Normal Blood Urea Nitrogen (BUN) Levels
The normal BUN level is approximately 7-25 mg/dL in adults, though this range varies slightly by laboratory and patient age. 1
Standard Reference Ranges
- General adult population: The typical normal range is 7-25 mg/dL 1
- Elderly patients (≥70 years): The normal range shifts higher to approximately 14-23 mg/dL for both males and females, reflecting age-related decline in renal function 2
Age-Related Considerations
BUN levels increase with advancing age, particularly after the seventh decade of life. 2
- Mean BUN levels in elderly subjects are significantly higher than in younger control subjects 2
- This elevation reflects physiological decline in renal function that correlates with age 2
- The age-related increase is more pronounced in males, where significant positive correlation exists between BUN and age 2
Clinical Context for Interpretation
BUN must be interpreted alongside creatinine and estimated glomerular filtration rate (eGFR) to distinguish normal variation from pathology. 1
Normal BUN/Creatinine Ratio
- The typical BUN:creatinine ratio is 10-15:1 3
- Ratios >20:1 suggest pre-renal azotemia, dehydration, or increased protein catabolism rather than intrinsic kidney disease 1, 3
Screening Thresholds
- BUN <15 mg/dL provides strong evidence against renal insufficiency (sensitivity 99.7% for detecting creatinine ≥2.0 mg/dL) 4
- BUN <20 mg/dL is a high-yield cutpoint with 98% sensitivity for excluding significant renal dysfunction 4
Factors Affecting BUN Independent of Kidney Function
Several non-renal factors can elevate BUN while creatinine remains normal:
- Volume depletion/dehydration: Increases proximal tubular reabsorption of urea 1, 3
- Heart failure: Decreased renal perfusion and neurohormonal activation elevate BUN disproportionately 1, 5
- High protein intake: Particularly >100 g/day increases urea production 3
- Hypercatabolic states: Sepsis, infection, high-dose corticosteroids, and critical illness 3
- Gastrointestinal bleeding: Protein load from blood digestion 3
Clinical Significance of Specific Thresholds
- BUN >19.6 mg/dL: Recognized as a severity marker in community-acquired pneumonia requiring ICU care 6
- BUN >20 mg/dL: Defines uremia as a minor criterion for ICU admission in severe pneumonia 6
- BUN >20-25 mg/dL: Associated with increased mortality risk in acute coronary syndromes, independent of creatinine-based GFR estimates 7
Important Caveats
Creatinine-based estimates may underestimate kidney dysfunction in certain populations, making BUN a valuable complementary marker:
- Elderly patients with reduced muscle mass produce less creatinine despite impaired kidney function 5, 2
- Malnourished patients similarly have lower creatinine production 5
- In these populations, BUN may detect renal impairment missed by creatinine alone 5, 7
BUN should never be used in isolation to assess kidney function—always correlate with creatinine, eGFR, and clinical context to distinguish physiologic variation from pathology. 1, 5