I've never been asked to show a certificate.
Though lots have asked if
'I'm clean'* after having a session with me. Somewhat horse/bolted:

I explain I get tested every two months and even if I had an all clear today
that would simply show that nothing was detected at the time of the test.
It doesn't show new infections just acquired prior to the test {HIV can take up to six months to show}
nor any picked up since then.
I stress safer sex and making both an informed choice on the available facts
and taking personal responsibility for ones own sexual health.
They take a deep breath and I then say
"No I don't have anything nasty, you'll be fine"
They then look relieved and go on their way.
Also same day STI/HIV certs are the standard in porn shoots in the UK and USA.
But they only reduce the likelihood of contracting something.
But these industries still get temporarily shut down each time a performer
shows up as HIV+ in a test...
>>>Latest shut down report<<<I hope my other comments register with them, later.
It's all about educating the client-base. Well, for me it is.
In summery, you don't need a cert, and they are rarely if ever asked for.
But regular STI testing is essential.
You are thinking and planning these very important aspects of what we do.
Which is great. Oh, also NHS GUM clinics give out free condoms and lube.
Just how cool is that!
*coz only messy untidy unwashed people get STI's... 