How else would you do it? Unlike an e.g. viral infection there is no positive test you can look at. Generally a disorder is considered something that significantly impacts someone's life, getting in the way of things like working, social life, life enjoyment. I don't think you can be totally objective about this, and you get into things like if i.e. autism is mild is it a disorder? It's pretty clear to me it should be considered as such after a certain level of severity, but maybe it shouldn't always be if it has minimal impact on the person