are legal brothel workers in Nevada and porn stars. Ladies who work for agencies are escorts, and sell their time, not sex. So it would be legally tricky for an agency to mandate testing for their ladies.  

I'm sure agency ladies would take it upon themselves to get tested. Most sexually active ladies involved in the industry do, as their bodies are their livelihoods.  

But I caution anyone to take the fact that someone gets tested regularly as a sign they are not infected with anything. You should play with the assumption that anyone you play with could have something, and take necessary precautions to play as safely as possible. Knowledge is power in this regard, and one would be wise to become familiar with STI info- photos, symptoms, what to look for, etc.  

I can tell you that even though brothel workers are tested regularly, they are not tested for Hepatitis, HSV, or HPV.

Also, porn stars test regularly and look at the problems they've had. Testing is not prophylactic.