BUN (blood urea nitrogen): what a high or low result means
Dr. Joan J. Morales Lappot, MD3 min read · Leer en español
In briefBUN sits right next to creatinine on most lab reports, and people often assume both tell the same story. They are related, but BUN is moved by many things besides the kidneys.
What BUN measures
When your body breaks down protein, the liver turns the leftover nitrogen into urea, and the kidneys remove it. BUN is the amount of that urea nitrogen in your blood. A typical range is about 7 to 20 mg/dL, depending on the lab.
Why it can be high
- Dehydration, the most common reason, often with a BUN-to-creatinine ratio above 20
- A high-protein diet or protein shakes
- Bleeding in the stomach or intestines
- Steroid medicines
- Reduced kidney function
Why it can be low
A low BUN is usually not a concern. It can come from a low-protein diet, drinking a lot of fluid, pregnancy or liver disease.
How I read it
I always look at BUN together with creatinine, your hydration, your diet and your medicines. A high BUN with a normal creatinine often just means you need more fluids. In advanced kidney disease, a very high BUN together with nausea, poor appetite or itching is one of the signals we use to talk about treatment options.