BUN (Blood Urea Nitrogen)
Complete guide to BUN testing — normal ranges, kidney function markers, and when to get tested.
What is BUN?
Blood Urea Nitrogen (BUN) measures the amount of nitrogen in your blood that comes from urea, a waste product formed when protein is broken down in the liver. Healthy kidneys filter urea out of the blood and into urine.
Why is it Important?
BUN is a key marker of kidney function. Elevated BUN can indicate:
- Kidney disease or failure
- Dehydration
- Heart failure
- Gastrointestinal bleeding
- High-protein diet or excessive protein breakdown
Normal Ranges
| Group | Range | Unit |
|---|---|---|
| Adult | 7 – 20 | mg/dL |
| Adult | 2.5 – 7.1 | mmol/L |
| BUN/Creatinine Ratio | 10:1 – 20:1 | - |
What Do Abnormal Results Mean?
High BUN
Common causes include:
- Acute or chronic kidney disease
- Dehydration (pre-renal azotemia)
- Congestive heart failure
- Gastrointestinal bleeding
- High-protein diet
- Steroid or tetracycline use
- Urinary tract obstruction
Low BUN
Less common, but can indicate:
- Malnutrition or low-protein diet
- Liver disease (reduced urea production)
- Overhydration
- Pregnancy
BUN/Creatinine Ratio
The ratio of BUN to creatinine helps distinguish between different causes of kidney dysfunction:
- High ratio (>20:1): Dehydration, heart failure, GI bleeding
- Low ratio (<10:1): Liver disease, malnutrition
- Normal ratio with both elevated: Kidney disease
When Should You Get Tested?
BUN is part of routine kidney function panels and comprehensive metabolic panels. Your doctor may order it if you have symptoms like fatigue, swelling, changes in urination, or risk factors for kidney disease.
This information is for educational purposes only and is not a substitute for professional medical advice.
Last reviewed: 2026-02-13