BUN (blood urea nitrogen): reference ranges

In shortThe normal result is generally 6–20 mg/dL (2.14–7.14 mmol/L of urea). High values often point at the kidneys, but dehydration and diet also move the number.

Reference ranges

Group	Reference range
Adults	6–20 mg/dL (2.14–7.14 mmol/L of urea)

Normal values vary among laboratories. BUN levels vary with sex and usually increase with age; infants and children run lower.

Check your value

Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.

A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.

What this marker is

BUN stands for blood urea nitrogen. Urea nitrogen is what forms when protein breaks down: the liver converts the nitrogen waste of protein metabolism into urea, the kidneys filter it out, and the BUN test measures the amount of urea nitrogen circulating in the blood. It is one of the standard checks of kidney function.

BUN and urea are related but not the same measurement. In the United States, labs report blood urea nitrogen in mg/dL; in many other countries labs report urea itself in mmol/L. The numbers are different even for the same blood sample — a BUN of 6–20 mg/dL corresponds to 2.14–7.14 mmol/L of urea — so the unit on the report matters before any comparison.

BUN is rarely read alone. Providers compare it with creatinine, because the pair separates kidney problems from other influences: dehydration, bleeding into the gut, or a very protein-rich diet can raise BUN while creatinine stays put. Levels also vary with sex and usually increase with age.

The test uses a blood sample from a vein, usually drawn from the inside of the elbow or the back of the hand. Many medicines can interfere with the result, so the provider needs the full medication list before the draw.

When it is high

Kidney disease and kidney failureGlomerulonephritis, pyelonephritis, acute tubular necrosis, and kidney failure all reduce urea excretion and raise BUN.

DehydrationToo little fluid in the body (hypovolemia) concentrates urea nitrogen in the blood — one of the most common non-kidney reasons for a high value.

Heart conditionsCongestive heart failure, a recent heart attack, and shock reduce blood flow to the kidneys and push BUN up.

Gastrointestinal bleedingBlood digested in the gut delivers a large protein load, and the resulting urea raises BUN.

Excess dietary proteinAn unusually high-protein diet increases urea production; burns and certain medicines can also raise the level.

Urinary tract obstructionA blockage that prevents normal urination backs waste up into the blood.

When it is low

Liver diseaseUrea is made in the liver, so liver failure lowers BUN — in people with liver disease the level may be low even when the kidneys are normal.

Low-protein diet and malnutritionToo little protein in the diet means less urea is produced.

Over-hydrationExcess fluid dilutes the blood and lowers the measured concentration.

What can skew the result

These factors can shift the value without any disease — worth considering before a retest:

Age and sexProtein intakeHydrationCertain medicines

How to prepare

Many medicines can interfere with the result. Your provider will tell you if any medicine should be paused before the test; do not stop or change medicines without talking to your provider first.

When a retest makes sense

An abnormal BUN is read together with creatinine — the comparison of the two helps tell kidney causes from dehydration, diet, or bleeding. The test is repeated after the influencing factor is addressed.

Who usually reviews this result

A result outside the reference range is usually reviewed by: your GP or primary care doctor; a nephrologist if kidney disease is suspected. Bring your lab report and any previous results to the appointment.

Names used on lab reports

Your report may use any of these names:
BUNBlood urea nitrogenUrea nitrogen testSerum BUNBUN to creatinine ratio

Units and conversion

Measured in: mg/dL

mg/dL = mmol/L (urea)

BUN mg/dL × 0.357 = urea in mmol/L. Example: 14 mg/dL = 5.0 mmol/L. This converts to urea: BUN measures only the nitrogen part of urea, so the two are related but not interchangeable. Results are rounded to 2 decimal places.

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Frequently asked questions

Is BUN the same as urea?

Not exactly. BUN measures the nitrogen portion of urea and is reported in mg/dL in the United States; many other countries report urea itself in mmol/L. The same blood sample gives different numbers under the two conventions — 6–20 mg/dL of BUN corresponds to 2.14–7.14 mmol/L of urea — so reports from different systems are compared only after conversion.

Can BUN be high with healthy kidneys?

Yes. Dehydration, gastrointestinal bleeding, a high-protein diet, burns, heart failure, a recent heart attack, and certain medicines can all raise BUN while the kidneys work normally. The creatinine comparison is how the provider tells the difference.

What does a low BUN mean?

Low values come from liver failure, a low-protein diet, malnutrition, or over-hydration. Because urea is made in the liver, people with liver disease can show a low BUN even with normal kidneys.

Sources

BUN (Blood Urea Nitrogen): MedlinePlus Medical Test — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
BUN - blood test: MedlinePlus Medical Encyclopedia — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
Basic metabolic panel: MedlinePlus Medical Encyclopedia — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.

BUN (blood urea nitrogen)