It is heartening to hear that supply is widening and prices are dropping. It is always good news to hear the loathsome drug warriors are losing their filthy war.
No test of that sort exists. It's redundant anyway. If someone wants to know if an individual is not capable of doing something they'd be better off performing an impairment test. The amount of some chemical is not going to tell them that definitively - it'll vary by individual and by drug, as well as a number of other factors. Workplaces for example typically rely on behavioural evidence of intoxication for alcohol, but use tests that detect nanograms, and then only of metabolites, for other drugs. The purpose of the testing is more than obvious - to maliciously hurt users of those drugs as well as by extension punishing their families.
you are probably right, the tests would have to be quite accurate.
This link seems to think it can pick up THC accurately enough to tell if you have used in the last 6 hours or not. If they can have similar for other drugs with that kind of accuracy i would have no issues in taking these kinds of tests
Link http://www.narcocheck.com/en/saliva-drug-tests/thc-marijuana-saliva-test.html