What Causes Elevation In Creatinine And BUN Levels?

I just received my annual lab work and my creatinine level is 1.17, my bun/creatinine ratio is 17, my GFR is 50, down from 59 a year ago and my Bun is 20. I am 59 year old female in good health on no prescription meds. Should I see a nephrologist or be alarmed? I do have only one functioning kidney with one atrophied from a childhood injury/fall. My labs have always been within the normal range as far as kidney function goes until now.