Blood Urea Nitrogen
A waste product of protein breakdown that the kidneys filter out.
| Typical reference range | 7–25 mg/dL |
Ranges vary by laboratory, method, age and sex: always use the range printed on your own report. Why labs differ.
What it measures
When protein is broken down, the liver converts the resulting ammonia into urea, which is filtered by the kidneys. BUN measures the nitrogen portion of urea in blood.
Why it matters
BUN reflects kidney filtration, hydration status and protein metabolism. It is less specific than creatinine or cystatin C for kidney function but helps assess dehydration and gastrointestinal bleeding.
What a high result can mean
High BUN can result from dehydration, high-protein diet, reduced kidney function, heart failure (reduced kidney blood flow), gastrointestinal bleeding, corticosteroids or catabolic states like severe illness.
What a low result can mean
Low BUN can reflect low protein intake, overhydration, pregnancy or reduced liver function (less urea synthesis).
What affects the result
Protein intake and hydration strongly affect BUN; a high-protein meal or dehydration can push it above range in healthy people. Interpret with creatinine (BUN/creatinine ratio) and eGFR.
Units and conversion
Usually reported in mg/dL.
- From mmol/L: multiply by 2.801 to get mg/dL
Also called on lab reports
BUN, Urea Nitrogen, Urea Nitrogen (BUN), Urea Nitrogen, Blood, Urea Nitrogen, Serum
Related tests: Kidney Function
This page is educational and does not replace advice from a healthcare professional.