Shouldn't really assume that. I mean, yes, it's unlikely, but given that most of the research is around prevention of damage, surely it's better to get tested early if you even remotely suspect there is a chance of it being Alzheimer's.

Yes, actually one should assume that; not doing so is irrational. There are many other diseases that one might have without even knowing they exist. It would be extremely foolhardy to get tested for them if/when you become aware of them.

"given that most of the research is around prevention of damage"

Given that aren't any preventative treatments, you're barking up the wrong tree.

"surely it's better to get tested early if you even remotely suspect there is a chance of it being Alzheimer's."

No, it's surely not.

The incidence of dementia for those aged 30-64 is about 54 per 100000. If you have a test that is 90% accurate and someone tests positive, according to Bayes you have 0.5% chance of actually having it.

Testing like that seems pointless to me.

Working: (0.9 * 0.00054) / (0.9 * 0.00054 + 0.1 * 0.99946)

Except that you are not picked from the whole population at large, but from a subset of it. Namely the subset that (weakly) believes they have dementia. This changes the incidence rate (or likelihood function) probably by at least an order of magnitude.

You're right. You have to update the probability with all the knowledge you have. Although I would say self diagnosis has a very, very high false positive rate, and if you know you're a hypochondriac that would make the probability drop, plus we don't even have a test for this that is 90% accurate. I'm not sure the difference is an order of magnitude :)

I think my point stands that it doesn't make sense to test for extremely rare diseases because of a self diagnosis.

Depends on the cost/benefit of being tested. Often non clinical early screening is a much better option.

For Alzheimer's that could be as simple as getting enough sleep for a solid 2-4 weeks. As the disease progresses really slowly waiting that time period has minimal impact, but A the pre-test is cheap and good for you, and B you rule out a lot of false positives.

> even remotely suspect there is a chance

Generally speaking, this is not a cost-effective approach to medicine.

Not even just the cost factor. You can do damage to your health from getting treatments for false positives.

There is no real test for alzheimer yet. It takes a while to establish a diagnosis. The closest thing that comes to qn actual test is the recently approved Amyvid, but then it simply shows the accumulation of Tau proteins and is not 100 percent predictive of Alzheimer.