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Blood Urea Nitrogen (BUN): Normal Range & Kidney Health

What BUN measures about kidney and liver function, normal reference ranges, and what abnormal levels indicate.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: · How we check our content

Normal range
6–24 mg/dL
Also called
Serum Urea (UK)
BUN:Creatinine ratio
10:1 to 20:1 (normal)

What Is BUN?

Blood urea nitrogen (BUN) measures the amount of urea nitrogen in the blood. Urea is a waste product formed when the liver breaks down protein. It's filtered by the kidneys: so BUN reflects both liver protein metabolism and kidney filtration.

Normal BUN Range

GroupNormal BUN (mg/dL)
Adults6–24
Elderly (over 60)7–23 (slightly higher is normal
Children5–18
Pregnant womenSlightly lower

Causes of High BUN

  • Kidney disease or acute kidney injury
  • Dehydration: less fluid to flush waste
  • High protein diet
  • Gastrointestinal bleeding: digested blood raises protein load
  • Congestive heart failure (reduced kidney blood flow)
  • Some medications (steroids, diuretics)

Why Urea Is a Weaker Kidney Marker Than Creatinine

Creatinine and urea are often quoted side by side as though they measure the same thing. They do not, and the difference is in what the kidney does with each after filtering it.

Creatinine is filtered and then largely left alone, so its level tracks filtration fairly directly. Urea is not left alone. The kidney reabsorbs a large share of it, and that reabsorption is not passive leakage — urea moves through dedicated transport proteins, and its handling is actively regulated as part of the mechanism that concentrates urine.2 Urea is in fact the dominant osmotic substance in urine, concentrated far above its level in blood.2

That regulation is the whole story behind a raised urea. When your body is short of water it conserves water, and it reclaims more urea along with it. Your urea rises because you are dehydrated, not because your kidneys are damaged. Creatinine, handled more simply, moves much less. So the two markers separate, and the gap between them carries the information.

Reading the Urea-to-Creatinine Ratio

This is why the ratio is quoted at all. Both markers rising together, in proportion, points towards the kidney itself. Urea rising well out of proportion to creatinine points to something upstream of the kidney — most often dehydration, but also heart failure or anything reducing blood flow through the kidney, where the same water-conserving response is switched on.

The pattern most easily missedA raised urea with a normal or near-normal creatinine, in someone with black tarry stools or unexplained anaemia, can indicate bleeding into the upper gut. Blood is protein; digested in the intestine it arrives as a large protein load and is converted to urea. The urea rises for a reason that has nothing to do with the kidneys, and the clue is the mismatch.

Other things move urea without kidney disease. A high-protein diet or protein supplements raise it. Steroids and any severely catabolic illness raise it by increasing protein breakdown. Bleeding, as above, raises it.

The reverse also holds, and a low urea is not automatically good news. Urea is manufactured in the liver, so significant liver disease lowers it. So does a genuinely low-protein diet, and so does pregnancy, where blood volume expands and filtration rises.

The practical conclusion is that urea is a poor test to judge kidney function by on its own. It is genuinely useful read alongside creatinine, where the relationship between the two says more than either number.

Causes of Low BUN

  • Severe liver disease: liver can't make urea
  • Malnutrition or very low protein intake
  • Overhydration

BUN-to-Creatinine Ratio

RatioInterpretation
10:1 to 20:1Normal
Above 20:1Pre-renal causes (dehydration, bleeding, heart failure)
Below 10:1Intrinsic renal disease or liver disease

FAQs

Is fasting required for BUN?
No. Though it may be included in panels where fasting is required for other tests.
What is the difference between BUN and creatinine?
Both are kidney waste products. BUN is affected by diet and liver function; creatinine is more specific to kidney filtration (GFR).
Can drinking more water lower BUN?
Yes. Dehydration concentrates BUN. Adequate hydration keeps BUN in the normal range if kidney function is intact.

Frequently Asked Questions

What does a high blood urea level mean?
Raised urea most often reflects reduced kidney filtration, but it also rises with dehydration, a high-protein diet, gastrointestinal bleeding, and some medicines. It is interpreted alongside creatinine and eGFR rather than on its own.
What does a low urea level mean?
Low urea is usually not a concern. It can occur in pregnancy, with a low-protein diet, with overhydration, or in significant liver disease, since the liver produces urea.
What is the urea-to-creatinine ratio used for?
A disproportionately high urea compared with creatinine suggests a pre-renal cause such as dehydration or bleeding into the gut, whereas both rising together points more towards intrinsic kidney disease.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

  1. Renal Function Tests. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK507821
  2. Uremia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK441859
  3. Bankir L, Yang B. New insights into urea and glucose handling by the kidney, and the urine concentrating mechanism. Kidney Int. 2012;81(12):1179–98. PMID 22456603 · doi:10.1038/ki.2012.67

Related reading

Medical Disclaimer: Elevated BUN should be interpreted alongside creatinine and eGFR. A single high BUN does not diagnose kidney disease, trends over time matter.