If someone has access to data and doesn't know how to interpret it, they should go out and find someone who can help them. At the very least, they should try not to jump to conclusions.

There is no ethical excuse for withholding information from someone about their own biology.

I would argue that knowledge without context is incomplete knowledge unless you can reasonably expect the receiver to have the context necessary. You aren't really being ethical if you aren't giving a truthful account of the information.

It kinda falls under "well, I didn't actually lie... I just didn't say more than absolutely necessary. It isn't my fault if they assumed wrongly."

In the example of early-onset alzheimer's, complete information would come with a recommendation of retesting, given the incidence of errors, a likelihood of the disease (was given), and whatever the accepted medical advice to follow afterwards is. It would probably be best to include warning signs of when to see the doctor about it if most doctors and insurances won't cover anything without symptoms.

You can have all the legal disclaimers on this as well.