As a layman who doesn't work with medical studies it always struck me that one of the bits of data that isn't (normally) collected along with everything else is genetic samples of all participants. It should be stored alongside everything else so that if the day comes when genetic testing becomes cheap enough it can be used to provide vastly greater insight into the study's results.
Even something as simple as a few strands of hair sealed in a plastic bag in a filing cabinet somewhere would be better than nothing at all.
That throws out anonymity. I don't see this getting approved, or people signing up for such studies, apart from those who don't care that there genetic data gets collected and stored.
Even if there is no name saved with the genetic sample, the bar for identification is low. The genes are even more identifying than a name after all. Worse, it contains deep information about the person.
Even something as simple as a few strands of hair sealed in a plastic bag in a filing cabinet somewhere would be better than nothing at all.