BUN (blood urea nitrogen) measures a waste product your liver makes from protein and your kidneys remove.
A typical adult result is about 6 to 20 mg/dL. Labs differ, and the number tends to rise with age.
High BUN most often reflects dehydration or reduced kidney filtering. Low BUN usually reflects low protein intake, extra fluid, or reduced liver output.
BUN can't judge kidney function alone. Creatinine and eGFR carry more weight.
November 11, 2026
BUN Blood Test: Normal Range and What High or Low Means
Key Takeaways
BUN (blood urea nitrogen) measures a waste product your liver makes from protein and your kidneys remove.
A typical adult result is about 6 to 20 mg/dL. Labs differ, and the number tends to rise with age.
High BUN most often reflects dehydration or reduced kidney filtering. Low BUN usually reflects low protein intake, extra fluid, or reduced liver output.
BUN can't judge kidney function alone. Creatinine and eGFR carry more weight.
A BUN blood test measures the amount of urea nitrogen in your blood. Urea is a waste product your liver makes when it breaks down protein, and your kidneys filter it out. A typical adult result is about 6 to 20 mg/dL.
Most people meet BUN as one flagged line on a metabolic panel. The useful thing to know is that the number has two inputs: how much urea you're making and how well you're clearing it. A high or low result can come from either side.
BUN test results at a glance
Result
Typical range
Common explanations
Usual next check
Low
Below about 6 mg/dL
Low protein intake, extra fluid, reduced liver output
Liver enzymes, albumin, diet review
Normal
About 6 to 20 mg/dL
Urea production and clearance in balance
None if creatinine is normal
High
Above about 20 mg/dL
Dehydration, reduced kidney filtering, high protein intake, bleeding in the gut
Creatinine, eGFR, repeat when hydrated
Ranges vary by lab. Read your number against the range on your report.
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BUN stands for blood urea nitrogen, the nitrogen portion of urea circulating in your blood. When you digest protein, your body produces ammonia, which is toxic. Your liver converts it to urea, and urea is the main route by which your kidneys carry that nitrogen out in urine.
"Urea nitrogen is a waste product that forms as your body breaks down proteins." - Source: MedlinePlus, U.S. National Library of Medicine
BUN is reported alongside creatinine and electrolytes such as carbon dioxide and chloride. It's rarely ordered alone.
What is a normal BUN level?
A normal BUN is generally 6 to 20 mg/dL, according to the MedlinePlus Medical Encyclopedia. Some labs set the upper limit a few points higher. MedlinePlus also notes that normal levels differ by sex and usually increase with age.
A result one or two points outside the range, with a normal creatinine, is common and often temporary.
What does a high BUN mean?
A high BUN means you're making more urea than usual, clearing less of it, or both. Sorting out which input moved is the whole job.
Dehydration and low blood flow
Dehydration is the most common reason for a mildly high BUN. When your body conserves water, your kidneys pull more urea back into the blood along with it. A stomach bug, a long flight, or not drinking enough water can do it.
Anything that lowers blood flow to the kidneys has the same effect. MedlinePlus lists congestive heart failure and shock among the causes, which is why BUN is often read with heart markers such as BNP.
Reduced kidney filtering
When the kidneys filter less, urea builds up. This is the cause people worry about, and it's more common than most realize.
More protein in means more urea out. A high-protein diet can nudge BUN up without any kidney problem. Whether years of very high protein intake harms healthy kidneys is still unsettled; a 2020 review in the Journal of the American Society of Nephrology says it's possible but not proven.
Bleeding in the stomach or upper intestine raises BUN for the same reason, because digested blood is a protein load. In a JAMA review, a BUN-to-creatinine ratio above 30 was one of the findings that raised the likelihood of a serious upper GI bleed (likelihood ratio 7.5).
Medications and other causes
MedlinePlus also lists certain medicines, burns, and a recent heart attack. A blockage in the urinary tract can raise BUN too.
What does a low BUN mean?
A low BUN means less urea is being made or it's diluted. MedlinePlus lists four causes: a low-protein diet, malnutrition, over-hydration, and liver failure.
The liver link is the one to know. Because the liver makes urea, BUN can run low in liver disease even when the kidneys are fine. That's when ALT, AST, and alkaline phosphatase get a closer look.
A slightly low BUN with otherwise normal labs is rarely a concern.
How does BUN fit with creatinine and eGFR?
BUN is the jumpy one. It moves with hydration, diet, and blood flow. Creatinine is steadier, and eGFR (an estimate of filtering calculated from creatinine) is the standard measure of kidney function.
Your lab results, explained
Talk to a board-certified clinician for clear answers and personalized guidance around your recent lab test results.
Dehydration, high protein intake, or bleeding in the gut
High
High
Reduced kidney filtering
Low
Normal
Low protein intake, extra fluid, or reduced liver output
These are starting points for a clinician. Many labs also print a BUN-to-creatinine ratio that sums up the same comparison in one number.
We'd rather repeat a mildly high BUN after a well-hydrated week than label anyone on one draw.
Does a high BUN cause symptoms?
A mildly high BUN doesn't cause symptoms. What you feel comes from the cause: thirst and dark urine with dehydration, for example.
Very high levels from advanced kidney disease can bring nausea, itching, deep fatigue, and trouble concentrating.
When to get care quickly
Call 911 or go to an emergency room if you're passing very little urine, vomiting blood, passing black stools, or have new confusion, chest pain, or shortness of breath.
What happens after a flagged BUN?
Follow-up starts with context, then a repeat test if needed.
Check creatinine and eGFR on the same panel. If both are normal, kidney disease is much less likely.
Think back over the two days before the draw. Low fluids, vomiting or diarrhea, and a very high-protein day all count.
Repeat the test when you're well hydrated.
Ask about a urine albumin test and a blood pressure and blood sugar review if BUN stays high or eGFR is low.
How Parsley clinicians approach BUN
Parsley Health clinicians are board-certified in conventional medicine with advanced training in functional medicine. A flagged BUN gets the standard kidney workup first, and medication when it's needed.
Parsley clinicians often look at hydration, protein intake, blood pressure, blood sugar and metabolic health, and medications together, since those are the inputs that move the number.
No single BUN number is dangerous in every person. A result that's far above the range, rising quickly, or paired with a high creatinine needs prompt medical attention. A mild bump with normal creatinine usually calls for a repeat test.
Can dehydration cause a high BUN?
Yes. Dehydration is the most common cause of a mildly high BUN, and the number usually settles once you're rehydrated. That's why a repeat test is often the first step.
Does a high-protein diet raise BUN?
It can. Urea comes from protein breakdown, so eating much more protein than usual tends to raise BUN modestly. Creatinine and eGFR help show whether the kidneys are involved.
Do I need to fast for a BUN blood test?
Not for BUN alone. If it's part of a panel that includes glucose or cholesterol, you may be asked to fast for those.
This article is for educational purposes only and isn't a substitute for medical advice, diagnosis, or treatment. Lab results vary from person to person. Talk with a licensed clinician about your own results and symptoms.
References & Sources:
Weiner ID, Mitch WE, Sands JM. Urea and ammonia metabolism and the control of renal nitrogen excretion. Clin J Am Soc Nephrol. 2015;10(8):1444-1458. doi:10.2215/CJN.10311013
Ko GJ, Rhee CM, Kalantar-Zadeh K, Joshi S. The effects of high-protein diets on kidney health and longevity. J Am Soc Nephrol. 2020;31(8):1667-1679. doi:10.1681/ASN.2020010028
Srygley FD, Gerardo CJ, Tran T, Fisher DA. Does this patient have a severe upper gastrointestinal bleed? JAMA. 2012;307(10):1072-1079. doi:10.1001/jama.2012.253
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At Parsley Health, we believe better health starts with trusted information. Our content is accurate, accessible, and compassionate—rooted in evidence-based research and reviewed by qualified medical professionals. For more details read about our editorial process.
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