It's interesting because what they actually test for are <span style = 'background-color: #dae8f4'>metabolite</span>s
(what the opioid breaks down into). Most common opioids break down into codeine, norcodeine, morphine, and normorphine. And that is what most tests actually look for.
However, out of
all the opioids, oxycodone and buprenorphine
are both semi-synthetic thebain derivatives and do not turn into those metabolite
(Thebain is just a minor chemical constituent of
opium by the way.)
And surprise, surprise...