A blood urea nitrogen or BUN-to-creatinine ratio helps physicians discover causes of abnormal BUN and creatinine levels, states WebMD. The kidneys remove creatinine, a waste product, from the blood through urine. When kidneys are not functioning properly, creatinine levels in the urine decrease while its levels in the blood increase.
A BUN test measures the amount of urea in the blood, and a blood creatinine level test measures the amount of creatinine in the blood. The BUN test and blood creatinine test results show the level of kidney function, explains WebMD. Blocked urine flow from the kidneys or kidney disease cause BUN and creatinine levels to rise, and dehydration causes BUN levels to rise higher than creatinine levels.