Tens of thousands of people every year are sent to jail based on the results of a $2 roadside drug test. Widespread evidence shows that these tests routinely produce false positives. Why are police departments and prosecutors still using them? by Ryan Gabrielson and Topher Sanders, ProPublica Graphics by Fathom. Photography by Todd Heisler, The New York Times. July 7, 2016 This story was co-published with The New York Times Magazine.

“…Police officers arrest more than 1.2 million people a year in the United States on charges of illegal drug possession. Field tests…help them move quickly from suspicion to conviction. But the kits — which cost about $2 each and have changed little since 1973 — are far from reliable. The field tests seem simple, but a lot can go wrong. Some tests…use a single tube of a chemical called cobalt thiocyanate, which turns blue when it is exposed to cocaine. But cobalt thiocyanate also turns blue when it is exposed to more than 80 other compounds, including methadone, certain acne medications and several common household cleaners. Other tests use three tubes, which the officer can break in a specific order to rule out everything but the drug in question — but if the officer breaks the tubes in the wrong order, that, too, can invalidate the results. The environment can also present problems. Cold weather slows the color development; heat speeds it up, or sometimes prevents a color reaction from taking place at all. Poor lighting on the street — flashing police lights, sun glare, street lamps — often prevents officers from making the fine distinctions that could make the difference between an arrest and a release….There are no established error rates for the field tests, in part because their accuracy varies so widely depending on who is using them and how. In Las Vegas, authorities re-examined a sampling of cocaine field tests conducted between 2010 and 2013 and found that 33 percent of them were false positives. Data from the Florida Department of Law Enforcement lab system show that 21 percent of evidence that the police listed as methamphetamine after identifying it was not methamphetamine, and half of those false positives were not any kind of illegal drug at all.”