BUN measures how well your kidneys filter urea, a waste product from protein breakdown. Combined with creatinine, it provides a complete picture of kidney function.
Blood Urea Nitrogen (BUN) measures nitrogen from urea in your blood. When you eat protein, your body breaks it down and produces ammonia as a byproduct. Your liver converts this toxic ammonia into urea, which is then filtered out by your kidneys.
According to the National Library of Medicine, BUN is part of the basic metabolic panel (BMP) and comprehensive metabolic panel (CMP), routinely used to assess kidney health.
Both BUN and creatinine assess kidney function, but they measure different things:
| Factor | BUN | Creatinine |
|---|---|---|
| Source | Protein metabolism (diet) | Muscle metabolism (constant) |
| Affected by diet | Yes (high protein raises BUN) | Minimally |
| Affected by hydration | Yes (dehydration raises BUN) | Less so |
| Kidney specificity | Less specific | More specific |
| Normal range | 7-20 mg/dL | 0.7-1.3 mg/dL |
May indicate liver issues, malnutrition, or overhydration
Healthy kidney function and protein metabolism
May indicate mild kidney impairment or dehydration
Significant kidney dysfunction or other serious condition
Select your BUN value for a detailed explanation of what it means and what to do next.
High BUN has multiple possible causes. The Mayo Clinic categorizes them as:
Acute or chronic kidney disease, kidney injury, kidney infection. Less filtration means more urea stays in blood.
Dehydration, heart failure, shock — reduced blood flow to kidneys impairs filtration.
High protein diet, GI bleeding (digested blood = protein load), severe burns, fever, catabolic states.
Kidney stones, enlarged prostate, bladder obstruction — blocks urea excretion.
Low BUN is less common but can indicate:
Liver failure, cirrhosis — liver can't convert ammonia to urea properly.
Low protein intake, anorexia, malabsorption — less protein to break down.
Excessive fluid intake, SIADH — dilutes BUN concentration in blood.
Increased blood volume dilutes BUN. Normal during pregnancy.
Upload your lab results to monitor BUN, creatinine, and eGFR trends. Catch changes early and protect your kidney health.
Start Tracking FreeMedical Disclaimer: This information is for educational purposes only and is not intended as medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for personalized recommendations about your health.