BUN/Creatinine Ratio
Differentiates prerenal from intrinsic renal causes of azotemia
Calculator
Why use it
Simple, widely available calculation to characterize azotemia etiology. Helps guide diagnostic workup and management: a high ratio suggests hypovolemia or GI bleeding, while a normal ratio in the setting of elevated creatinine points toward intrinsic renal disease.
About
The BUN/Creatinine ratio is calculated by dividing serum blood urea nitrogen (BUN) by serum creatinine. The ratio helps differentiate the causes of azotemia (elevated BUN and/or creatinine): prerenal causes (dehydration, heart failure, GI bleeding, high protein intake) typically elevate BUN disproportionately compared to creatinine, producing a ratio > 20. Intrinsic renal disease typically maintains a ratio of 10–20.
When to use
- Elevated BUN and/or creatinine on laboratory panel
- Suspected dehydration vs. intrinsic renal failure
- Upper GI bleeding evaluation (blood in gut elevates BUN via protein digestion)
- Heart failure with worsening renal function
- ICU patients with oliguria or rising creatinine
Formula
Pearls & pitfalls
- Normal ratio is 10–20. Ratio > 20 suggests prerenal; < 10 may indicate malnutrition or hepatic failure.
- Upper GI bleeding elevates BUN via digestion of blood protein without proportional creatinine rise.
- Corticosteroids increase protein catabolism and can elevate BUN independently of renal function.
- Tetracyclines and other catabolic drugs may also raise BUN disproportionately.
- In malnutrition or liver disease, urea synthesis is impaired, causing a low BUN with normal creatinine (low ratio).
- FENa is generally superior to BUN/Cr ratio for distinguishing prerenal AKI from ATN.
Next steps
Ratio > 20: Prerenal causes likely (dehydration, heart failure, GI bleeding, high protein) — trial of IV fluids if volume depleted; endoscopy if GI bleed suspected. Ratio 10–20: Normal; consider intrinsic renal disease or post-renal if creatinine elevated. Ratio < 10: Consider malnutrition, liver disease, SIADH, or rhabdomyolysis.
Evidence
Standard clinical biochemistry. Morgan et al. (Crit Care Med 1980) and Dossetor (Can Med Assoc J 1966) established clinical utility. Extensively validated in GI bleeding literature (Cotton et al., 1992).