BUN/Creatinine Ratio
Calculates the BUN-to-creatinine ratio to help differentiate prerenal azotemia from intrinsic acute kidney injury. BUN rises disproportionately to creatinine in prerenal states due to increased proximal tubule urea reabsorption.
mg/dL
Serum BUN. If using SI units (mmol/L urea), divide by 2.8 to get mg/dL BUN.
mg/dL
Serum creatinine in mg/dL. If using umol/L, divide by 88.4.
References
- Diagnostic performance of serum blood urea nitrogen to creatinine ratio for differentiating prerenal from intrinsic acute kidney injury — BMC Nephrology/PMC (2017)
- Fractional Excretion of Sodium (FENa) — Clinical Journal of American Society of Nephrology/PMC (2022)
Note: For SI unit conversion: BUN in mmol/L divided by creatinine in umol/L * 1000/113 = approximate ratio equivalent. When both are in mmol/L, BUN:Cr >10 (with urea:creatinine) suggests prerenal.
Warnings & Limitations
- BUN/Cr ratio has poor sensitivity and specificity for differentiating prerenal vs intrinsic AKI in ED settings (PMID: 28376817).
- Ratio affected by non-renal factors: GI bleeding, steroids, protein intake, catabolic state, malnutrition, liver disease.
- Use FeNa or FeUrea for more reliable differentiation.
- In SI units (mmol/L), a ratio >100 suggests prerenal.