If you find yourself the subject of a police stop based on suspicion that you are driving under the influence of drugs or alcohol, including marijuana, at some point you might find yourself being subjected to a roadside drug test. This can take the form of a test to measure the presence of drugs in your body via a breathalyzer or a saliva test, or a test of a substance itself to determine if it is an illegal drug. You might be wondering just how accurate such tests are, or at least you should, because a growing body of studies suggest that false positives are more common than they should be, especially when the test indicates the presence of illegal drugs.

Police reliance on roadside drug testing is understandable in that such tests are a relatively inexpensive way to verify or reinforce the results of a borderline performance on your part of standardized field sobriety tests. But we have all heard stories about false positive test results; just how prevalent are they, and what can you do if you are the victim of a false positive?

False positives can happen more often that you think

What happens if during a stop of your car a police officer sees something that is perfectly legal for you to possess but he or she suspects is an illegal drug, and a test of the substance indicates that it is in fact an illegal drug? This disturbing prospect has been documented in at least one study, which along with other anecdotal evidence suggests that innocent items like spices, plants, vitamins, non-prescription medications, chocolate, coffee and even the air itself can, to the test kit, look like marijuana, cocaine, heroin, or methamphetamines.

How much marijuana is too much to be able to drive?

One of the claimed advantages of the saliva test for drugs is that it is less subject to being “gamed” by the test subject, such as sample alteration or substitution. Another is that unlike a blood draw, taking a saliva sample is at least arguably “non-invasive” from the standpoint of your expectation of privacy against unreasonable searches and seizures under the Fourth Amendment to the U.S. Constitution.

These claimed benefits need to be weighed against the real possibility that – particularly with regard to marijuana – the test can be of only limited value when it comes to measuring “intoxication.” According to a study by the AAA Foundation, it is scientifically impractical to establish a meaningful level for the effect on driving of THC levels following marijuana use: it noted that drivers could pass field sobriety tests and yet still be intoxicated according to the established concentration levels.

In a state like Oklahoma, which has a strict “per se” law for drugged driving (meaning that any detectable presence of the active chemical of marijuana or its metabolites is illegal when driving), this may not be as significant a drawback as it is in other states that have adopted a minimum measurement threshold for these substances; the driver’s best bet here is to not exhibit other symptoms and behaviors of marijuana use to trigger suspicion on the part of the police officer in the hope that the saliva field test is not administered.

“You can explain it all, down at the courthouse”

The problem with a false-positive drug test result or with running afoul of Oklahoma’s strict per se law against having traces of marijuana in your system while driving (THC metabolites, being fat-soluble, can linger in your system for days or even weeks even after the last use of marijuana) is that it can take time to correct the error while the justice system marches on against you. In the interim, you can be incarcerated if you cannot post bail, you can lose your job, and your public and private reputation can both suffer.

Until such time as more fool-proof means exist to test for the presence of illegal drugs in your possession or in your body, the preservation of your liberty when you are the innocent victim of a wrong test result will be something that requires assistance from skilled legal criminal defense counsel.