The only way that would make sense is if:
1) they already know what the problem is in some cases and
2) the patient cops to using the substance-in-question.
So they could come to a conclusion without the need for the screen, and maybe just take a sample for later or something. Turn it over to the CDC and let them deal with it.
But if that were the case, why don't they come out with it and 'splain?
So yeah, it's fishy if true. I'm putting it all firmly in the rumor category. Pretty good one though, if you're trying to prove conspiracy and discredit them. But if I were them, I'd be ...... at such a rumor if it were false.