Normal Values of Blood Urea Nitrogen (BUN)
The normal range for Blood Urea Nitrogen (BUN) is 10 to 20 mg/dL in adults. 1
Understanding BUN Values
- BUN is an important laboratory marker that reflects kidney function, but it should be interpreted alongside serum creatinine for a more complete assessment of renal status 1
- The normal BUN to serum creatinine ratio is typically 10-15:1, with ratios >20:1 often suggesting pre-renal causes of azotemia 2
- BUN levels can be affected by various factors beyond kidney function, including protein intake, catabolic state, volume status, and certain medications 2
Clinical Significance of BUN Values
Elevated BUN (>20 mg/dL)
- BUN values >28 mg/dL have been associated with increased mortality in critically ill patients, even after correction for other factors including renal failure 3
- Elevated BUN has been shown to be an independent predictor of long-term mortality in older, medically stable patients 4
- In patients with acute coronary syndromes, elevated BUN (>20 mg/dL) is associated with increased mortality independent of serum creatinine-based estimates of glomerular filtration rate 5
Severely Elevated BUN (>100 mg/dL)
- BUN levels ≥100 mg/dL, especially with uremic symptoms, typically indicate the need for urgent hemodialysis 6
- Disproportionate elevation of BUN relative to creatinine (BUN:Cr >20:1) with BUN ≥100 mg/dL is often multifactorial and most common in elderly patients and those with high protein intake 2
- When initiating hemodialysis for severely elevated BUN, shorter initial dialysis sessions with reduced blood flow rates may be advisable to prevent dialysis disequilibrium syndrome 6
Monitoring BUN in Clinical Practice
- For patients with kidney disease, regular monitoring of BUN is recommended, with frequency depending on disease severity 7
- In critically ill patients with elevated BUN, more frequent monitoring (every 4-6 hours initially) may be necessary 6
- For patients on hemodialysis, both pre-dialysis and post-dialysis BUN samples should be obtained during the same session to assess dialysis adequacy 6
Common Pitfalls in BUN Interpretation
- Improper blood sampling technique (dilution with saline or heparin) can artificially lower BUN values 6
- BUN can be elevated due to non-renal factors such as high protein intake, gastrointestinal bleeding, corticosteroid use, or catabolic states 2
- Low BUN may be seen in severe liver disease (due to decreased urea synthesis) or with low protein intake, and doesn't necessarily indicate good kidney function 1
- Fractional sodium excretion <1% is consistent with pre-renal azotemia but may not be present in all cases of disproportionate BUN elevation 2