Services!
It can mean being granted access to services. That was the first thought that came to my mind at this topic's title.
I received a detailed written report of all testing done and that the process pointed to that I have autism.
My report also included 1. a list of all conditions that could be mistaken for autism, explaining that they were dismissed (and why), 2. the results of my IQ testing to see how it correlates to autism or might affect it, 3. as well as a short summary of some of my symptoms and abilities that became apparent during the testing and 4. might be relevant for treatment of my autism. There might have been something else, but I forgot.
This report was the prerequisite for my ASD-specific treatment. It was my sole reason to undergo the diagnostic process and receive an official diagnosis. I came to the same conclusion with my ADHD. I realised I needed access to treatment after all and in order to get it, I had to be diagnosed officially first.
I'm not a resident of the US, but my diagnoses also mean I can't get private insurance. It's not much of a problem where I live though. I also informed myself about other possible negatives consequences the diagnosis of autism might have for me. I felt it was a tough decision, until I figured that I likely wouldn't ever have a job, have kids or that I might get in trouble with law for autistic behaviour if I didn't get the diagnosis.
So, it wasn't that tough of a decision for eventually. Autism might get used against me in wrong, unfunded ways, but without the diagnosis I very likely wouldn't be doing any of the things I do now. I'd have missed out on a lot of great achievements.
Personally, I think a diagnosis is useful if you need access to treatment or services. If you don't need either, you might still desire a diagnosis to be certain that you have a known disorder and don't continue to feel like the world's only weirdo who's completely unlike the other almost-7 billion people.
To tell the truth, personally, I don't feel very certain about a diagnosis just because someone said so and I was sure I had autism and ADHD when I underwent testing. I know myself best, but I also realised the need to confirm my perception of my impairment and symptoms. My opinion is that there just isn't a guarantee that one is always right and always has an accurate perception that is untainted by emotions and lack of knowledge.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett