it would be highly unlikely for xanax to remain in the system after a week. it clears very fast. the real question may be "why did he take xanax? ". many will take this sedative to come down from a different drug that makes them high like methamphetamines. it would not be a waste of money to drug-test your son if you suspect drug use but one negative test does not vindicate him. you may have to randomly test him from time to time to be sure. and make sure he is observed getting the specimen. many teens have some tricks.
i think my son took xanax. if i test him how long after could it show? don't want to waste the money if its been to long. i suspect my son taking xanax a week ago. will i be wasting my money on a test if i do a home test now. can he still show positive if he did take xanax after a week?