Blood Urea Nitrogen (BUN)
As part of a routine comprehensive or basic metabolic panel (CMP or BMP) during a health exam; when you have signs and symptoms that may be due to kidney disease or you have a condition that may cause or be worsened by kidney dysfunction; at regular intervals when you are being treated for kidney disease or damage
A blood sample drawn from a vein
Urea is a waste product formed in the liver when protein is metabolized into its component parts (amino acids). This process produces ammonia, which is then converted into the less toxic waste product urea. This test measures the blood urea nitrogen (BUN) level in the blood. Sometimes, a BUN to creatinine ratio is calculated to help determine the cause of elevated levels.
Nitrogen is a component of both ammonia and urea. Urea and urea nitrogen are referred to somewhat interchangeably because urea contains nitrogen and because urea/urea nitrogen is the "transport method" used by the body to rid itself of excess nitrogen. Urea is formed in and released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and released into the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood. However, when the kidneys cannot filter wastes out of the blood due to disease or damage, then the level of urea in the blood will rise.
The kidneys are a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. Within them are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a microscopic cluster of looping blood vessels, called glomerulus. The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what still can be used by the body. The remaining waste forms urine.
Most diseases or conditions that affect the kidneys or liver have the potential to affect the amount of urea present in the blood. If increased amounts of urea are produced by the liver or if the kidneys are not working properly and have difficulty filtering wastes out of the blood, then urea levels will rise in the blood. If significant liver damage or disease inhibits the production of urea, then BUN levels may fall.
How is the test used?
The BUN test is primarily used, along with the creatinine test, to evaluate kidney function in a wide range of circumstances, to help diagnose kidney disease, and to monitor people with kidney dysfunction or failure. It also may be used to evaluate your general health status when ordered as part of a renal panel, basic metabolic panel or comprehensive metabolic panel.
If your creatinine and BUN tests are found to be abnormal or if you have an underlying disease that is known to affect the kidneys, such as diabetes or high blood pressure, then creatinine and BUN tests may be used to monitor for kidney dysfunction and the effectiveness of treatment.
Blood creatinine and BUN tests may also be ordered to evaluate kidney function prior to some procedures, such as a CT (computed tomography) scan, that may require the use of drugs that can damage the kidneys if not cleared rapidly.
When is it ordered?
- When you have a routine health checkup
- When you are acutely ill and seek medical care at an emergency room
- During a hospital stay
- Blood and/or protein in your urine
- Swelling or puffiness (edema), particularly around the eyes or in the face, wrists, abdomen, thighs, or ankles
- Urine that is foamy, bloody, or coffee-colored
- A marked decrease in the amount of urine
- Problems urinating, such as a burning feeling or abnormal discharge during urination, or a change in the frequency of urination, especially at night
- Mid-back pain (flank), below the ribs, near where the kidneys are located
- High blood pressure (hypertension)
BUN also may be ordered:
- At regular intervals to monitor kidney function when you have a chronic disease or condition such as diabetes, congestive heart failure, or when you have had a heart attack
- At regular intervals to monitor kidney function and treatment when you have been diagnosed with kidney disease
- Prior to and during certain drug treatments to monitor kidney function
- Along with a creatinine when a CT scan is planned
- At regular intervals to monitor the effectiveness of dialysis
What does the test result mean?
BUN and creatinine levels that are within the ranges established by the laboratory performing the test suggest that your kidneys are functioning as they should. Increased BUN and creatinine levels may mean that your kidneys are not working as they should. Your healthcare practitioner will consider other factors, such as your medical history and physical exam, to determine what condition, if any, may be affecting your kidneys.
BUN Reference Range
The reference ranges1 provided here represent a theoretical guideline that should not be used to interpret your test results. Some variation is likely between these numbers and the reference range reported by the lab that ran your test. Please consult your healthcare provider.
AGE CONVENTIONAL UNITS2 SI UNITS3 0-18 years Not available due to wide variability. See child's lab report for reference range. Adult 6-20 mg/dL 2.1-7.1 mmol/L >60 years 8-23 mg/dL 2.9-8.2 mmol/L
1 from Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. 5th edition, St. Louis: Elsevier Saunders; 2011.
2 Conventional Units are typically used for reporting results in U.S. labs
3 SI Units are used to report lab results outside of the U.S.
Some examples of conditions that can increase BUN levels include:
- Kidney disease, kidney damage, or kidney failure; for more details, read the article on Kidney Disease.
- Decreased blood flow to the kidneys, caused by conditions such as congestive heart failure, shock, stress, recent heart attack, or severe burns
- Conditions that can block the flow of urine, such as kidney stones
- Increased protein breakdown or significantly increased protein in the diet
Low BUN levels are not common and are not usually a cause for concern. Decreased BUN may be seen in severe liver disease, malnutrition, and sometimes when you are overhydrated (too much fluid volume), but the BUN test is not usually used to diagnose or monitor these conditions.
If one kidney is fully functional, the BUN level may be normal even when the other kidney is not functioning properly.
My BUN result is slightly out of range. What does this mean?
Your BUN result is interpreted by your healthcare provider within the context of other tests that you have had done as well as other factors, such as your medical history. A single result that is slightly high or low may or may not have medical significance. This is why a healthcare practitioner may repeat a test on you and why they may look at results from prior times when you had the same test performed.
However, a result outside the range may indicate a problem and warrant further investigation. Your healthcare provider will evaluate your test results in the context of other relevant factors to determine whether a result that falls outside of the reference range means something significant for you.
For more, read the article Reference Ranges and What They Mean.
What is a BUN to Creatinine ratio?
Occasionally, a healthcare practitioner will look at the ratio between a person's BUN and blood creatinine to help determine what is causing these concentrations to be higher than normal. The ratio of BUN to creatinine is usually between 10:1 and 20:1. An increased ratio may be due to a condition that causes a decrease in the flow of blood to the kidneys, such as congestive heart failure or dehydration. It may also be seen with increased protein, from gastrointestinal bleeding, or increased protein in the diet. The ratio may be decreased with liver disease (due to decrease in the formation of urea) and malnutrition.
What other tests are used with the blood urea nitrogen test to check how my kidneys are functioning?
BUN and creatinine are the primary tests used to check how well the kidneys are able to filter waste products from your blood. Your healthcare provider may also order other tests to evaluate kidney function. Examples include:
- Renal panel
- Urine protein—a few different tests may be used to screen for protein in the urine:
- Urine albumin and albumin/creatinine ratio (ACR)—these detect small amounts of albumin in the urine, which may indicate kidney dysfunction.
- Urinalysis—this is a routine test that can detect protein in the urine as well as red blood cells and white blood cells. These are not normally found in the urine and, if present, may indicate kidney disease.
- Urine total protein or urine protein to creatinine ratio (UP/CR)—detects not just albumin, but all types of proteins that may be present in the urine.
- Estimated glomerular filtration rate (eGFR)— a calculation based on the results of a blood creatinine test or a cystatin C test, along with other variables such as age, sex, and race (e.g., African-American, non-African American), depending on the equation used. The glomerular filtration rate refers to the amount of blood that is filtered by the glomeruli per minute. As kidney function declines, the filtration rate decreases.
How does BUN change with age?
Is there anything else I should know?
BUN levels can increase with the amount of protein in the diet. High-protein diets may cause abnormally high BUN levels while very low-protein diets can cause an abnormally low BUN.
A wide variety of drugs can cause an increase in BUN. Drugs that can decrease BUN include chloramphenicol and streptomycin. Inform your healthcare provider of any medications you are taking.
Both decreased and increased BUN concentrations may be seen during a normal pregnancy.