Urea Nitrogen (BUN)

Blood urea nitrogen (BUN) measures the amount of nitrogen in your blood that comes from urea, a waste product your body makes when it breaks down protein. In most adults, a normal BUN sits at roughly 7 to 20 mg/dL.

Ranges vary between labs, so compare to the range on your own lab report.

What BUN measures

When your body breaks down protein from food and tissue, it produces ammonia, which your liver converts into urea. Your kidneys filter that urea out of your blood and pass it into your urine. BUN reports how much urea nitrogen remains circulating, giving you a snapshot of both protein turnover and how well your kidneys are clearing waste.

What high or low levels mean

A high BUN often reflects dehydration or reduced blood flow to your kidneys, since less fluid concentrates the waste your kidneys handle. It can also rise with a very high protein intake, reduced kidney function, or gastrointestinal bleeding, where digested blood adds a protein load. A low BUN is less common and may point to low protein intake, liver disease that limits urea production, or overhydration that dilutes your blood. Because BUN shifts with fluid status and diet, one result rarely tells the whole story. Clinicians read it alongside creatinine and the BUN-to-creatinine ratio to separate kidney issues from dehydration or diet. Follow up if your BUN is repeatedly outside range or paired with rising creatinine, swelling, or reduced urination.

BUN FAQ

Does drinking water lower your BUN?

Staying well hydrated can lower a BUN that was elevated by dehydration, because added fluid dilutes the urea in your blood. If your BUN is high from kidney or other causes, water alone will not correct it.

Can a high-protein diet raise your BUN?

Yes. Eating large amounts of protein gives your body more nitrogen to process into urea, which can nudge your BUN toward the upper end of normal or slightly above, even when your kidneys are healthy.

Is BUN the same as a kidney function test?

Not on its own. BUN is one piece of kidney assessment, but it is affected by hydration and diet, so it is interpreted together with creatinine and estimated GFR for a fuller picture.

Read alongside creatinine, estimated GFR, and the BUN-to-creatinine ratio for a complete view of your kidney health.

Sources

1. Cleveland Clinic. "Blood Urea Nitrogen (BUN) Test." https://my.clevelandclinic.org/health/diagnostics/17684-blood-urea-nitrogen-bun-test

2. Medscape. "Blood Urea Nitrogen (BUN): Reference Range, Interpretation." https://emedicine.medscape.com/article/2073979-overview