Will coffee affect a kidney test?

I have read both viewpoints, that coffee harms or helps the kidneys, and there are medical studies that would need to bare scrutiny to get to the essential truth of the matter. However, coffee is a gentle diuretic. Diuretics are given to people who suffer from Kidney failure. It would seem logical that coffee/caffeine would be a be a good choice for someone who is having trouble flushing their kidneys, certainly as good a choice as prescription drugs, considering it is natural, cheap, and never proven to be greatly harmful.