What Causes The Presence Of Hydrocodone In Drug Screen Test While On Oxycodone?

as a patient under pain management I was told I failed my drug screen. Is there a way to clear my name as this was clearly a mistake. They claim they found a drug in my screen that I do not have. On oxycodone and the lab claims hydroycodone was found. Chain of custody was broken during that visit and my screen was left unattended. Any advice? This Email is my husband's address.