I guess that as it's a diagnostic tool, it's meant to be a guide for diagnosticians as to whether someone is on the spectrum, but it's not meant to definitively state whether someone IS on the spectrum or not. It even states that, at the discretion of the diagnostician, if someone scores below the threshold yet seems to be ASD, then their professional opinion should prevail over the score.
When I went for an ASD assessment, I believe this was the test that I did - or, at least, I did a test that was very close to this one. It took about an hour to administer and while I was never told the score, I do know that I scored above the threshold.
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Said the apple to the orange,
"Oh, I wanted you to come
Close to me and
Kiss me to the core."
Think you're ASD? Get thee to a professional!