Get a better test.
Not necessarily a false positive.
All kinds of things will legitimately set off a meter, notably including many fertilizers. Wheel a chair down a sidewalk where the adjacent grass has recently been sprayed, and you have a perfectly legitimate positive.
How they handled the situation is ridiculous, but probably not the meter response.
A MythBusters episode I recently watched demonstrated clearly that one poppyseed muffin will routinely give a positive for drug use. People get fired all the time for a single positive reaction on the test. It’s not the fault of the test, which accurately measures what it’s supposed to, it’s the fault of the idiots interpreting and applying the results.
I was held back for a false-positive once. I think the TSA was punishing me for opting out their body scanners! I just stood there, almost dchallenging them to arrest me, charge me, or whatever. I did not care what they did. Finally, they said there were probably some medicines I was taking causing the false positive, and they let me go.