BUN (Blood Urea Nitrogen)

What is BUN?

BUN stands for Blood Urea Nitrogen. It measures the amount of nitrogen in your blood that comes from urea, a waste product created when your body breaks down protein.

Here's how it works: You eat protein → Your body breaks it down → Ammonia is produced (toxic) → Your liver converts ammonia to urea (less toxic) → Your kidneys filter urea out of the blood.

When kidneys aren't working properly, urea builds up in the blood, causing BUN to rise.

Normal Ranges

BUN Level	Interpretation
7–20 mg/dL	Normal
Below 7	May indicate low protein intake or liver disease
20–40	Mildly elevated
Above 40	Significantly elevated

Normal ranges can vary by laboratory and individual factors like age and muscle mass.

Why It Matters

Kidney Function Assessment

BUN, alongside creatinine and eGFR, helps assess how well your kidneys are filtering waste. Rising BUN can indicate declining kidney function.

Dehydration Detection

BUN rises with dehydration because there's less fluid to dilute the urea in your blood. This makes BUN a useful marker for hydration status.

Protein Metabolism

Very low BUN can indicate inadequate protein intake or liver disease (liver can't convert ammonia to urea).

BUN/Creatinine Ratio

Doctors often calculate the BUN-to-creatinine ratio to help identify the cause of abnormal values:

• Normal ratio: 10:1 to 20:1
• High ratio (BUN elevated more than creatinine): Often dehydration, high-protein diet, or gastrointestinal bleeding
• Low ratio (creatinine elevated more than BUN): May indicate kidney disease, low protein diet, or liver disease

What Affects BUN

Increases BUN

• Dehydration — Most common cause of mild elevation
• High-protein diet — More protein breakdown
• Kidney disease — Reduced filtering ability
• Gastrointestinal bleeding — Blood proteins are digested
• Certain medications — Steroids, some antibiotics
• Heart failure — Reduced kidney blood flow

Decreases BUN

• Overhydration — Dilutes blood
• Low-protein diet — Less protein breakdown
• Liver disease — Can't produce urea efficiently
• Malnutrition
• Pregnancy — Blood volume increases

High BUN: What to Do

Mildly elevated BUN is often not concerning, especially if creatinine is normal and you're well-hydrated.

If BUN is elevated:

1. Recheck after adequate hydration
2. Check creatinine and eGFR
3. Review medications with your doctor
4. Consider recent protein intake

How Often to Test

• Routine checkups: Usually included in comprehensive metabolic panel
• Chronic kidney disease: Every 3–6 months or as directed
• During acute illness: As needed to monitor kidney function

Related Biomarkers

• Creatinine
• eGFR
• Electrolytes (sodium, potassium)