There is a much more accurate test called Cystatin C that can be used instead, and it isn't subject to being thrown off by creatine, muscle mass, etc.

IF you tell your doc, they can adjust their interpretation of the test.

Edit: see comment below (i.e. better to stop taking creatine at least a week before a test).

If you have a good dr they'll just ask you to stop taking it for two weeks, then do a blood test.