I know a lot about drug testing. I worked for nearly two years in a criminal drug testing laboratory. I was a member of the lab staff, meaning I was in the lab constantly. We did a lot of things there including accepting, preparing and analyzing samples, writing and sending out reports, reviewing data, maintaining case files, and acting as expert witnesses who testified in court regarding drug test results.
Unfortunately, my knowledge on what causes false positives or how they are caused is extremely limited. In my capacity working for a criminal drug testing laboratory where we were trained as expert witnesses it is very unfortunate that what I was taught and learned is that false positives are extremely rare occurrences in drug screens and never occur in confirmation tests.
That's what we were told to say in court anyway. If that's true though, I remember seeing a several instances where some 83 year old woman was taking multiple hallucinogens such as LSD, ketamine, and cocaine. She musta been a real partier.
I remember another time where we had a problem with our mass-screening machine. This machine was the size of a small pickup truck, cost for the machine alone was around $800,000 USD. And the reagents were expensive as hell. They flew technicians in from overseas whenever it needed repair. Supposed to be top-of-the-line.
I did reports so I got to see all of data coming out of the machine and started noticing something odd. After several attempts to mention it to some coworkers and superiors it was explained to me that that particular machine was completely devoid of any potential for false positives. The technology was so advanced it completely eliminated that possibility.
It was a few months later (with this odd anomaly in the data continuing during that time) that some 'problem' was noticed by some of my superiors. So they flew in a tech from overseas, it was worked on, and that data anomaly I saw disappeared and I had to 'fix' a lot of old reports and send them back out.
And no, not all of the reports were fixed and sent back out. I never knew exactly how it was determined which reports should be fixed and resent and which ones shouldn't. Funnily enough most of the ones that were fixed were ones for which we had to testify on at some later date.
To be honest this incident is the main reason I no longer work in that field.