What is the normal Blood Urea Nitrogen (BUN) level in adults?

Normal Blood Urea Nitrogen (BUN) Levels in Adults

The normal BUN range in adults is approximately 7-20 mg/dL, though this can vary slightly by laboratory and patient age, with elderly patients (≥70 years) having a higher normal range of 14-23 mg/dL. 1

Standard Reference Ranges

• General adult population: The typical normal BUN range is 7-20 mg/dL in most clinical laboratories 2
• Elderly patients (≥70 years): Normal range shifts higher to 14-23 mg/dL for both males and females, reflecting age-related decline in renal function 1
• Screening threshold: BUN values <15-20 mg/dL provide strong evidence against renal insufficiency (creatinine ≥2.0 mg/dL), with negative likelihood ratios of 0.005 and 0.03 respectively 2

Age-Related Considerations

• Physiologic changes with aging: Mean BUN levels are significantly higher in elderly subjects compared to younger controls, with a positive correlation between BUN and age in males after the seventh decade 1
• Muscle mass effects: Elderly patients tend to have lower muscle mass, which can affect the BUN:creatinine ratio and interpretation of values 3

Clinical Context for Interpretation

Normal BUN:Creatinine Ratio: The typical ratio is 10-15:1 3

Factors That Elevate BUN Beyond Renal Function

• Protein metabolism: High protein intake (>100 g/day), increased catabolism, gastrointestinal bleeding, and high-dose corticosteroids can elevate BUN disproportionately 3
• Volume status: Hypovolemia, congestive heart failure, and shock states increase proximal tubular reabsorption of urea, raising BUN independent of GFR 3, 4
• Nutritional status: Hypoalbuminemia (<2.5 g/dL) is associated with disproportionate BUN elevation 3

Clinical Significance of Abnormal Values

• BUN ≥28 mg/dL: Associated with significantly increased mortality in critically ill ICU patients, independent of creatinine and other markers of renal failure 5
• BUN 20-25 mg/dL: Associated with 1.9-fold increased mortality risk in acute coronary syndrome patients with normal to mildly reduced GFR 4
• BUN ≥25 mg/dL: Associated with 3.2-fold increased mortality risk in the same population 4

Special Populations

Dialysis Patients

• Pre-dialysis BUN targets: Vary by dialysis frequency, with average pre-dialysis targets ranging from 5,500 mg/dL·min for thrice-weekly dialysis to 2,100 mg/dL·min for six-times-weekly dialysis 6
• Time-averaged BUN: Used to calculate equivalent renal clearance (EKR) and assess dialysis adequacy 6

Critical Care Settings

• Initial assessment: BUN should be obtained STAT along with electrolytes, creatinine, and glucose in patients with diabetic ketoacidosis, hyperosmolar hyperglycemic state, or other critical illnesses 6
• Prognostic value: Elevated BUN has independent prognostic significance for mortality even after adjusting for severity scores (APACHE2, SAPS2) and creatinine 5

Common Pitfalls

• Over-reliance on BUN alone: BUN reflects more than kidney function—it is affected by protein intake, hydration status, liver function, and tissue catabolism 7
• Ignoring age-adjusted ranges: Using standard adult ranges in elderly patients may lead to under-recognition of elevated values 1
• Assuming pre-renal azotemia: Disproportionate BUN:Cr elevation (>20:1) is frequently multifactorial in ICU patients and not simply indicative of renal hypoperfusion 3
• Sample timing in dialysis: Improper BUN sampling technique (dilution with saline or heparin) artificially lowers values and affects dialysis adequacy calculations 7