The on site damp only does random testing I believe, not pre employment drug and alcohol testing.
On site testing can do random, post incident and reasonable suspicion testing. In fact the organisations DAMP has to specify this. The only element that can be exempted is random which we don't do simply because there is no past evidence to support it being required. For pre-employment we send the dude/dudette off to the path lab for the drug screen and do a breath test with our own device. A pain in the backside but not too difficult.