BUN (Blood Urea Nitrogen)
Waste product from protein breakdown
20 of 22 providers
Kidney Function
dehydration
What is BUN (Blood Urea Nitrogen)?
BUN (Blood Urea Nitrogen) measures the amount of urea nitrogen in your blood. Urea is a waste product formed in your liver when protein is broken down into amino acids. The liver converts toxic ammonia (from protein breakdown) into urea, which is then carried by blood to the kidneys where it is filtered out and excreted in urine.
Unlike creatinine which is produced at a constant rate from muscle, BUN is highly variable and influenced by protein intake, hydration status, liver function, tissue breakdown (fever, steroids, GI bleeding), and kidney function. This makes BUN less specific than creatinine for assessing kidney function alone, but the BUN/creatinine ratio helps differentiate between kidney disease and other causes of elevated BUN.
BUN rises in three main scenarios:pre-renal (dehydration, heart failure, bleeding—kidney function actually normal), renal (actual kidney disease with reduced filtration), and post-renal (urinary obstruction). Low BUN can indicate liver disease (reduced urea production), malnutrition, or overhydration. The pattern of BUN elevation combined with creatinine and the BUN/creatinine ratio provides diagnostic insight.
Why BUN Complements Creatinine for Kidney Assessment
- Hydration status indicator:BUN rises with dehydration and falls with overhydration more than creatinine, making it useful for assessing volume status
- Pre-renal vs renal distinction:BUN/creatinine ratio >20 suggests pre-renal azotemia (dehydration, heart failure) rather than intrinsic kidney disease
- GI bleeding detection:Digested blood acts as protein load, elevating BUN disproportionately to creatinine (ratio often >30)
- Protein metabolism:Reflects dietary protein intake, protein catabolism, and liver urea synthesis capacity
- Medication dosing:Like creatinine, used to adjust drug dosing for kidney-cleared medications
- Uremia screening:Very high BUN (>80-100 mg/dL) indicates uremic syndrome requiring dialysis consideration
Optimal vs Standard Ranges
Optimal8-20 mg/dL▼
- Indicates balanced protein metabolism, adequate hydration, and normal kidney function
- BUN/creatinine ratio typically 10-20:1
- Values in this range reflect appropriate protein intake, liver synthesis, and kidney excretion without significant tissue breakdown or dehydration
Borderline Elevated20-30 mg/dL▼
- May indicate mild dehydration, high protein intake, increased protein catabolism, or early kidney dysfunction
- Check BUN/creatinine ratio:if >20, suspect dehydration or GI bleeding;if 10-20, suspect kidney disease
- Rule out recent high-protein meal, dehydration, or medication effects (steroids, tetracycline)
- Retest after hydration
- If persistent with normal creatinine, likely benign
Moderately Elevated30-60 mg/dL▼
- Requires investigation
- If BUN/creatinine ratio >20:Likely pre-renal (dehydration, heart failure, GI bleeding)
- Rehydrate and recheck
- If ratio 10-20:Suggests kidney disease
- Check urinalysis, kidney ultrasound, consider nephrology referral
- Evaluate for acute kidney injury or CKD
- If ratio <10:Consider liver disease or severe malnutrition (though BUN would typically be low)
Severely Elevated (Azotemia/Uremia)>60 mg/dL▼
- Indicates significant kidney dysfunction, severe dehydration, or major GI bleeding
- BUN >80-100 mg/dL causes uremic symptoms (nausea, confusion, pericarditis) and may require dialysis
- Urgent evaluation needed
- Check creatinine, electrolytes (especially potassium), urinalysis
- If BUN/creatinine ratio >30, investigate GI bleeding (melena, hematemesis) or severe volume depletion
- If ratio normal, indicates advanced kidney failure requiring nephrology consultation
Scientific Evidence
BUN/Creatinine Ratio for Pre-renal Azotemia
BUN/creatinine ratio >20 has 70-85% sensitivity for diagnosing pre-renal azotemia (dehydration, heart failure, hypoperfusion) versus intrinsic renal disease. In pre-renal states, kidneys increase urea reabsorption disproportionately to creatinine, raising the ratio. This simple calculation helps distinguish reversible causes (respond to fluids) from intrinsic kidney damage. Ratio >30 strongly suggests GI bleeding.
BUN as Mortality Predictor in Heart Failure
Elevated BUN is a strong independent predictor of mortality in heart failure patients, even more predictive than creatinine. BUN >43 mg/dL is associated with 2-3x higher mortality risk. Elevated BUN in heart failure reflects neurohormonal activation, reduced renal perfusion, and increased protein catabolism—all markers of disease severity.
GI Bleeding Detection via BUN
Upper GI bleeding causes BUN elevation (typically >40 mg/dL) with BUN/creatinine ratio often >30-36 due to digestion and absorption of blood proteins in small intestine. This pattern helps identify occult GI bleeding before hemoglobin drops significantly. Lower GI bleeding (colon) causes less BUN elevation as blood passes through faster with less digestion.
Source:Srygley FD, et al. Does this patient have a severe upper gastrointestinal bleed? JAMA. 2012;307(10):1072-1079.
Low-Protein Diet Effects on BUN in CKD
Protein restriction to 0.6-0.8 g/kg/day in CKD patients significantly reduces BUN, uremic symptoms, and may delay dialysis initiation by 6-12 months. However, overly restrictive protein (<0.6 g/kg) risks malnutrition. The MDRD study showed protein restriction slowed GFR decline and improved uremia, though adherence was challenging. Dietitian supervision essential.
Source:Levey AS, et al. Effects of dietary protein restriction on the progression of advanced renal disease in the Modification of Diet in Renal Disease Study. Am J Kidney Dis. 1996;27(5):652-663.
Uremic Syndrome and Dialysis Initiation
Uremic syndrome typically manifests when BUN exceeds 80-100 mg/dL, though symptoms vary by individual and chronicity. Classic uremic symptoms (nausea, confusion, pericarditis, bleeding) indicate need for dialysis initiation. However, early dialysis (based solely on eGFR <10-15 without symptoms) does not improve outcomes compared to symptom-driven initiation. Dialysis decisions should incorporate symptoms, not just BUN/creatinine levels.
Source:Cooper BA, et al. A randomized, controlled trial of early versus late initiation of dialysis. N Engl J Med. 2010;363(7):609-619.
Which Providers Test BUN (Blood Urea Nitrogen)?
Full Provider Comparison
| Provider | Includes | Annual Cost | Biomarkers |
|---|---|---|---|
 Superpower | ✓ | $199 | 100+ (150 with ratios) |
 WHOOP Advanced Labs | ✓ | $349 | 65 |
 Labcorp OnDemand | ✓ | $398 | 30+ |
| ✓ | $486 | 40+ | |
| ✓ | $444 | 288 | |
| ✓ | $349 | 100+ | |
| — | $761 | 54 | |
 Function Health | ✓ | $365 | 160+ |
| ✓ | $250 | 65 | |
| ✓ | $495 | 70+ | |
| ✓ | $895 | 100+ | |
| ✓ | $1950 | 150+ | |
| ✓ | $375 | 80+ | |
| — | $Varies | 75+ | |
| ✓ | $190 | 100+ | |
| ✓ | $99 | 50 | |
| ✓ | $124 | 60 | |
| ✓ | $199 | 50 | |
| ✓ | $499 | 120+ | |
| ✓ | $4188 | 70-80+ | |
| ✓ | $375 | 85 | |
| ✓ | $700 | 128 |
Frequently Asked Questions
What is BUN (Blood Urea Nitrogen)?
Waste product from protein breakdown
What is the optimal range for BUN (Blood Urea Nitrogen)?
The standard reference range for BUN (Blood Urea Nitrogen) is dehydration. Optimal ranges may differ based on individual health goals and expert recommendations.
Which blood test providers include BUN (Blood Urea Nitrogen)?
20 out of 22 blood testing providers include BUN (Blood Urea Nitrogen) in their panels. This biomarker is widely available across major providers.
What category does BUN (Blood Urea Nitrogen) fall under?
BUN (Blood Urea Nitrogen) is categorized under Kidney Function. This category includes biomarkers that help assess related aspects of health and wellness.
Medical Disclaimer
This information is for educational purposes only and is not medical advice. Always consult with a qualified healthcare provider about your specific health needs.
Last reviewed:2026-02-20