
A BUN (blood urea nitrogen) test measures the amount of urea nitrogen in your blood. Urea nitrogen is a waste product that forms when your body breaks down protein; it is carried in the blood and normally removed by the kidneys through urine.
Why it’s ordered
A BUN test is commonly done to check how well the kidneys are working. It may be ordered as part of a routine checkup, to help monitor a kidney condition or its treatment, or to screen people at higher risk (for example, those with a family history of kidney problems, diabetes, high blood pressure, or heart disease). It is often included in a comprehensive metabolic panel (CMP).
General reference range
MedlinePlus gives a general normal result of about 6 to 20 mg/dL. This is a general reference only. Normal values vary from one laboratory to another and can also differ by age and sex (BUN tends to increase with age and can vary by sex), as well as by the testing method a lab uses. The range printed on your own lab report is what applies to your result, and a single value outside the general range is not a diagnosis on its own. Talk to your provider about your specific results.
If it’s high
A higher-than-normal BUN may suggest the kidneys are not clearing waste as well as usual, but it can also be seen with a number of non-kidney situations. General associations MedlinePlus lists include dehydration, a high-protein diet or excess protein in the digestive tract, gastrointestinal bleeding, congestive heart failure, a recent heart attack, shock, urinary tract obstruction, certain medicines, and various kidney diseases or kidney failure. This is a general, non-exhaustive list, not a diagnosis. Only your clinician can interpret what an elevated result means for you.
If it’s low
A lower-than-normal BUN is less commonly a focus, but per MedlinePlus it may be seen with a low-protein diet, malnutrition, liver disease or liver failure, or overhydration (too much fluid). These are again general possible associations, not a complete list and not a diagnosis.
A result is not a diagnosis
Reference ranges vary by laboratory, age, sex, and method — the range on your own report is what applies to you. A single value out of range doesn’t confirm any condition; your clinician interprets it alongside your symptoms, history, and other results. This page is general information, not medical advice.
Medicines that can move this result
ACE inhibitors relax the outflow vessel of the kidney's filter, which drops filtration pressure slightly, so urea clears more slowly. The label describes the rise as minor and reversible once the drug is stopped, and more common alongside a diuretic.
Source ↗Heavy fluid loss means less blood volume reaching the kidney, so urea gets reabsorbed rather than excreted and BUN climbs. Dehydration, not kidney damage, is the usual cause, and the label calls the rise reversible.
Source ↗As a diuretic it lowers circulating fluid volume, slowing how fast the kidney clears urea. The label calls this transient and flags it as most noticeable when kidney function was already reduced.
Source ↗Tetracycline antibiotics have an "anti-anabolic" effect — they tip the body toward breaking protein down rather than building it, and urea is the waste product of that breakdown, so BUN can rise without the kidneys being injured. The label notes this is not a problem with normal kidney function, but can tip into azotemia when function is significantly impaired.
Source ↗Oral corticosteroids speed the breakdown of the body's own protein, and the nitrogen waste from that protein is what a BUN test measures — so the number can rise while the kidneys work normally. A high-protein diet does the same thing, which is why diet is worth mentioning to whoever reads the result.
Source ↗NSAIDs cut the prostaglandins that hold blood flow through the kidney steady, so filtration falls and urea backs up. The label names the people this matters most for: impaired kidney function, heart failure, liver dysfunction, anyone on a diuretic or ACE inhibitor, and the elderly.
Source ↗IV vancomycin can injure the kidney's tubules and the tissue around them — interstitial nephritis, less often acute tubular necrosis — and the label names a rising BUN as one of the blood signals that this is happening. It is why drug levels and kidney tests are checked during a course.
Source ↗Aminoglycoside antibiotics accumulate in the cells lining the kidney tubule and can damage them, so BUN and creatinine are tracked through treatment as an early warning.
Source ↗Do not stop a prescribed medicine over a lab number. These are recognised associations, not a finding about your result — your value may have nothing to do with any medicine you take, and the list is not complete. Bring it to whoever prescribed the drug; they can weigh it against why you are taking it.
Frequently asked
- What does the BUN (blood urea nitrogen) test measure?
- A BUN (blood urea nitrogen) test measures the amount of urea nitrogen in your blood. Urea nitrogen is a waste product that forms when your body breaks down protein; it is carried in the blood and normally removed by the kidneys through urine.
- What is a normal BUN (blood urea nitrogen) test range?
- MedlinePlus gives a general normal result of about 6 to 20 mg/dL. This is a general reference only. Normal values vary from one laboratory to another and can also differ by age and sex (BUN tends to increase with age and can vary by sex), as well as by the testing method a lab uses. The range printed on your own lab report is what applies to your result, and a single value outside the general range is not a diagnosis on its own. Talk to your provider about your specific results.
- What does a high result mean?
- A higher-than-normal BUN may suggest the kidneys are not clearing waste as well as usual, but it can also be seen with a number of non-kidney situations. General associations MedlinePlus lists include dehydration, a high-protein diet or excess protein in the digestive tract, gastrointestinal bleeding, congestive heart failure, a recent heart attack, shock, urinary tract obstruction, certain medicines, and various kidney diseases or kidney failure. This is a general, non-exhaustive list, not a diagnosis. Only your clinician can interpret what an elevated result means for you.
- What does a low result mean?
- A lower-than-normal BUN is less commonly a focus, but per MedlinePlus it may be seen with a low-protein diet, malnutrition, liver disease or liver failure, or overhydration (too much fluid). These are again general possible associations, not a complete list and not a diagnosis.
Other lab tests
Sources
General reference, not medical advice, and not a substitute for your clinician. Lab reference ranges and interpretation depend on the laboratory and on your individual situation — discuss your results with a licensed healthcare professional.