The Consequences of False Positive Test Results

Recently, it was reported Karlos Cashe spent 90 days in jail because powdered drywall was mistaken for cocaine.  Both cocaine and powdered drywall are white.  Both cocaine

Photos of Chemical/Pharmaceutical Drugs
White Powder – Drywall? Cocaine? Something else entirely? By Rillke – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=22833840

and powdered drywall can take the form of a powder.  And both cocaine and powdered drywall apparently test presumptively positive for cocaine using field test kits that screen for illegal drugs.

Understanding Presumptive Tests

Presumptive tests “presume,” but do not prove, the presence of certain substances.  There are presumptive tests for blood, saliva, semen, and yes, there are presumptive tests for a host of different illegal substances, such as cocaine, heroin, methamphetamine, LSD, etc.  While a presumptive test will generally provide a positive test result for the substance it is marketed for, (in Mr. Cashe’s case, cocaine), it will also yield a positive test result for other substances (in Mr. Cashe’s case, drywall).

False Positives

In addition to drywall, there are any number perfectly legal, common items which yield a positive test result for illegal substances.  By way of example only, a short list of substances previously yielding positive results for the presumptive presence of illegal drugs includes:


Error Rates

There is no precise known error rate.  However, there is no question false positives occur with presumptive field drug tests.  For example, in Florida, one lab found 21 percent of the evidence police submitted as “methamphetamine” was not actually methamphetamine.

Criminal Charges

It is not uncommon in Minnesota for the state to file charges based on a presumptive drug test result.  The substance should then be sent to an accredited laboratory for confirmatory testing.  In some parts of the country, such as Harris County, Texas, courts won’t accept a plea of guilty without a confirmatory drug test.  This assures the courts innocent people aren’t swept up due to presumptive testing alone.  Unfortunately, this does not address crime lab backlogs, and the amount of time defendants spend in jail, awaiting test results.


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