Not very reliable, as it turns out.
That is according to the nonprofit investigative newsroom ProPublica, which published a critical review of drug field tests in 2016. The investigators discovered that the $2 drug tests most police use haven’t been updated in decades. The results aren’t admissible in court. They can only be used to develop probable cause for arrest; an admissible drug test must be done later, at a crime lab.
Moreover, drug field tests routinely produce false positives. The test for cocaine is especially prone to produce inaccurate results, as the chemical in the test reacts to at least 80 other compounds ranging from acne medications to household cleaners.
Other tests use more than one reactive chemical, and the test tubes have to be broken in the correct order for the test to be valid. Far too often, police break the wrong tubes and gain false-positive results. Environmental factors such as heat and cold can change or prevent the reaction.
Even lighting plays a role. Sun, glare from street lamps, and flashing police lights can make it hard for officers to make the fine color distinctions that are necessary for the tests to work.
Just how inaccurate are these tests? That is hard to say. The accuracy varies widely depending on how they are used, and officers often make mistakes. There is no central agency that regulates the tests, and nobody is keeping comprehensive records about their accuracy.
However, ProPublica obtained data from the Florida Department of Law Enforcement lab system that indicated that 21% of the evidence police said was methamphetamine was actually not meth. Half the time, the supposed meth was not even an illegal substance.
These field tests are putting people in prison. If a police officer tests something in your possession and it comes up positive as an illegal drug, you’re going to be arrested even though the police know that these tests are inaccurate much of the time.
Once you’ve been arrested, you may be assigned bail that you can’t afford based on the positive drug test. You could be held in jail for weeks or months while you await your chance to dispute the positive test. Many people are held long enough that they simply give up and plead guilty.
After the ProPublica series, more than 250 people have been exonerated of drug crimes. It was found that the substances in their possession were not illegal drugs at all. Recently, five more were exonerated in Las Vegas.