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Blood Test Tracker

Blood Urea Nitrogen (BUN)

BUN is a waste product from protein breakdown that your kidneys clear from the blood, so it gives a rough read on kidney function and hydration.

AI-verified against cited sources. Awaiting expert reviewAlso called BUN, UREA NLOINC 3094-0

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The usual range for BUN depends on sex. Choose female or male above to see where you stand.

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What it is

When your body breaks down protein, the liver turns the leftover nitrogen into urea. Urea travels in your blood to the kidneys, which filter it into urine. A BUN test measures the nitrogen part of the urea in your blood. It is usually run with creatinine as part of a basic or comprehensive metabolic panel.

Why it matters

BUN helps show how well your kidneys are clearing waste, but it is not a stand-alone kidney test. It also moves with how much protein you eat, how much fluid your body has and how much protein it is breaking down. That is why it is read together with creatinine and eGFR. A single out-of-range BUN has many possible explanations, and many of them are temporary.

If it’s low

A low BUN is common and often has an ordinary explanation. Eating little protein, having extra fluid in the body and pregnancy can all lower it. Less often it reflects advanced liver disease, because the liver is where urea is made.

Common causes

  • Pregnancy
  • Low protein intake or malnutrition
  • Extra fluid in the body, including IV fluids or drinking a lot of water
  • Conditions that make the body hold extra water, such as SIADH
  • Severe liver disease

What can help

  • Ask your clinician whether your diet, pregnancy or fluid intake could explain a low result
  • If you are eating very little protein or losing weight without trying, mention it at your next visit

If it’s high

A high BUN often has a simple explanation, such as not drinking enough fluid or eating a lot of protein. It can also be a sign that the kidneys are clearing waste more slowly than usual. Your clinician will usually read it together with your creatinine and eGFR.

Common causes

  • Dehydration, including from vomiting, diarrhea or heavy sweating
  • A recent high-protein meal or a high-protein diet
  • Older age (BUN tends to rise as we get older)
  • Corticosteroids, and some other medicines such as tetracycline antibiotics taken together with diuretics
  • Illness, surgery, burns or other stress that makes the body break down more protein
  • Bleeding in the digestive tract
  • Heart failure or other conditions that reduce blood flow to the kidneys
  • Kidney disease or injury, or a blockage in the urinary tract

What can help

  • Ask your clinician whether your fluid intake, recent meals or medicines could explain the result
  • Ask whether to recheck BUN along with creatinine and eGFR once you are well hydrated

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on sex and age. Choose women or men to see the one that applies.

Children and teens 1 ranges

See BUN by age and sex, with the U.S. survey numbers where they exist.

How common is your level?

NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.

How BUN varies with age in the U.S.Shaded: middle 90% and middle 50% of people. Line: the median. mg/dL. Point at any age, or focus the chart and use the arrow keys, to read both sexes in words.

Use the left and right arrow keys to read each age. Shift moves five years.

Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.

Explore BUN by age and sex: every percentile as a table, with a download.

What can affect your result

  • Protein intake: a high-protein meal or diet can raise BUN, and a low-protein diet can lower it
  • Hydration: dehydration raises BUN, and extra fluid lowers it
  • Age and sex: BUN tends to rise with age, and adult ranges run a little higher for men than for women
  • Pregnancy tends to lower BUN
  • Medicines such as corticosteroids can raise it by increasing protein breakdown
  • Bleeding in the digestive tract, burns, surgery and severe illness can raise it
  • Labs use slightly different ranges, so compare your result with the range printed on your own report
  • A recent high-protein meal or a high-protein diet can raise BUN, and very low protein intake can lower it.
  • Dehydration raises BUN, and extra fluid (including IV fluids) lowers it.
  • Corticosteroids and some other medicines can raise BUN by increasing protein breakdown.
  • Labs reject grossly hemolyzed samples, and tubes containing sodium fluoride, oxalate, EDTA or citrate are not suitable for this test.

Follows changes in hydration, protein intake and kidney function within a day or two.

When to talk to a clinician

Talk with your clinician soon if your BUN is flagged high, especially if your creatinine is also high or rising. Get prompt care if you are passing much less urine than usual, have swelling in your legs or face, are vomiting blood, or have black or bloody stools. Call 911 or your local emergency number for chest pain, trouble breathing, confusion or fainting.

Questions you could ask

  • How does my BUN compare with my past results?
  • Do any of my medicines or supplements affect my BUN?
  • Could dehydration, my diet or a medicine explain my high BUN?
  • Should we look at my creatinine and eGFR along with it?
  • Should we recheck it, and when?
  • Could my protein intake or fluid intake explain a low BUN?

Questions people ask

What is the difference between BUN and urea?

They measure the same waste product in two ways. BUN counts only the nitrogen in urea and is reported in mg/dL in the US. Many countries report whole urea instead, usually in mmol/L. To convert BUN in mg/dL to urea in mmol/L, multiply by 0.357. Urea in mg/dL is about 2.14 times the BUN value.

Can dehydration or a high-protein meal raise my BUN?

Yes. Both can raise BUN even when your kidneys are working well. If your result is high, your clinician may read it with your creatinine and eGFR, or suggest a recheck when you are well hydrated.

Is BUN or creatinine the better kidney test?

Creatinine is generally seen as the more specific of the two, because BUN is affected by more things outside the kidneys, such as protein intake and bleeding. The two are usually read together, and eGFR, which is calculated from creatinine, gives a fuller picture.

Is a low BUN a problem?

Often it is not. Low protein intake, extra fluid and pregnancy are common reasons. If it is very low, or comes with symptoms or liver results that are off, your clinician can look further.

Do I need to fast for a BUN test?

Usually not, since BUN itself does not need fasting. If other tests on the same blood draw need it, such as glucose or cholesterol, follow those instructions.

Show the science and sources

Units

  • 1 mg/dL = 1 mg/dL
  • 1 mg/dL = 0.357 mmol/L

Open the BUN unit converter

Is a change real?

BUN naturally varies about 13.9% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 43% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Critical appraisal and meta-analysis of biological variation estimates for kidney related analytes (Jonker N et al., EFLM WG-BV).

For clinicians and researchers

Urea is made in the liver from ammonia released by amino acid breakdown, filtered freely at the glomerulus and partly reabsorbed along the tubules, more so when the body is volume-depleted. BUN is therefore a less specific marker of GFR than creatinine. It rises with protein load, gastrointestinal bleeding and catabolic states (glucocorticoids, surgery, trauma, burns, parenteral nutrition) and falls with low protein intake, pregnancy, volume expansion (including SIADH) and severe liver disease. When blood flow to the kidneys falls, BUN tends to rise faster than creatinine (Labcorp), and in progressive chronic kidney disease azotemia appears only after a large share of kidney tissue is lost (Labcorp cites about three-quarters). Labcorp uses an enzymatic (urease) method. Sodium fluoride tubes (ARUP) and oxalate, EDTA or citrate plasma (Labcorp) are unsuitable. US labs report urea nitrogen in mg/dL. The UK and much of the world report whole urea, usually in mmol/L, and some countries report urea in mg/dL. Those results belong to the separate marker urea: urea mmol/L = BUN mg/dL × 0.357, and urea mg/dL = BUN mg/dL × 2.14. A BUN printed in mmol/L (used by some labs) follows the same 0.357 factor.

LOINC 3094-0 (primary), 14937-7, 6299-2, 59570-2, 35234-4. Specimen: serum-or-plasma.

Sources

  1. Blood Urea Nitrogen (BUN), Serum (test ID BUN), Mayo Clinic Laboratories (test catalog copy hosted by Schneck Medical Center)
  2. Urea Nitrogen (BUN) (test 001040), Labcorp
  3. Pediatric Testing Reference Ranges, Labcorp (2022)
  4. Comp. Metabolic Panel (14), sample report (test 322000), Labcorp (2022)
  5. Basic Metabolic Panel (8), sample report (test 322758), Labcorp (2022)
  6. Urea Nitrogen, Serum or Plasma (test 0020023), ARUP Laboratories
  7. BUN (Blood Urea Nitrogen) Test, MedlinePlus (National Library of Medicine)
  8. Acute Kidney Injury (AKI), Merck Manual Professional Edition, Merck Manual (2026)
  9. Nitrogen (PubChem CID 947), molecular weight 28.014, PubChem (National Library of Medicine)
  10. Urea (PubChem CID 1176), molecular weight 60.056, PubChem (National Library of Medicine)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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