We’ve been testing for THC (the main psychoactive part of cannabis) for a very long time. Our problem is not detecting it, but doing so with a portable machine in a non-invasive way.
As far as existing tests go, urine tests are commonly used for athletes or other employees undergoing drug tests. THC can be detected for anywhere from 1-30 days after use, depending on the frequency of use and the body fat of the individual (since THC is fat soluble).
False positives can result from consuming a variety of things: hemp seeds, ibuprofen, naproxen, and even Prevacid (an antacid). Luckily, a blood test can differentiate between true and false positives. Unluckily, blood tests for casual users of cannabis are only effective at detecting THC for about 1 day after consumption.
An alternative is a hair test. For that, the top 1.5 inches of a strand of head hair is tested. Body hair can also be used, though finding a 1.5-inch piece of leg or arm hair may be difficult. THC can be detected in hair up to 90 days post-consumption, although hair treatments like perms or dyes can affect results. This method is very sensitive and does not create false positives, though it does take longer than a urine test.
The quick and portable test that the Canadian government has settled on using is saliva-based. It’s called the Dräger DrugTest 5000. It requires only 0.28 mL of saliva, produces results in minutes, and even though very little THC passes from the blood into the saliva, the limit of detecting is low enough to still detect the compound.
The DrugTest 5000 is used in Australia, Germany and the UK. But it alone will not be the sole method of measuring intoxication of drivers. In addition to measuring the THC in a driver’s saliva, police will be allowed to perform field sobriety tests, and watch for telltale signs that someone is intoxicated.
Besides cannabis, the DrugTest 5000can detect opiates, benzodiazepines, cocaine, amphetamines and methamphetamines, though it does so for a hefty price: about $6000 per unit. It is only usable when the temperature falls between 4 and 40 ˚C, and can show false results if the subject has recently eaten or smoked.
This study took blood and saliva samples from 369 drivers and tested them using the DrugTest 5000 and traditional blood test methods (UHPLC-MS-MS). The DrugTest 5000 was correct in its assessment about 85% of the time for THC. This means that a false positive or negative reading would be given roughly once in every eight tests. Not really the best numbers.
The rate of false negatives is much better for the DrugTest 5000 when detecting methamphetamine (6.1%), opiates (0%) or cocaine (0%), although the rates of false positives (38.4%, 65.5% and 87.1%) are still quite high. False positives would at least be revealed as falsepositives upon blood test, a better alternative to letting an intoxicated driver free on the roads, but a 1 in 8 chance of a false reading is not what I’d hope for from the technology used by the Canadian police.