If you have watched episodes of law-enforcement-related reality television, you may have seen police testing substances that they find in the vehicle of a driver that they have pulled over. These tests typically involve placing a suspect powder into a pouch containing one or more glass vials that contain chemicals which react to illegal drugs by changing color, such as blue indicating the presence of cocaine.

It is tempting to think of chemical drug tests as being reliable, if not fool-proof. After all, chemistry is based on science, so a positive result from a roadside test should be compelling evidence, right?

Not necessarily. Research suggests that for a number of reasons, roadside drug tests are not as reliable as they may seem; indeed, many cases have emerged in which false positive test results have led to drivers and passengers all over the nation being wrongly convicted.

How big is the problem of roadside drug test unreliability?

Every year in the United States more than a million people are arrested for drug possession, including possession of illegal drugs in vehicles that have been subjected to traffic or other stops. Of this number, about 100,000 eventually plead guilty to drug charges based on test results from the drug test kits that police use to make a preliminary finding of the presence of controlled substances. This happens despite a general policy in almost every police jurisdiction that roadside drug tests by themselves cannot qualify as evidence in a criminal trial: they need to be backed up by another form of testing, such as mass spectographs performed in a laboratory.

The trouble is, many drivers do not know about this limitation on the evidentiary use of roadside drug tests. When they see the color change, some of them assume that they cannot fight this “evidence” — and that makes them susceptible to pressure to accept a plea deal in which they plead guilty to illegal drug possession in exchange for a reduction in charges, a reduction of sentence, or both. These unfortunates may think that they are getting the best deal they can under the circumstances, but they can be terribly mistaken in that assumption.

What can go wrong with a roadside drug test?

What can make a roadside drug test unreliable? There are multiple things that can go wrong:

• Legal substances can trigger a false-positive. For tests that use cobalt thiocyanate to detect cocaine, more than 80 other substances can produce the same blue color change that cocaine does, including household cleaners and even acne medications.
• Lack of proper training / improper test administration. If the police officer who administers the test does not carry it out properly — such as by breaking one of three chemical vials in a test kit out of order — that can produce an inaccurate result.
• Weather and lighting conditions. Hot or cold temperatures during the administration of the test, or attempts to read the color change in poor lighting conditions, can also produce a false positive reading.

Estimates vary, but according to available information possibly one of every three cocaine test results generated by a roadside test kit is a false positive and one of every five that detects the presence of methamphetimenes is also inaccurate. Although the general rule requiring these tests to be backed up at trial is meant to minimize the prejudical effect of a false positive reading, The problem is that nationally about 90 percent of illegal drug convictions are not the result of a trial but of a plea bargain — and in 90 percent of jurisdictions across the country, the court will permit a guilty plea based solely on the result of a roadside drug test. Moreover, in about two-thirds of guilty-plea cases the lab responsible to perform a more accurate verification test does not do so.

How can you fight a roadside drug test result?

If you have been subject to having something in your vehicle tested by a roadside drug test kit and it produces a positive result, you should not admit to anything and should insist on consulting with a criminal defense attorney, preferably one who is experienced in defending DUI and other substance-related cases. These tests are definitely not “settled science,” and before you consider any kind of plea negotiation you should allow your defense attorney the opportunity to examine the validity of the test result and to challenge it if any of the factors that could produce a false positive were present at the time it was administered. The potential long-term negative consequences to you of pleading guilty to a drug charge are serious enough, and the risk of a mistake high enough, that you owe it to yourself to present a vigorous legal defense.