How to Calculate BUN/Creatinine Ratio
The BUN/creatinine ratio is simply calculated by dividing the blood urea nitrogen value by the serum creatinine value. 1
Formula and Normal Range
- BUN/Creatinine Ratio = Blood Urea Nitrogen (mg/dL) ÷ Serum Creatinine (mg/dL) 1, 2
- The normal BUN/Creatinine ratio typically ranges from 10:1 to 15:1 (or 10-15) 2
- A ratio >20:1 is often considered elevated and may suggest certain clinical conditions 2, 3
Clinical Significance
- An elevated BUN/creatinine ratio (>20:1) has traditionally been associated with pre-renal causes of kidney dysfunction, though this interpretation has limitations 2, 3
- In a large retrospective study, patients with AKI and high BUN/creatinine ratio (>20) actually had higher hospital mortality compared to those with lower ratios, challenging the traditional interpretation 3
- The ratio should be interpreted in context with other clinical and laboratory findings, not as a standalone diagnostic tool 1
Factors Affecting the Ratio
- Age: BUN levels tend to increase with age (r=0.172, p<10^-5), which can affect the ratio 4
- Body Mass Index: Higher BMI tends to correlate with higher serum creatinine levels (r=0.118, p<10^-5) 4
- Smoking status: Smokers tend to have lower BUN levels compared to non-smokers (Z=-4.52, p<10^-5) 4
- Protein intake: Higher protein intake can increase BUN levels without affecting creatinine, elevating the ratio 5
- Hydration status: Dehydration can increase BUN disproportionately to creatinine 1, 2
Clinical Applications
- The ratio can help assess nutritional status in patients with chronic kidney disease 5
- In heart failure patients, a BUN/creatinine ratio higher than age-specific and sex-specific normal ranges is associated with worse prognosis, independent of both individual BUN and creatinine values 6
- In dialysis patients, BUN levels (which affect the ratio) are used to calculate protein catabolic rate and assess dialysis adequacy 7
Pitfalls to Avoid
- Do not rely solely on the BUN/creatinine ratio for diagnosis of kidney dysfunction; estimated GFR using formulas like MDRD or CKD-EPI provides more accurate assessment 7, 1
- Be cautious when interpreting the ratio in elderly patients, as age-related changes in muscle mass can affect creatinine levels and potentially mask renal dysfunction 7, 1
- A disproportionately elevated BUN:creatinine ratio (>20:1) is often multifactorial, especially in elderly and critically ill patients, and may not simply indicate pre-renal azotemia 2