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
| Group | Normal BUN (mg/dL) |
|---|---|
| Adults | 6–24 |
| Elderly (over 60) | 7–23 (slightly higher is normal |
| Children | 5–18 |
| Pregnant women | Slightly 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)
Causes of Low BUN
- Severe liver disease — liver can't make urea
- Malnutrition or very low protein intake
- Overhydration
BUN-to-Creatinine Ratio
| Ratio | Interpretation |
|---|---|
| 10:1 to 20:1 | Normal |
| Above 20:1 | Pre-renal causes (dehydration, bleeding, heart failure) |
| Below 10:1 | Intrinsic 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.
Medical Disclaimer: Elevated BUN should be interpreted alongside creatinine and eGFR. A single high BUN does not diagnose kidney disease — trends over time matter.