BUN (Blood Urea Nitrogen)
Also written as: BUN, urea, serum urea
A protein waste product cleared by the kidneys, used with creatinine to assess kidney function and hydration.
What it measures
The amount of urea nitrogen in the blood. Urea is a waste product the liver makes when protein is broken down, and the kidneys filter it out into urine. Its level reflects the balance between how much is produced and how well it is cleared.
Why a doctor orders it
It is a standard part of metabolic panels used to check kidney function and fluid status. Clinicians almost always read it alongside creatinine, another kidney marker, because the two together are more informative than either alone.
What is generally associated with a higher result
Broadly associated with reduced kidney filtering, dehydration, a high protein intake, and bleeding into the digestive tract. It also tends to drift up with normal aging and with some common medications, such as steroids.
What is generally associated with a lower result
Broadly associated with a low protein diet, liver conditions that reduce urea production, overhydration, and pregnancy. Mildly low values are generally not pursued.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- Hydration - dehydration concentrates it, drinking a lot of fluid dilutes it
- A high protein meal or protein supplements in the day before the draw
- Pregnancy, which normally lowers it
- Common medications, including steroids and some diuretics
- Intense exercise shortly before the draw
Questions to take to your appointment
- How does this look next to my creatinine, and what does the ratio between them suggest?
- Could my hydration or diet on the day of the draw explain the result?
- Do any of my current medications affect this value?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
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