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

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).

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.