The DSM (currently being used is DSM-IV - which is the fourth version) is the Diagnostic and Statistical Manual that contains the criteria for all of the various disorders. Every so often, a group of psychiatrists get together and decide what needs to be updated, added, removed or changed and they come out with a new version. I believe, but am not sure, that they are working on the DSM-V now or in the near future. I've heard talk, anyway.
The thing is when you have no definitive medical tests that can diagnosis one thing or another, I feel that you need to take the time to really look at everything going on and treat things as a rule out basis. Not only with mental health, but with physical health, too, where there are no definitive tests. It seems that our medical profession is too quick to jump to a conclusion. Whether that's catering to insurance or a desire to provide answers to patients, or a combination of both, I don't know. Personally, I'd rather wait for answers and know they are right.