BUN · low result
Low BUN: what it means (and why it is usually nothing)
Short answer
A low BUN — below about 6–7 mg/dL — is usually unremarkable. Urea comes from dietary protein and is made by the liver, so a low-protein or vegetarian diet, over-hydration before the draw, pregnancy (plasma volume expands and the kidneys clear more), a small body size and youth all lower it without meaning anything is wrong. The exceptions are significant liver disease, which makes less urea, and severe malnutrition. A low BUN with normal creatinine, liver tests and albumin needs no follow-up; it is the company it keeps that decides.
Medically reviewed by Youmna DiStefano, MD · Last reviewed November 2026
When does this result need same-day care?
- A low BUN is not urgent on its own. Seek care for the condition it may accompany — jaundice, abdominal swelling or confusion (liver), or rapid unintended weight loss.
What is the normal range for BUN?
- age 18+: 7.00 – 20.0 mg/dL
Can reflect low-protein intake, liver disease, or overhydration.
Full reference table, citations and testing options: BUN biomarker page.
What are the common causes of low BUN?
- Low-protein, vegetarian or vegan diets, and small body size — less protein in, less urea made
- Over-hydration — drinking large volumes before the draw, intravenous fluids, SIADH
- Pregnancy, particularly the second and third trimesters
- Liver disease — cirrhosis, severe hepatitis, acute liver failure; the liver makes urea, so a failing liver makes less
- Malnutrition and malabsorption — too little protein absorbed to make urea from
- Anabolic states and growth — children, bodybuilding phases with anabolic steroids
- Rare inherited urea-cycle disorders — present from childhood with high ammonia
What symptoms go with low BUN?
- None from the BUN itself
- Symptoms belong to the cause when there is one — jaundice, swelling and confusion with liver disease; weight loss and weakness with malnutrition
What do clinicians usually check next?
- Nothing if creatinine, eGFR, albumin and liver tests are normal and you eat a low-protein or plant-based diet
- A liver panel (ALT, AST, bilirubin, albumin) when the low BUN is unexplained or there are liver symptoms
- A nutrition review and albumin or prealbumin when intake may be inadequate
- Repeat with normal fluid intake if a very high water intake preceded the draw
- Do not repeat for the BUN itself; it will be on the next routine metabolic panel
Follow-up tests available at TestWell: ALT Test (Alanine Aminotransferase) ($16.99), Prealbumin Test ($29.99).
How do you recheck BUN?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Basic Metabolic Panel | Test · 8 biomarkers | $17.99 | Yes | 1–3 business days |
Common questions
Should I worry about a low BUN?
Usually not. It reflects diet or hydration far more often than disease.
What is a low BUN level?
Below the lab's lower limit, usually about 6–7 mg/dL. Values of 4–6 are common in people who eat little meat, drink a lot of water, are pregnant or are small; values below that with abnormal liver tests or low albumin are the ones that get a closer look.
Can a low BUN mean liver disease?
It can accompany significant liver disease, because the liver is where urea is made — but then bilirubin, albumin, ALT or AST are also abnormal and the diagnosis rests on those. An isolated low BUN with a normal liver panel is not a liver finding.
Does a vegetarian or vegan diet lower BUN?
Yes — less dietary protein means less urea, and plant-based eaters commonly run a BUN of 5–8. It is expected and harmless, and worth remembering so a 'low' flag is not mistaken for a problem.
Does drinking a lot of water lower BUN?
Yes, temporarily — extra water dilutes the blood and the kidneys clear more urea, so a very high intake before the draw can push BUN under the range. It corrects within hours and has no consequences.
Is a low BUN normal in pregnancy?
Yes. Plasma volume rises by up to half and the kidneys filter more, so BUN typically falls to 5–8 mg/dL during pregnancy. Pregnancy-specific reference ranges account for it.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.