Wednesday, July 13, 2016

Drug tests use cobalt thiocyanate?

I promise it's not Recreational Drug Week on the blog, but I did want to point out this long New York Times Magazine article about field tests for drugs, and how those field tests can give false positives (and how those false positives can ruin people's lives): 
The field tests seem simple, but a lot can go wrong. Some tests, including the one the Houston police officers used to analyze the crumb on the floor of Albritton’s car, use a single tube of a chemical called cobalt thiocyanate, which turns blue when it is exposed to cocaine. But cobalt thiocyanate also turns blue when it is exposed to more than 80 other compounds, including methadone, certain acne medications and several common household cleaners. Other tests use three tubes, which the officer can break in a specific order to rule out everything but the drug in question — but if the officer breaks the tubes in the wrong order, that, too, can invalidate the results. The environment can also present problems. Cold weather slows the color development; heat speeds it up, or sometimes prevents a color reaction from taking place at all. Poor lighting on the street — flashing police lights, sun glare, street lamps — often prevents officers from making the fine distinctions that could make the difference between an arrest and a release. 
There are no established error rates for the field tests, in part because their accuracy varies so widely depending on who is using them and how. In Las Vegas, authorities re-examined a sampling of cocaine field tests conducted between 2010 and 2013 and found that 33 percent of them were false positives.
(Later in the article, a pretty decent description of mass spectrometry.)

I'm surprised that there hasn't been a switch to an antibody-based test, but I am guessing there are both cost and technical issues there. 


  1. CJ--I think the antibody tests are generally much slower. Google found a procedure for a cocaine assay that requires 8-10 minutes to establish a negative result. That may not sound like much, but if you're a cop standing on the roadside at night wondering if one of those passing cars will veer too close and pile up on the back of your cruiser, the extra 7.5-9.5 minutes is an eternity.

  2. A 33% false positive rate is unacceptable. It disrupts too many people's lives and overburdens limited forensic lab resources. I'm also concerned with the false negative rates.

    1. Good training and procedures are required to minimize issues yet this is a test that is not highly specific to cocaine so 33% false positive is not too surprising and would suspect pretty much any amine might trigger color. After all this is supposed to be a cheap and rapid screen test in the field that should lead to more detailed and precise verification in forensic lab setting if positive so unless people are willing to bare the significant costs in more highly accurate testings or be taken to holding cell while such tests are conducted it would seem an acceptable means to determine if further elucidation is warranted.

      Maybe based on all the pot head responses in recent post perhaps the bigger risk is being taken in for smell of marijuana.

  3. Here's a link to an article about the test results with cocaine and other substances. As stated by Anonymous (7/13 @ 3:52 PM)  the value of the test is in reducing the workload of the forensic lab by eliminating samples that can't be cocaine.