Home Biomarkers Blood Urea Nitrogen (BUN)

Blood Urea Nitrogen (BUN)

Kidney Biomarker

Sample Needed

Collection Type: Blood

Body System

Related System: Kidney

Overview

Blood Urea Nitrogen (BUN) measures the amount of nitrogen in the blood that comes from urea, a waste product formed in the liver when the body breaks down protein. The BUN test evaluates how well the kidneys are filtering and excreting urea and can indicate kidney function, hydration status, and protein metabolism. Elevated BUN may suggest kidney impairment, dehydration, high protein intake, gastrointestinal bleeding, or heart failure; low BUN can reflect liver disease, malnutrition, or overhydration. Results vary with age (older adults often have slightly higher BUN), pregnancy (often lower), muscle mass and dietary protein; gender differences are minimal.

People also test for

Test Preparation

No special preparation is required

Why Do I Need This Test

Profile: Kidney - Symptoms: reduced urine output, swelling, fatigue, confusion, shortness of breath, changes in urine color or volume - Diagnoses/monitoring: acute kidney injury, chronic kidney disease, dehydration, monitoring kidney function during treatment - Reasons for abnormal levels: decreased renal clearance, increased protein catabolism or intake, liver dysfunction, volume status changes - Biological meaning: high BUN = impaired excretion or increased production; low BUN = reduced production (liver) or dilution - Behaviors: high-protein diet, dehydration, certain medications; family history of kidney disease or diabetes may prompt testing

Run our symptom checker to see if this test is right for you

Symptom Checker

Understanding Test Results

Values 7–20 mg/dL are considered normal.
Mild elevation (21–40 mg/dL) commonly reflects dehydration, high protein intake, recent GI bleeding, or early kidney impairment.
Moderate elevation (41–80 mg/dL) suggests significant renal dysfunction or marked pre-renal causes (severe dehydration, heart failure) and may require prompt evaluation.
Very high values (>80 mg/dL) indicate severe kidney failure or advanced uremia and often need urgent intervention.
Low values (<7 mg/dL), especially <3 mg/dL, can indicate severe liver disease, malnutrition, or overhydration.
Always interpret BUN with serum creatinine and BUN:creatinine ratio: a ratio >20:1 favors pre-renal causes (dehydration, hypovolemia), whereas a low ratio (<10:1) can point to intrinsic renal damage or low protein states.
Clinical context and repeat testing guide management.

Normal Range

7-20 mg/dL OR 2.5-7.1 mmol/L

FAQs

Q: What does a high urea nitrogen BUN mean?

A: Blood urea nitrogen (BUN) measures urea in the blood. A high BUN usually indicates reduced kidney clearance or increased protein breakdown—common causes include dehydration or low blood flow to kidneys (prerenal), intrinsic kidney disease, urinary obstruction (postrenal), high protein intake, gastrointestinal bleeding, or certain drugs. Evaluation includes repeat testing, creatinine/eGFR, hydration status and medication review; see a clinician for diagnosis and treatment.

Q: What if blood urea nitrogen is low?

A: Low blood urea nitrogen (BUN) can reflect overhydration, low dietary protein or malnutrition, reduced hepatic urea production (severe liver disease), pregnancy, or certain hormonal states. It’s often asymptomatic but may signal liver dysfunction, malnutrition, or lab/volume issues. Interpretation requires creatinine, liver tests and clinical context. Management focuses on correcting hydration, improving protein intake or treating underlying disorders.

Q: What is normal BUN level by age?

A: Normal BUN ranges by age: newborns about 3–12 mg/dL; infants/children about 5–18 mg/dL; adults (≈18–60 years) about 7–20 mg/dL; older adults (>60 years) may be slightly higher, roughly 8–25 mg/dL. Individual lab reference ranges vary interpret with clinical context and consult a healthcare provider for abnormal results.

Q: Why is BUN low or high?

A: BUN (blood urea nitrogen) rises with decreased kidney perfusion or function (dehydration, acute/chronic renal failure, urinary obstruction), high protein intake or catabolism, gastrointestinal bleeding, and heart failure. BUN is low in liver failure or advanced liver disease (reduced urea synthesis), malnutrition or low‑protein diets, overhydration, pregnancy, or anabolic states. Interpretation should consider creatinine and clinical context.

Q: Is high urea serious?

A: High urea (elevated BUN) can be serious: it often indicates impaired kidney function, dehydration, severe infection, heart failure, or internal bleeding. Mild, transient rises may be reversible; very high levels or symptoms—reduced urine, confusion, swelling, breathlessness—require urgent evaluation. Treatment targets the underlying cause (fluids, medications, dialysis if needed). See a healthcare professional promptly for tests and management.

Q: How to avoid blood urea nitrogen?

A: To prevent elevated blood urea nitrogen (BUN): stay well hydrated, eat a balanced diet with moderate protein (follow renal-diet advice if needed), control blood pressure and blood sugar, avoid or limit NSAIDs, excess alcohol and high‑protein supplements, and don’t overexercise. Review and adjust medications with your clinician and have regular kidney-function tests to detect problems early.