Drug metabolites are measured repeatedly and precisely, as is a forensic bite analysis . Not sure how you can claim those are unreliable when they are used daily. GC/MS is a good start for drug testing..
Drug metabolites are measured repeatedly and precisely, assuming no one mislabeled anything and the machine is well-calibrated and the technicians are honest even though they know who chooses which lab to use and the sample was collected fastidiously and not contaminated with anything.
But drug metabolites aren't drugs. Eat a bagel with poppy seeds and you can test positive for opioids. Take certain decongestants and you can test positive for meth.
Law enforcement does all sorts of sketchy things in order to get a conviction. Why is it a surprise that they use pseudoscience that juries eat up because they've seen it used in their favorite police procedural TV show hundreds of times?