They could test for the levels of that drug in your system to see if it is being taken as prescribed. I think they are more concerned with the abuse of a drug and if your levels would be higher than they are supposed to be than if they are lower.
In A Urine Drug Screen Does Flexeril And Zanaflex Show-up The Same Or Are They Different? If You Don't Take Percolates As Much As Prescribed Will That Show-up Also As Not Having Enough In Your System?
A drug test doesnt test for each drug out there, they are in a family of drugs, thats what is tested for, and yes you can test positive for not having enough of the drug you are prescribed into you system, depends on the test they give you, if its done in the office or if its sent out. I believe the Xanaflex and the Flexeril are in the same family, I'm not 100% sure with that tho.