Optimal Result:
6 - 24 mg/dL,
or
2.14 - 8.57 mmol/L.
Medically reviewed on February 14, 2026
Normal BUN for adults is 7–20 mg/dL (ranges vary by age — see table below)
BUN measures how well your kidneys filter protein waste from your blood
High BUN often means dehydration, high protein intake, or reduced kidney function
Low BUN can indicate liver disease, malnutrition, or overhydration
BUN is most useful when interpreted alongside creatinine and eGFR
Blood urea nitrogen (BUN) is a measure of how much nitrogen — from protein breakdown — remains in your blood, and it is one of the primary indicators of kidney health. Healthy kidneys continuously filter urea from the blood and excrete it in urine. When kidneys are damaged or under stress, BUN rises.
Here is how it works: When you eat protein, your body breaks it down into amino acids, producing ammonia as a byproduct. Your liver converts that ammonia into urea — a less toxic compound — and releases it into the bloodstream. Your kidneys then filter urea out and excrete it in urine. A BUN test measures how much urea nitrogen remains in your blood at the time of the test.
Because BUN depends on both protein intake and kidney function, it is always interpreted alongside creatinine and eGFR for a complete picture of kidney health.
Normal BUN varies by age. The table below shows standard reference ranges. Note that some labs use slightly different ranges — always compare your result to the range printed on your lab report.
| Age Group | Normal Range (mg/dL) | Normal Range (mmol/L) | Notes |
|---|---|---|---|
| 0–2 years | 4–15 | 1.4–5.4 | Lower due to rapid growth |
| 3–16 years | 9–18 | 3.2–6.4 | — |
| 17–64 years | 8–22 | 2.9–7.9 | Most labs use 7–20 mg/dL |
| 65+ years | 10–28 | 3.6–10.0 | Higher due to reduced kidney reserve |
HealthMatters displays the optimal result range of 6–24 mg/dL, which represents the range where most healthy adults cluster. Your lab's reference range may differ slightly.
The BUN test is part of the Comprehensive Metabolic Panel (CMP), a standard blood panel typically ordered at routine health checkups. A BUN test helps your doctor:
People with early kidney disease often have no symptoms. A BUN test can detect kidney stress before symptoms appear, when treatment is most effective.
A high BUN level means your kidneys may not be clearing urea efficiently, or your body is producing more urea than usual due to increased protein breakdown. High BUN alone is not diagnostic — doctors always interpret it alongside creatinine and eGFR to identify the cause.
⚠️ When to see a doctor
A one-time slightly elevated BUN is often benign (dehydration, recent high-protein meal). If your BUN is consistently above 25 mg/dL, rising over time, or accompanied by changes in creatinine or eGFR — speak to your doctor. Persistent high BUN alongside low eGFR is a red flag for chronic kidney disease (CKD).
The BUN/creatinine ratio is more informative than BUN alone. It helps distinguish between kidney disease, dehydration, and GI bleeding.
| BUN/Creatinine Ratio | Likely Interpretation | Common Causes |
|---|---|---|
| > 20:1 (high) | Pre-renal or non-kidney cause | Dehydration, GI bleeding, heart failure, high-protein diet |
| 10–20:1 (normal) | Normal kidney perfusion | — |
| 10–20:1 + both elevated | Intrinsic kidney disease | CKD, AKI, glomerulonephritis |
| < 10:1 (low) | Liver or nutritional issue | Liver disease, malnutrition, overhydration |
Both BUN and creatinine are waste products filtered by the kidneys, but they differ in important ways:
| Feature | BUN | Creatinine |
|---|---|---|
| Source | Protein metabolism (diet + body) | Muscle breakdown (constant rate) |
| Affected by diet? | Yes — high protein raises BUN | Minimally (very high meat intake slightly raises it) |
| Affected by hydration? | Yes — dehydration raises BUN significantly | Less so |
| Liver involvement? | Yes — liver disease lowers BUN | No direct liver link |
| Kidney specificity | Less specific (many non-kidney causes) | More specific to kidney filtration |
| Best used for | Screening + ratio analysis | Estimating GFR, staging CKD |
BUN can rise or fall for many reasons unrelated to kidney disease. This is why it is never interpreted in isolation:
Treatment depends entirely on the underlying cause. Do not attempt to manage an abnormal BUN without medical guidance. Common approaches:
No. Urea is the chemical compound (NH₂)₂CO produced by the liver from ammonia. BUN measures specifically the nitrogen portion of urea in the blood. Since urea contains two nitrogen atoms, BUN is approximately 2.14 times the urea concentration. In the UK and Europe, labs typically report urea (mmol/L) rather than BUN (mg/dL).
BUN above 100 mg/dL is considered severely elevated and may indicate acute kidney injury or end-stage kidney disease. However, "danger" depends on context — a BUN of 40 mg/dL in a person with no prior kidney disease is more alarming than a BUN of 40 mg/dL in someone with known stable CKD. Always consult your doctor.
Yes — dehydration is one of the most common causes of elevated BUN. When you are dehydrated, blood volume decreases, concentrating all blood markers including BUN. A BUN/creatinine ratio above 20:1 in a dehydrated patient often resolves completely with fluid repletion within 24 hours.
For adults aged 17–64, yes — a BUN of 25 mg/dL is mildly above the typical reference range of 8–22 mg/dL. However, for adults over 65, a BUN of 25 mg/dL falls within the normal range (10–28 mg/dL). Context matters: check your lab's reference range, hydration status, protein intake, and whether creatinine is also elevated.
Low BUN (below 6–7 mg/dL) most commonly indicates liver disease (reduced urea production), severe malnutrition or very low protein intake, or overhydration (dilution effect). Pregnancy (especially first trimester) can also cause mildly low BUN due to expanded blood volume. Low BUN should be evaluated alongside liver enzymes and albumin.
The BUN/creatinine ratio helps identify why BUN is abnormal. A ratio above 20:1 with elevated BUN suggests a pre-renal cause (dehydration, GI bleeding, heart failure). A ratio of 10–20:1 with both values elevated suggests intrinsic kidney disease. A ratio below 10:1 suggests liver disease, malnutrition, or overhydration.
Yes. Protein metabolism produces ammonia, which the liver converts to urea. A high-protein diet — including excess protein shakes, meat, or supplements — can raise BUN 10–20% above baseline. This is a benign dietary effect, not a kidney problem, provided creatinine and eGFR remain normal.
The BUN test does not require fasting. However, a large high-protein meal in the hours before the test can mildly elevate your result. If you are having a full CMP (which includes BUN), your doctor may ask you to fast for 8–12 hours because other markers in the panel (glucose, triglycerides) do require fasting.
Testing.com. Blood Urea Nitrogen. testing.com
Lyman JL. Blood urea nitrogen and creatinine. Emerg Med Clin North Am. PubMed
Mayo Clinic. Blood Urea Nitrogen (BUN) Test. mayoclinic.org
National Kidney Foundation. Understanding Lab Values. kidney.org
Gounden V, Bhatt H, Jialal I. Renal Function Tests. StatPearls. NCBI
MedlinePlus. BUN (Blood Urea Nitrogen). medlineplus.gov
Seki M et al. Blood urea nitrogen is independently associated with renal outcomes in CKD. BMC Nephrol. 2019. doi:10.1186/s12882-019-1306-1
Inker LA, Perrone RD. Assessment of Kidney Function. UpToDate. uptodate.com
A high blood urea nitrogen (BUN) level means your kidneys may not be clearing urea - a waste product from protein metabolism - as efficiently as they should, or that your body is producing excess urea due to increased protein breakdown. BUN is a useful signal, but it is not specific on its own. Doctors always interpret it alongside other markers to understand the underlying cause.
BUN can rise for many reasons, including:
Dehydration (one of the most common causes)
High-protein diet or increased protein breakdown
Gastrointestinal bleeding
Severe infection, burns, trauma, or surgery
Medications such as corticosteroids, tetracyclines, or diuretics
Heart failure or conditions that reduce blood flow to the kidneys
Kidney disease, both acute and chronic
If BUN rises to very high levels, you may notice:
Swelling in the legs, hands, or face
Fatigue or weakness
Itching or dry skin
Muscle cramps
Nausea or poor appetite
Changes in urine frequency or volume
These symptoms usually appear later in the course of kidney dysfunction.
Creatinine is the most important companion marker.
Both BUN and creatinine elevated: Suggests kidney dysfunction (acute or chronic), reduced kidney perfusion, or severe dehydration.
High BUN with normal creatinine: More likely due to dehydration, high protein intake, GI bleeding, steroid use, or increased catabolism.
This ratio helps identify why BUN is high.
>20:1 (high ratio): Often dehydration, GI bleed, heart failure, shock, or high-protein intake.
Normal ratio (10–20:1) with both elevated: Likely intrinsic kidney disease.
Low ratio: Possible liver disease, malnutrition, or overhydration.
Low eGFR + high BUN strongly suggests reduced kidney filtration (CKD or AKI).
Normal eGFR + high BUN typically points to non-kidney causes such as dehydration, protein load, GI bleeding, or medications.
Kidney dysfunction often affects electrolytes:
High potassium (hyperkalemia) + high BUN: Possible advanced CKD or reduced kidney perfusion.
Low sodium (hyponatremia) may occur in heart failure or overhydration.
Metabolic acidosis (low CO2/bicarbonate): Common in chronic or acute kidney injury.
Because urea is produced in the liver:
High BUN + abnormal liver markers may indicate combined liver–kidney involvement.
Low BUN + poor liver function suggests reduced urea production due to liver dysfunction.
These can help confirm kidney disease:
Proteinuria or albuminuria + high BUN: Damage to kidney filtering units (glomeruli).
Hematuria: Possible glomerulonephritis or infection.
Anemia + high BUN: Suggests chronic kidney disease, as the kidneys also help regulate red blood cell production.
High BUN + low hemoglobin may also indicate GI bleeding.
Treatment depends entirely on the cause—not just the number. Below are common approaches:
Increase water intake
IV fluids if severe
Avoid excess caffeine or diuretics unless medically needed
Expected effect: BUN often normalizes quickly.
Reduce excessive protein intake
Address causes of catabolism (infection, fever, trauma, hyperthyroidism)
Review supplements (e.g., protein shakes, creatine)
Identify and treat the bleeding source
Medications to reduce stomach acid
Endoscopic treatment if needed
Note: GI bleeding can dramatically raise BUN even when kidneys are normal.
Control blood pressure (ACE inhibitors/ARBs are often first-line)
Tight blood sugar control if diabetic
Manage electrolytes
Reduce sodium intake
Treat underlying causes such as obstruction, autoimmune disease, or infection
Avoid nephrotoxic medications (NSAIDs, contrast dyes where possible)
Optimize heart failure treatment (diuretics, ACE inhibitors, beta blockers)
Reduce fluid overload
Manage blood pressure and vascular health
If steroids, tetracyclines, diuretics, or other drugs are contributing:
Adjust dosage or switch medications if appropriate
Discuss with your doctor before making any changes
If kidney function declines significantly:
Dialysis may be necessary
Address anemia, bone health, and electrolyte imbalances caused by CKD
Monitoring BUN over time - alongside creatinine, eGFR, electrolytes, and urinalysis - can:
Detect early kidney stress
Distinguish hydration issues from true kidney disease
Guide dietary changes and medication adjustments
Help evaluate the severity of existing kidney conditions
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Low BUN (typically below 6–7 mg/dL) is less common but can indicate liver problems, malnutrition, or overhydration. Because the liver produces urea, a damaged liver produces less of it — leading to low BUN even when the kidneys are working normally.
Low BUN should be evaluated alongside albumin, ALT, and AST to distinguish liver disease from nutritional deficiency.
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