You need to request them to have 6-MAM and or gas chromatography–mass spectrometry
test. If this is confusing, Im sorry. I
drug test people every week where I work so I'm informed on all this stuff. False positives do happen so you need to be informed on what to do. You can always print this and show your MD. If one of the opiates you are on is Codeine, then you may have your culprit.
The specific detection of heroin as separate from other opiates is facilitated by its unique metabolites.