You should not plead to the violation. Depending on the technology used, false positives are possible. Contact an attorney who is familiar with the type of testing used.
Depending on where you are located and your history, the range is the maximum possible penalty to a lecture from the judge. Make sure you talk with a lawyer before you make any decisions about how to respond.
It will depend on what type of case you have, but at the very least I would expect that a VOP petition will be filed and you will be taken into custody until the VOP is admitted or proven. If you aren't having the same attorney continue to represent you, you need to secure other counsel right away.