There is no blood test for left-handedness that could come back as “positive”. It doesn’t exist.

It still tends to run in families, but

You cannot directly test for it. A doctor has to sit down and observe the person. Do they use their left hand a lot? Do they use it more than their right hand? Is their writing with their left hand faster and clearer than with their right hand? Do they instinctively grab things with their left hand?

If the answer is yes, then they fit the criteria for left-handedness

Similarly with autism, a professional must observe the candidate and look for a set of criteria. There is no physical test, where you draw a blood sample and look at it under a microscope and Yup, AUTISM! There is no ultimate yes-or-no binary. You just have to have autistic traits and someone with an expensive degree goes “Yep, you tick enough boxes, you’re autistic.”

And that’s that.

Like, you don’t need a professional to tell you that you are left-handed. You don’t need an official diagnosis to know that you are autistic.

Signed,

- An officially diagnosed officially autistic person who supports self-diagnosis