Lab test glossary · Kidney
Blood Urea Nitrogen: what it means
Also known as: BUN, urea nitrogen
BUN reflects kidney function and hydration; it can rise with dehydration or reduced kidney filtering and is read alongside creatinine.
What it measures
BUN measures the nitrogen in urea, a waste product from protein breakdown that the kidneys remove. It's affected by hydration, protein intake, and kidney function, so it's usually interpreted together with creatinine rather than alone.
Typical range
Often roughly 7–20 mg/dL.
Reference ranges differ between labs and are printed on your own report — always read your result against that range, not a general one.
What a high result can mean
- Dehydration
- Reduced kidney function
- High-protein diet or gastrointestinal bleeding
A high value is a starting point for a conversation, not a diagnosis — many causes are benign or temporary.
What a low result can mean
- Overhydration
- Low-protein diet or liver disease
Why the trend matters
BUN swings with hydration, so its trend and its ratio to creatinine are more useful than a single reading.
When to talk to your doctor
Discuss a persistently high BUN with your doctor, especially alongside an elevated creatinine or low eGFR.
This page is general education about a common lab test — it isn’t medical advice and can’t interpret your specific result. What a value means depends on your history, symptoms, and the rest of your labs, which is a conversation for you and your clinician.