BUN (Blood Urea Nitrogen)
A blood test that measures the amount of urea nitrogen in the bloodstream, used as a key indicator of kidney function and hydration status.
Last updated: April 10, 2026
BUN is one of the most widely used tests for evaluating kidney health. Urea is a waste product formed when the liver breaks down protein. The kidneys filter urea out of the blood and excrete it in urine. When the kidneys are not working properly, urea builds up in the bloodstream. A normal BUN level for adults is 7–20 mg/dL. BUN is almost always tested alongside creatinine because the two tests together give a more complete picture of kidney function.
Why Is BUN Tested?
The BUN test measures how well the kidneys are removing waste from the blood. Every time the body digests protein — whether from food or from the body's own tissues — the liver converts the nitrogen-containing waste into urea. Healthy kidneys then filter urea from the blood and send it to the bladder for removal in urine.
BUN is a standard component of the Comprehensive Metabolic Panel (CMP) and the Basic Metabolic Panel (BMP). It is one of the first tests ordered when kidney problems are suspected. However, BUN is not specific to the kidneys alone — it can also be affected by liver function, protein intake, hydration, and other factors.
A healthcare provider may order a BUN test as part of a routine health screening, to evaluate kidney function before starting certain medications, to monitor existing kidney disease, to assess dehydration, or to investigate symptoms such as swelling, fatigue, decreased urination, or confusion. It is frequently ordered alongside creatinine because the BUN-to-creatinine ratio helps distinguish between kidney-related and non-kidney-related causes of abnormal results.
Normal Reference Ranges
Reference ranges may vary by laboratory. Always use the ranges on your lab report. The values below are general guidelines.
| Category | Range | Unit |
|---|---|---|
| Adults | 7 – 20 | mg/dL |
| Adults (SI units) | 2.5 – 7.1 | mmol/L |
| Children | 5 – 18 | mg/dL |
| Older Adults (60+) | 8 – 23 | mg/dL |
| Status | BUN Level (mg/dL) |
|---|---|
| Normal | 7 – 20 |
| Borderline / Mildly Elevated | 21 – 30 |
| High | Greater than 30 |
What Does a HIGH BUN Level Mean?
A BUN level above 20 mg/dL may indicate that the kidneys are not filtering waste efficiently, but many non-kidney factors can also raise BUN.
Kidney-related causes:
- Chronic kidney disease (CKD) — gradual loss of kidney function over time
- Acute kidney injury — sudden decrease in kidney function from infection, medication, or reduced blood flow
- Kidney obstruction — kidney stones, tumors, or enlarged prostate blocking urine flow
Non-kidney causes (prerenal):
- Dehydration — the most common cause of mildly elevated BUN; concentrated blood leads to higher readings
- High-protein diet — eating large amounts of protein increases urea production
- Gastrointestinal (GI) bleeding — blood in the GI tract is digested like protein, raising urea levels
- Congestive heart failure — reduced blood flow to the kidneys causes urea to accumulate
- Burns or severe infections — increased tissue breakdown raises protein metabolism
- Medications — corticosteroids, tetracycline antibiotics, and some diuretics can raise BUN
- Fever or stress — increased metabolic rate can temporarily raise BUN
Severity and context matter. A BUN of 22 mg/dL with a normal creatinine in a person who had a steak dinner the night before is very different from a BUN of 60 mg/dL with elevated creatinine and reduced urine output. The BUN-to-creatinine ratio (normally 10:1 to 20:1) is particularly helpful. A ratio above 20:1 often points to a prerenal cause like dehydration, while a ratio in the normal range with both values elevated suggests intrinsic kidney disease.
What Does a LOW BUN Level Mean?
A BUN level below 7 mg/dL is considered low and is less common than high BUN. Low levels are generally less alarming but can sometimes indicate an underlying condition.
Possible causes of low BUN include:
- Liver disease — the liver produces urea, so severe liver damage (cirrhosis, hepatitis) can reduce urea production
- Low-protein diet — inadequate protein intake means less urea is produced
- Malnutrition — severe caloric or protein deficiency
- Overhydration — excessive fluid intake dilutes BUN in the blood
- Pregnancy — increased blood volume and higher kidney filtration rates during pregnancy can lower BUN
- Syndrome of inappropriate antidiuretic hormone (SIADH) — causes the body to retain too much water, diluting blood levels
In practice, slightly low BUN in a healthy person with adequate hydration and a balanced diet is rarely a concern. A healthcare provider will evaluate low BUN in the context of liver function tests, protein intake, and overall health status.
Factors That Can Affect Your Results
BUN levels are influenced by more factors than creatinine, which is why the two tests are almost always ordered together. Knowing what can shift BUN helps put results in proper context.
- Diet: High-protein meals, especially red meat, can raise BUN within hours. Vegetarian or low-protein diets tend to produce lower BUN levels. This is one of the biggest non-disease influences on the test.
- Hydration: Dehydration is one of the most common reasons for a mildly elevated BUN. Even mild dehydration from not drinking enough water before the test can push results above the normal range.
- Age: BUN tends to increase with age. Older adults often have slightly higher levels due to decreased kidney function and changes in body composition. Reference ranges for adults over 60 are slightly higher.
- Gender: Men tend to have slightly higher BUN levels than women, partly due to differences in muscle mass and protein metabolism.
- Pregnancy: BUN typically decreases during pregnancy due to increased blood volume and higher glomerular filtration rate. Lower BUN in pregnancy is expected and normal.
- Medications: Corticosteroids, tetracycline, and some chemotherapy drugs can raise BUN. Certain IV fluids can lower it by diluting the blood.
- Exercise: Intense or prolonged exercise increases protein breakdown and can temporarily raise BUN levels.
- GI bleeding: Blood in the stomach or intestines is digested as protein, which can significantly raise BUN without any kidney problem being present.
Should I Be Concerned About My BUN Level?
A BUN level that is slightly outside the reference range on its own is often not a major concern. BUN is one of the most diet- and hydration-sensitive blood tests, so a mildly elevated or low result may simply reflect what was eaten or how much water was consumed before the blood draw.
BUN is most informative when interpreted alongside creatinine. If both BUN and creatinine are elevated, kidney impairment is more likely. If BUN is high but creatinine is normal, a non-kidney cause such as dehydration or high protein intake is more probable. The BUN-to-creatinine ratio adds another layer of diagnostic value.
A single abnormal BUN reading will typically prompt a repeat test after ensuring proper hydration and a normal diet. Persistent abnormalities across multiple tests are taken more seriously and may lead to additional kidney function testing.
Common Next Steps
Depending on the result, a healthcare provider may recommend:
- Creatinine and eGFR — creatinine and estimated glomerular filtration rate provide a more specific assessment of kidney function (BUN vs. creatinine comparison)
- BUN-to-creatinine ratio — calculated from both results; helps distinguish prerenal causes (dehydration, heart failure) from intrinsic kidney disease
- Urinalysis — examines urine for protein, blood, and other markers of kidney damage
- Electrolyte panel — checks sodium, potassium, and chloride levels, which are often affected alongside BUN
- Hydration assessment — if dehydration is suspected, rehydrating and retesting may resolve the abnormality
- Liver function tests — if low BUN is found, liver function may be evaluated since the liver produces urea
- Dietary review — assessing protein intake, especially if BUN is elevated with normal creatinine
- Repeat testing — a single abnormal result is often repeated after a period of proper hydration and normal diet
Frequently Asked Questions
What is a normal BUN level?
A normal BUN level for adults is 7–20 mg/dL (2.5–7.1 mmol/L). Older adults may have a slightly higher normal range (up to about 23 mg/dL). Children generally have slightly lower levels. The exact range depends on the laboratory, so always refer to the values on your lab report.
What does BUN stand for?
BUN stands for blood urea nitrogen. Urea is a waste product that forms when the liver breaks down protein. The nitrogen component of urea is what the test measures. Healthy kidneys filter urea out of the blood and remove it through urine, so measuring BUN helps assess how well the kidneys are performing this filtering job.
What is the BUN-to-creatinine ratio?
The BUN-to-creatinine ratio is calculated by dividing the BUN value by the creatinine value. A normal ratio is approximately 10:1 to 20:1. A ratio above 20:1 with elevated BUN but normal creatinine often suggests a prerenal cause like dehydration, GI bleeding, or heart failure. A ratio below 10:1 may suggest liver disease, malnutrition, or certain rare conditions. The ratio adds important context that neither test provides on its own.
Is BUN the same as creatinine?
No, they are different tests that both assess kidney function. BUN measures urea nitrogen from protein breakdown, while creatinine measures a waste product from muscle metabolism. Creatinine is generally considered a more specific marker of kidney function because it is less affected by diet and hydration. However, the two tests together provide more information than either alone. See the BUN vs. creatinine comparison for a detailed breakdown.
Can dehydration affect BUN levels?
Yes, and significantly so. Dehydration is one of the most common causes of elevated BUN. When the body is dehydrated, the kidneys reabsorb more water and urea, concentrating urea in the blood. Mild dehydration from not drinking enough water before a blood draw can be enough to push BUN above the normal range. Rehydrating and retesting often resolves the elevation.
Does eating protein before the test affect BUN?
Yes. Because BUN is a byproduct of protein metabolism, eating a high-protein meal — especially red meat — within 12–24 hours before the test can raise BUN levels. This is one key difference from creatinine, which is much less affected by diet. If a BUN result is mildly elevated and the person recently consumed a large amount of protein, the healthcare provider may consider this when interpreting the result.
Is BUN included in routine blood work?
Yes. BUN is included in both the Comprehensive Metabolic Panel (CMP) and the Basic Metabolic Panel (BMP). These are among the most frequently ordered laboratory tests and are commonly part of annual physicals, pre-surgical evaluations, and hospital admissions.
Can GI bleeding raise BUN without kidney disease?
Yes. Gastrointestinal bleeding is a well-known non-kidney cause of elevated BUN. When blood enters the stomach or intestines (from ulcers, for example), the blood proteins are digested just like dietary protein, producing urea. This can raise BUN significantly while creatinine remains normal. A high BUN-to-creatinine ratio (above 20:1) with normal creatinine is a classic pattern that can prompt investigation for GI bleeding.
What does a low BUN level indicate?
A low BUN level (below 7 mg/dL) may indicate liver disease (since the liver produces urea), malnutrition, a very low-protein diet, overhydration, or pregnancy. Slightly low BUN in a healthy person is usually not concerning, but significantly low levels — especially when combined with abnormal liver function tests — may warrant further evaluation.
How is BUN different from urea?
BUN and urea measure the same substance but express it differently. In the United States, the test reports the nitrogen portion of urea (BUN, in mg/dL). In many other countries, the test reports the total urea molecule (in mmol/L). To convert urea (mmol/L) to BUN (mg/dL), multiply by 2.8. The two tests are clinically equivalent — the difference is simply in how the result is expressed.
Sources
- MedlinePlus. BUN (Blood Urea Nitrogen) Test. U.S. National Library of Medicine.
- National Kidney Foundation. Understanding Your Lab Values. 2024.
- Hosten AO. BUN and Creatinine. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition.
- KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.
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Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice. Reference ranges vary by laboratory, testing method, and individual factors. Always discuss your results with a qualified healthcare provider.