The BUN blood test measures the amount of urea nitrogen in the blood and is commonly used to assess kidney function. It may also provide helpful information about liver health and hydration status.
High BUN levels can occur with kidney or liver problems, dehydration, or a high-protein diet. In more severe cases, very high levels may lead to uremia, a condition that can be harmful to the body.
The BUN blood test is often ordered along with creatinine to better evaluate how well the kidneys are filtering waste from the blood. It can also help monitor kidney disease, interpret symptoms like fatigue or swelling, and guide treatment such as hydration, diet changes, or dialysis.
What is urea?
Urea is a substance made by the liver from the proteins you eat. Its levels can change when there are problems with the liver or kidneys.
Proteins from food are broken down in the liver and turned into urea, which is then filtered out of the blood by the kidneys and removed from the body in the urine.
Why it's ordered
The BUN blood test is mainly used to evaluate kidney function and monitor the progression of kidney disease.
The test may also be ordered when there are symptoms of excess urea or kidney problems, such as extreme fatigue, high blood pressure, foamy or bloody urine, or swelling in the legs.
Reference values
According to the National Kidney Foundation, normal BUN levels in adults typically range from about 7 to 20 mg/dL, although values may vary slightly between laboratories.
Preparing for the test
No fasting or special preparation is needed before a BUN blood test.
How it's done
The BUN blood test is done by collecting a small blood sample, which is sent to a laboratory for analysis.
What results mean
BUN results should be interpreted by the healthcare provider who ordered the test, along with your other lab results.
1. High BUN
High BUN levels in the blood may mean that the liver is producing a large amount of urea or that the kidneys are not working properly.
According to the National Kidney Foundation, BUN is one of the lab values that may be monitored in people with chronic kidney disease, along with other kidney health numbers such as creatinine and eGFR.
The main causes of high BUN levels are:
-
Kidney failure
-
Reduced blood flow to the kidneys
-
Severe burns
-
Dehydration
If BUN levels are high, it is important to see a healthcare provider to determine the cause. Treatment may involve drinking more fluids, making dietary changes, taking medications, or, in more severe cases, having dialysis.
When high BUN is due to dehydration, increasing fluid intake is usually recommended. If it is related to diet, a registered dietitian can help adjust food choices to lower BUN levels while reducing the risk of nutritional deficiencies.
2. Low BUN
Low BUN levels are usually not a cause for concern and may occur due to:
-
Low protein intake
-
Malnutrition
-
Pregnancy
-
Poor intestinal absorption
-
Overhydration
-
Inability of the liver to metabolize protein, such as in liver failure or cirrhosis
Although a low BUN is uncommon and usually not linked to serious conditions, results should always be interpreted by a healthcare provider.