Marijuana test may be responsible for thousands of false positives

More than 800,000 people are arrested on marijuana charges each year in the United States, many on the basis of an error-prone test...

The Duquenois test was developed in the late 1930s by a French pharmacist, Pierre Duquenois, while he was working for the United Nations division of narcotics. In 1950, he completed a study for the UN which claimed that his test was "very specific" for marijuana; it was adopted by the UN and crime labs around the world as the preferred test for marijuana.

After undergoing several modifications, including the use of chloroform, the test became known as the Duquenois-Levine test, and became widely popular. Though scientists would show in the 1960s and 1970s that the D-L test was nonspecific, meaning it rendered false positives, it remains today the most commonly used test for marijuana -- used in many of the 800,000 marijuana arrests that take place each year...

Despite its widespread use, as early as the 1960s, the D-L test had been proven incapable of definitively identifying the presence of marijuana in a seized substance. A 1968 article in the Chemistry and Pharmacy Bulletin of Japan reported that the D-L tests "lack in adequate specificity." In 1969, M. J. de Faubert Maunder, a chemist in the Ministry of Technology, a UK government agency, documented the unreliability of the D-L test in an article in the Bulletin on Narcotics, noting that test results depended heavily on the subjective judgment of the analyst -- and thus could easily vary dramatically from lab to lab. "[A] positive test is not recorded until this colour has been identified," he wrote, "and because it is almost impossible to describe in absolute terms it is best recognised by experience." Moreover, he reported finding twenty-five plant substances that would produce a D-L test result barely distinguishable from that of Cannabis and cautioned that the D-L test "should never be relied upon as the only positive evidence."

[Thanks 23 Wolves!]