. >> reporter: ryan custer is a scientist in san francisco, operating blood donation centers like this one, they provided leading scientific research and it was his work which helped the fda formulate the policy change. >> this is a major change , in the way that blood donors are selected in the u.s. and everybody will be treated equally when they come to donate, they will be asked the same questions and those answers will determine whether somebody is eligible to be a donor, not your sexual orientation. >> reporter: all perspective donors , gay or straight, who report a new sexual partner, more than one in the past three months or a no -- >> a lot of gay and bisexual men , and there are women who take it. and all of us, we are all categorically excluded, even though we have all but zero risk of getting hiv. >> reporter: prep might delay detection or potentially get false negatives and more study is needed. >> the concern is what happens if somebody, for whatever reason, isn't adhering to taking prep, or if they have had exposure , could we detect that infection in a blood donation? th