The Bottom Line: The normal BUN:Cr ratio is 10-15:1. If the ratio is 20:1 or higher, prerenal azotemia is likely to be present. In relation to upper GI bleeding, a ratio greater than 36 has a sensitivity of 90% and specificity of 27%.
Summary: If either the creatinine or BUN concentrations are above the upper limit of the reference interval, it is advised that the BUN to creatinine ratio (BUN/Cr) be calculated. The normal BUN/Cr ratio is between 10:1 and 20:1. The ratio is helpful in determining the cause of renal impairment.
References: Ernst AA, Haynes ML, Nick TG, Weiss SJ. “Usefulness of the blood urea nitrogen/creatinine ratio in gastrointestinal bleeding.” American Journal of Emergency Medicine 1999 Jan; 17(1):70-72.
Feinfeld DA, Bargouthi H, Niaz Q, Carvounis CP. “Massive and disproportionate elevation of blood urea nitrogen in acute azotemia.” International Urology and Nephrology 2002; 34(1):143-145.
Winter WE. The Kidney. In: Laposata M. eds. Laboratory Medicine: The Diagnosis of Disease in the Clinical Laboratory. New York, NY: McGraw-Hill; 2014