Blood Urea Nitrogen (BUN)
BUN measures the amount of nitrogen in your blood from urea, a waste product created when the liver breaks down protein. The kidneys normally filter urea out of the blood and excrete it in urine. BUN rises when kidneys are not functioning well, but it is less specific than creatinine because diet, hydration, and liver function also affect it.
Reference Ranges
Male
7 – 20
mg/dL
Female
7 – 20
mg/dL
Child
5 – 18
mg/dL
Ages 1–12
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
Low BUN can indicate low protein intake, severe liver disease (liver cannot produce urea), overhydration, or malnutrition. It is generally not as clinically significant as high BUN.
What High Levels Mean
High BUN may indicate kidney disease, dehydration, high-protein diet, gastrointestinal bleeding (blood is digested as protein), heart failure, or urinary obstruction. The BUN-to-creatinine ratio helps distinguish kidney causes from non-kidney causes.
Frequently Asked Questions
What does the BUN/creatinine ratio tell?
A normal ratio is 10:1 to 20:1. A high ratio (>20:1) with normal creatinine suggests dehydration, GI bleeding, or high protein intake — not kidney disease. A high ratio with high creatinine suggests true kidney impairment.
Should I worry about a slightly high BUN?
A mild elevation (21–25 mg/dL) is often due to dehydration or a high-protein meal before the test. If creatinine and eGFR are normal, isolated mild BUN elevation is usually not concerning. Stay hydrated and retest.
Can diet affect BUN levels?
Yes. A high-protein diet (meat, eggs, protein supplements) increases urea production and raises BUN. A low-protein diet or vegetarian diet tends to lower BUN. This is why BUN alone is not a reliable kidney function marker.
This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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