What this test measures
Urea is produced in the liver from protein breakdown and excreted by the kidneys. BUN (Blood Urea Nitrogen) measures the nitrogen content of urea in mg/dL — the standard Indian unit. A few labs report "urea" directly in mg/dL; conversion is BUN × 2.14 = urea.
Unlike creatinine, BUN is heavily influenced by diet (protein intake), hydration, GI bleeding (digested blood becomes urea), and liver function. The BUN/Creatinine ratio is therefore more informative than BUN alone.
Why it matters
BUN is rarely used in isolation — it is part of the Kidney Function Test panel. The most useful interpretation is the BUN/Creatinine ratio: a ratio > 20 suggests "pre-renal" causes (dehydration, GI bleeding, heart failure) rather than intrinsic kidney disease. Low BUN with normal creatinine in a hospitalised patient can signal liver failure or severe protein malnutrition.
How to prepare
No fasting required. Stay normally hydrated. Avoid heavy meat / protein meals the day before — they transiently raise BUN. Continue medications unless your doctor advises otherwise; mention any recent steroid or tetracycline use, which can raise BUN.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Blood Urea Nitrogen (BUN) (mg/dL)[1][2] | 7 – 20 | Low BUN may reflect low protein intake, severe liver disease (urea is made in the liver), or over-hydration. Pregnancy normally lowers BUN slightly due to expanded plasma volume. | High BUN can reflect reduced kidney clearance, dehydration, high-protein diet, GI bleeding (digested blood adds nitrogen), heart failure, or certain drugs (steroids, tetracyclines). Pair with creatinine — a BUN/Creatinine ratio > 20 favours dehydration or GI bleed; ratio 10–20 with both raised suggests true kidney dysfunction. |
BUN / Creatinine ratio interpretation
| BUN/Creatinine ratio | Likely cause | Notes |
|---|---|---|
| < 10 | Hepatic synthesis problem, low protein intake, over-hydration | Check liver function and dietary protein |
| 10 – 20 | Normal / intrinsic kidney disease | Use eGFR to assess kidney function |
| > 20 | Pre-renal (dehydration, GI bleed, heart failure) | Re-hydrate, evaluate for GI bleed if symptoms; check heart function |
Frequently asked questions
Do I need to fast for a BUN test?
No fasting is needed. Avoid a heavy protein / meat meal the night before, which transiently raises BUN.
My BUN is high but creatinine is normal — what does that mean?
A high BUN with normal creatinine usually points to a pre-renal cause: dehydration, GI bleeding, heart failure, high-protein diet, or steroid use. Re-hydrate and re-test; if it persists, your doctor will investigate.
What is the difference between BUN and urea?
BUN is the nitrogen portion of urea molecules. Urea is the molecule itself. Some labs report BUN (mg/dL), others report urea (mg/dL). Convert with: urea = BUN × 2.14.
Should I do BUN alone or a full KFT?
A full Kidney Function Test (KFT) including creatinine, electrolytes, uric acid and eGFR gives much more information for the same blood draw. Standalone BUN is rarely ordered.
Can a high-protein diet raise BUN?
Yes — a heavy protein / meat meal the day before testing can transiently raise BUN by 5–10 mg/dL. Sustained high-protein diets (e.g. competitive bodybuilding, certain weight-loss diets) can chronically raise BUN.
Why is BUN low in liver disease?
Urea is synthesised in the liver. Severe liver disease (advanced cirrhosis, fulminant hepatitis) reduces urea synthesis — BUN drops despite kidneys functioning normally.
How often should I check BUN?
As part of an annual KFT for at-risk adults (diabetes, hypertension, age 60+); more often if kidney disease is already known.
Related Kidney / Electrolytes tests
Tests commonly ordered alongside BLOOD UREA NITROGEN (BUN), or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — BUN Test · accessed 2026-05-29T00:00:00.000Z
- KDIGO 2024 CKD Guideline · accessed 2026-05-29T00:00:00.000Z
- NCBI StatPearls — Blood Urea Nitrogen · accessed 2026-05-29T00:00:00.000Z
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