Police have historically relied on the Duquénois-Levine (D-L) colorimetric field test to presumptively identify botanical material as Cannabis for the purpose of arresting a suspect for possession of marijuana.
The reagent used in the D-L test reacts with a particular part of the THC molecule producing an intense violet color that is readily identifiable by law enforcement at the roadside. However, the D-L test is not specific for just THC. Many other cannabinoids, including hemp-derived extracts such as CBD, will yield similar purple hues in the presence of the reagent.
Of course, these extracts are not marijuana under Iowa law, but instead are hemp products that are legal to possess at both the federal and state level thanks to the 2018 Farm Bill, S.F. 599 and H.F. 2581.
A driver who possesses legal hemp-derived CBD oil still faces arrest and prosecution because a police field test will mistake this extract for marijuana even when there is no THC! That’s right. Broad spectrum hemp-derived CBD with zero THC will still yield a false positive result.
Manufacturers of D-L field test kits are keenly aware of the problem. The maker of the NARK II D-L reagent that is advertised as a presumptive field test for marijuana, hash oil and THC warns law enforcement:
NOTICE: NOT FOR USE WITH CBD OILS. In states where the sale of CBD (cannabinoid) oil and hemp is legal, this test should NOT BE USED as everything tested will be positive. NARK II Duquenois-Levine Reagent is a qualitative test, not quantitative. It does not distinguish between 3% or 20% THC. It only establishes that, presumptively, THC is present.
You read that correctly. Don’t use this to test legal CBD oil because it will test positive for marijuana, hash oil or THC.
Normally, the presumptive D-L field test will be supplemented with other forensic laboratory testing, such as thin layer chromatography (TLC), to confirm the presence of a controlled substance.
It remains to be seen, however, whether TLC testing performed by the DCI laboratory is any better at differentiating between CBD and marijuana.