^ That's completely and absolutely false. The classical test, the NIDA 5, tests for amphtamines, cannabinoids, cocaine, opiates, and PCP. Then companies can exand upon those to add, barbiturates, hydrocodone, methqualone, benzos (getting more common), methadone, propoxyphene, ethanol and MDMA.
Meanwhile, Metaxalone shouldn't turn up on anything.