I took a drug screening a couple years ago I was taking hydro and Valium. They called me up and told me I tested positive for Valium asked if I had a script and told me what to do to prove to them I had the script. (if you want the details please ask)

However two months ago I had to take another drug screening and I'm currently taking Hydro and Xanax. They told me I tested positive for opiates and to please provide the script.

So Xanax either didn't show up on the test or they didn't test for it. They do use two test. The EMIT test and the GCMS test.

The GC/MS is typically used to confirm "positive" EMIT test results. GC/MS will indicate precisely what chemical is present. This is necessary because the EMIT will only indicate whether something similar to what's being tested was found. The GC/MS is difficult and more costly, which is why the EMIT is given first.