The ratio of BUN to creatinine is usually between 10:1 and 20:1. The ideal ratio of BUN to creatinine falls between 10-to-1 and 20-to-1. The BUN/Creatinine ratio blood test is used to diagnose acute or chronic renal ( kidney) disease or damage. In medicine, the BUN-to-creatinine ratio is the ratio of two serum laboratory values, the blood urea nitrogen and serum creatinine. A blood urea nitrogen (BUN)/creatinine ratio (BCR) >20 (0. A BUN-to-creatinine ratio can help your doctor check for problems, such as dehydration, that may cause abnormal BUN and creatinine levels.