What kidney problems did your parents have and do they know the cause? At age 47 and a 50 to 60 GFR I'm not sure I would hit the panic button yet. You are correct that labs will vary, but if you see a good nephrologist they can run labs in connection with a 24-hour urine test. It tells them much more about the function of your kidneys (both are run in combination). I suspect unless there is a genetic component about the only way a nephrologist will know for sure what is causing the problem is to take a kidney biopsy, which is something to very careful with. If it were me and If you are close to Mayo I would be finding nephrologist there for at least a second opinion. Also, diet is very important. While it may not be for everyone, here is what I did: 1) became a vegan, 2) no processed foods, 3) calorie-restricted diet of about 1000 calories a day 4) take a plant-based protein powder for my protein and be sure not to exceed my daily target amount and 5) walk 3 to 5 miles a day (you are getting the same benefit from spinning). It seems to have worked for my sick kidneys -- which are much worse than yours. My GFR is about 17 up from 13 two years ago (previously spent three years on dialysis). By the way, phosphorous and potassium are not a problem unless they are high on your lab. I'm not sure they cause stress on your kidneys as animal protein does. Finally, if you have labs drawn within a day or so of a heavy workout, that will surely impact your creatinine which is a major component of calculating your GFR. Also, be sure you are hydrated before your labs. By the way, you should sure talk with your doctor before doing any of this. My doc approved all of this for me (although several of the items were my idea).