johnd, as Naledi mentioned , there is a window period for certain std's, before they can show up on a blood test...so only fools , these days will do bareback, on any pay for sex platform, or any other platform for that matter.
maybe these links can provide some basic info...Call them!
i-base.info/guides/testing/what-is-the-window-period
clicks.co.za/health/article-view/do-you-have-an-std