I know that both DXM and PCP are classified as dissociatives, but I think that's as far as their relation goes. I doubt that Dextromethorphan would come up as a false positive for PCP, because then anyone who took cough medicine in the past week would look like a drug addict. I'm not sure though...

"DXM is not specifically tested for in drug tests, but there is speculation that it could trigger the pcp test which is standard."

Speculation, but I haven't found anything to suggest that it definately does. Just list that you've taken cough syrup in the past week for a cold, DXM isn't illegal:

http://www.erowid.org/chemicals/dxm/dxm_testing.shtml
kr1Nx01 Reviewed by kr1Nx01 on . dxm turns out to be pcp on tests i have read that you can get a false positive for pcp if you use dxm recreationally. Does anyone know how long dxm stays in your system or pcp? Rating: 5