The police pull you over, claim to find illegal drugs in your car, and use a field test to examine them right there on the side of the road. The chemical — cobalt thiocyanate — starts out pink and then turns blue. That means, the police tell you, that the substance was cocaine. You’re arrested immediately.

How accurate is that test? Is your fate really determined by a chemical that changed colors on the side of the road?

Studies have found that the tests actually are not all that accurate. The problem with cobalt thiocyanate, for instance, is that it doesn’t just turn blue for cocaine, but for over 80 more compounds as well. Some are used in household cleaning solutions and acne medications.

To try to avoid false positives, police sometimes use a test that has a set of three different tubes with chemicals in them, and they can open the tubes one at a time. The problem there is that mistakes are easy. The order in which the tubes are used is vital, and mistakes can lead to inaccurate results.

Outside factors come into play as well. For instance, the color change is often faster when it’s warm and slower when it’s cold, and the temperature has even been shown to stop the colors from changing. If the lighting isn’t great — with the flashing police lights on a dark night — police may think the color has changed when it hasn’t.

In short, there are a multitude of reasons why a positive test may not mean that cocaine — or any other drug — has been identified. If you’ve been arrested based on a faulty test, be sure you know your legal rights.

Source: New York Times, “How a $2 Roadside Drug Test Sends Innocent People to Jail,” Ryan Gabrielson and Topher Sanders, accessed Sep. 27, 2017