I disagree that HOTA should be opt out but let's that's not the topic of discussion.

There is no way a bone marrow transplant can be made opt out since it requires the person to attend a test in person first and then subsequent attend in person for the marrow to be harvested. This is unlike an organ transplant under HOTA when the person is already dead and unable to protest or not attend.

You can write a letter to each and every fellow turning 18 to request for them to come down but if you say it is compulsory unless you opt out, then isn't my non-attendance already a sign that I am opting out?