Well my friend who is a CNA here in MN said:

Dreamo: but yea they don't do randos unless the suspect... and you have to be tested to be hired
Dream: i probably just barely passed
Dream: O:-)
Dream: i thought i had about a month before it was going to happen, and it only ended up being less than a week
KL: so they suspected?
Dream: no no
Dream: you have to have one initially
Dream: to be hired
Dream: but after that they would only do it if they suspected that you were on drugs or something happened at work kind of thing