Duquenois-Levine is a test used by police in many jurisdictions, including Fairfax County, Virginia, to identify substances believed to be marijuana. Its critics call it unreliable, since it is meant to be a screening test, and it is not scientifically "specific" to marijuana.
Yes, all Virginia agencies use this test.
I wouldn't call it a BAD test, but the exact language would be to say that it is not "specific" for marijuana and thc. Instead, it will show positive if there is marijuana, but it will also show positive for other substances, including plain brown coffee.
Now, let's think about what that means. Assume that coffee is the only substance that would create a false positive. Ok,--- then the cop just needs to testify that it smelled like weed, and not like coffee, and it looked like weed, and not coffee. It tested positive, and it was not coffee, so it must be weed. Fair enough, but it is still a pretty sloppy way to do business.
In toto, they are taking a test that would not pass muster in a laboratory environment, standing alone, and they are using it in the field and in the courtroom as if it WERE a specific test.