BUN stands for Blood Urea Nitrogen. Urea nitrogen is a waste product created in the liver when the body breaks down proteins. The blood carries this waste to the kidneys, which filter it out and excrete it in urine.
Because the kidneys are responsible for clearing urea from the blood, a BUN test is primarily used to evaluate how well your kidneys are working. However, BUN is easily influenced by other factors like diet and hydration, so it is almost always interpreted alongside creatinine.
Typical BUN reference ranges (mg/dL)
| Adults | 7 – 20 | Ranges vary slightly by lab |
|---|---|---|
| Adults over 60 | Slightly higher | Kidney function naturally declines with age |
BUN levels can fluctuate day-to-day based on how much protein you eat and how much water you drink.
What a high BUN means
A high BUN level can indicate that your kidneys aren't filtering waste efficiently. This could be due to chronic kidney disease or acute kidney injury.
However, a high BUN doesn't always mean kidney damage. The most common cause of a mildly elevated BUN is simply dehydration. When you lack fluid, your blood volume drops, and the concentration of urea nitrogen rises. High protein diets, certain medications (like corticosteroids), and gastrointestinal bleeding can also cause BUN to rise even if the kidneys are perfectly healthy.
The BUN-to-Creatinine Ratio
Because BUN can be affected by dehydration but creatinine is more stable, doctors look at the ratio between the two. A high BUN-to-creatinine ratio (greater than 20:1) usually points to dehydration or decreased blood flow to the kidneys. A normal ratio with both values elevated points more strongly to intrinsic kidney disease.
When to actually worry — and when not to
- If both your BUN and creatinine are significantly elevated, it indicates decreased kidney function that requires medical evaluation.
- A mildly high BUN with a normal creatinine in a person who feels well is very often just dehydration. Try drinking more water and ask your doctor if a repeat test is necessary.
Common questions
What does a low BUN mean?
A low BUN is generally not a cause for concern. It is often seen in people who eat a low-protein diet, are very well hydrated, or are pregnant.
Does a high protein diet raise BUN?
Yes. Because urea nitrogen is a byproduct of protein breakdown, eating a very high-protein diet (like a carnivore or keto diet) can cause your BUN to run higher than average.
One value never tells the whole story.
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Upload your reportMedical disclaimer: This article is for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Reference ranges vary between laboratories — always compare your result to the range on your own report, and consult a qualified healthcare professional about your results and any symptoms.