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. 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. 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. By 1978, the Department of Justice had determined that field tests "should not be used for evidential purposes," and the field tests in use today remain inadmissible at trial in nearly every jurisdiction. But this has proved to be a meaningless prohibition. Most drug cases in the United States are decided well before they reach trial, by the far more informal process of plea bargaining.