Blood urea nitrogen (BUN)
Waste product of protein breakdown that the kidneys remove in urine
Also known as
Definition
Urea forms when the body breaks down protein. It is carried in the blood to the kidneys and removed in urine. A small amount in the blood is normal; if the kidneys are not working well, urea can build up. Results are given either as urea or as blood urea nitrogen (BUN), which counts only the nitrogen part of urea. The test is often part of a larger lab panel and is used together with other values to assess kidney function.
| Parameter | Value |
|---|---|
| Unit | mg/dl or mmol/l |
| Reference Range | Typically about 17–43 mg/dl of urea in adults (2.8–7.1 mmol/l). The ranges differ by age and sex; the German institute IQWiG gives 15–40 mg/dl for women under 50 and 21–43 mg/dl over 50, and 19–44 mg/dl for men under 50 and 18–55 mg/dl over 50. As urea nitrogen (BUN), the adult range corresponds to roughly 8–20 mg/dl; MedlinePlus gives 6–20 mg/dl BUN. Labs differ; the range printed on your report is what counts, and your doctor interprets the result. |
Agree on your personal target range with your doctor.
Conversion: urea (mg/dl) = BUN (mg/dl) × 2.14; BUN (mg/dl) = urea (mg/dl) × 0.467; urea (mmol/l) = urea (mg/dl) × 0.167. The ranges by age and sex come from IQWiG (Gesundheitsinformation.de) and the usual adult range of 17–43 mg/dl from Apotheken Umschau (both in German); the BUN figure is converted from them and rounded.
↓ What a low value means
A low level can be linked to a low-protein diet, malnutrition, liver disease up to liver failure, or over-hydration. In liver disease the level can be low even when the kidneys are working normally. Discuss any deviations with your doctor.
↑ What a high value means
A raised level can be a sign that the kidneys are not working well, for example in kidney disease or kidney failure. It can also be linked to dehydration, a very high-protein diet, bleeding in the digestive tract, congestive heart failure, a recent heart attack, burns, shock, a blockage of the urinary tract or certain medicines. An abnormal result does not always mean a condition that needs treatment. Urea is only one measure of kidney function; your doctor may add further values such as creatinine and GFR.
When to test
Often as part of a larger metabolic panel at a routine check-up, to monitor known kidney disease and when the risk of kidney problems is raised, for example with diabetes, high blood pressure, heart disease, obesity, a family history, smoking or age over 60. Early kidney disease often causes no symptoms. Usually no preparation is needed; if other values are tested on the same sample, you may need to fast. Never stop medicines on your own.
Frequently asked questions
What is the difference between urea and BUN? +
Both describe the same substance. BUN (blood urea nitrogen) counts only the nitrogen part of urea, so BUN values in mg/dl are lower: urea in mg/dl is about BUN × 2.14. When comparing reports, check which form your lab uses.
Why is urea often measured together with creatinine? +
Urea is only one measure of kidney function. If kidney disease is suspected, further tests may follow, such as creatinine, another waste product removed by the kidneys, and GFR, which estimates how well the kidneys filter the blood.
Can diet and fluid intake affect the level? +
Yes. A very high-protein diet and dehydration can be linked to a higher level; a low-protein diet, malnutrition or over-hydration to a lower one. A single abnormal value is therefore not proof of kidney disease; your doctor interprets the result.
Does the level change with age? +
Urea levels can differ by sex and usually rise with age. That is why the reference range on your report is what counts.
Blood urea nitrogen (BUN)
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.