"There are only two possible reasons for a police department to be using a test like that [with a known 70% false positive rate]. The first is ignorance, but of a degree and nature that is quite scandalous. The second is to commit fraud upon the courts, by claiming to have evidence of drug possession where no drugs exist. In light of the FBI hair test scandal (https://www.washingtonpost.com/local/crime/fbi-overstated-fo...) I think the latter explanation is fairly likely."
[1]: https://ecf.ksd.uscourts.gov/cgi-bin/show_public_doc?2013cv2... (pdf)
"In fact, as plaintiffs point out in their submissions, Deputy Burns testified that, at the time he tested the plant material he discovered in plaintiffs’ trash, he had no knowledge that anything other than marijuana could test positive on a marijuana field test kit and that he was “not aware” of the possible occurrence of false positive test results. Deputy Blake’s testimony on those issues is substantially the same. Moreover, Sheriff Denning testified that, since he came to the Johnson County Sheriff’s Office in 1978, the Office has conducted “thousands” of field tests and the only false positive results of which he is aware are the results at issue in this case."
Apparently the judge is under the impression that this justifies his ruling, rather than damns both him and the entire police department.