Blood Urea Nitrogen (BUN) Test
A blood test that measures the amount of urea nitrogen in your blood to evaluate how well your kidneys are functioning. Urea nitrogen is a waste product that forms when your liver breaks down protein from the foods you eat, and healthy kidneys filter it out of your body through urine. Your doctor may order this test during a routine checkup, to monitor known kidney disease, to check for dehydration, or to investigate symptoms such as swelling in your legs or feet, frequent urination, or unexplained fatigue. No fasting is usually required, though you may be asked to avoid eating large amounts of meat or protein for 24 hours before the test. Normal levels typically range from 7 to 20 mg/dL (2.5 to 7.1 mmol/L), and your doctor will interpret your results alongside other tests such as creatinine and eGFR.
{"The BUN test measures waste product (urea) in your blood that kidneys normally filter out","Elevated BUN levels may indicate kidney problems or dehydration","Normal BUN levels range from 7 to 20 mg/dL, but reference ranges vary by laboratory","The test requires a simple blood draw","Results are often interpreted together with other kidney function tests like creatinine"}
Key facts at a glance
- LOINC
- 35234-4
- Short code
- BUN
- Synonyms
- BUN SerPl-msCnc
- Category
- chemistry
- Sample
- serum
- Container
- Serum separator tube (SST/Gold top) or plain red top tube
- Units
- not specified
- Ref low
- 7
- Ref high
- 20
- Critical range
- 5 not specified – 100 not specified
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- ≥ 0 years
- Method
- Enzymatic (Urease) or Colorimetric Method
- CPT
- 84520
- SNOMED
- 271565004
- Specimen volume
- 1-2 mL serum minimum
- Stability
- Room temperature: 24-48 hours; Refrigerated (2-8°C): 7 days; Frozen (-20°C): 30 days. Avoid repeated freeze-thaw cycles.
- Hemolysis max
- 50
- Lipemia max
- 300
- Icterus max
- 20
- TAT
- 1–4 hours
- Order
- routine
What is this test?
The Blood Urea Nitrogen (BUN) test measures the amount of urea nitrogen in your blood. Urea nitrogen is a waste product formed when your liver breaks down protein from the foods you eat. Normally, your kidneys filter urea nitrogen out of your blood and it leaves your body through urine. This test helps doctors evaluate how well your kidneys are working.
Why is this test performed?
Your doctor may order a BUN test as part of a routine health checkup or to: monitor kidney function if you have known kidney disease, check for dehydration, evaluate the effectiveness of kidney treatments or dialysis, or investigate symptoms like frequent urination, swelling in your legs or feet, or unexplained fatigue.
How to prepare for the test
For a BUN test, you typically do not need to fast. However, your doctor may ask you to avoid eating large amounts of meat or protein for 24 hours before the test, as this can affect results. Inform your doctor about all medications, vitamins, or supplements you are taking, as some medicines can influence BUN levels. Drink plenty of water unless otherwise instructed.
What happens during the test?
A healthcare professional will draw a small blood sample from a vein in your arm using a needle. You may feel a brief pinch or sting. The sample is then sent to a laboratory for analysis. The entire process takes only a few minutes.
Understanding your results
Normal BUN levels typically range from 7 to 20 mg/dL (2.5 to 7.1 mmol/L), but reference ranges may differ depending on the laboratory and your individual health factors. Higher than normal BUN levels may indicate kidney problems, dehydration, heart failure, or excessive protein intake. Lower than normal BUN levels may suggest liver disease or malnutrition. Your doctor will interpret your results in context with other tests.
Reference range
7-20 mg/dLnot specified
No special preparation or fasting required
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
The BUN test is primarily used to assess kidney function. Elevated BUN levels (azotemia) can result from kidney disease, acute kidney injury, dehydration, heart failure, or urinary tract obstruction. Decreased BUN levels may indicate severe liver disease, malnutrition, or overhydration. The BUN-to-creatinine ratio can help doctors determine whether kidney issues are pre-renal (before the kidneys), renal (within the kidneys), or post-renal (after the kidneys). This test is frequently ordered alongside creatinine and estimated glomerular filtration rate (eGFR) for comprehensive kidney assessment.
Frequently asked questions
Does the BUN test diagnose specific kidney diseases?
Can I eat before a BUN test?
What can cause falsely elevated BUN results?
Is the BUN test dangerous?
How often should I have my BUN levels checked?
Sources
- 1MedlinePlus - BUN TestSource
- 2
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 10, 2026. · Read our editorial policy