I used to not only perform drug screens for large companies, I also taught people the entire COC rigamaroo....

Saliva tests have a very high incidence of technical error, if the technician does not get a good enough 'scraping' of the cheek then a proper sample cannot be obtained. False Positves are rare from technical error though, unless they got the papers mixed up. Saliva tests are also not recognized by many courts, including DOT guidelines, because there is only 1 test run rather than a test, the a confirmation.

Depending on your state you could take it to court and WIN, but not worth it for a pre-employment, just keep that in mind in case of an injury....