What Is BUN in a Blood Test?

BUN stands for Blood Urea Nitrogen. It measures the amount of urea nitrogen in your blood. Urea is a waste product formed when your liver breaks down protein, and your kidneys normally filter it out into your urine.

Normal Range

• Adult normal: 6–24 mg/dL
• The BUN-to-creatinine ratio helps distinguish the cause of an elevated BUN. A normal ratio is about 10:1 to 20:1.

What Causes High BUN?

• Kidney disease — impaired filtration; BUN rises when the kidneys cannot clear waste efficiently.
• Dehydration — concentrated blood raises BUN without necessarily indicating kidney damage.
• High-protein diet — more protein breakdown produces more urea.
• GI bleeding — digested blood is a large protein load.
• Heart failure — reduced kidney perfusion.
• Certain medications — corticosteroids and some antibiotics can raise BUN.

What Causes Low BUN?

• Liver disease — impaired urea production.
• Malnutrition — low protein intake.
• Overhydration — dilutional effect.

How Is BUN Interpreted?

BUN is never interpreted in isolation. Providers compare it to creatinine (the BUN:creatinine ratio) to determine whether an elevated BUN is due to kidney dysfunction (both rise), dehydration (BUN rises more than creatinine), or a non-renal cause. It is part of a standard metabolic panel and is ordered routinely in hospital, annual physical, and emergency settings.