Cannabis drug testing
Cannabis drug testing describes various drug test methodologies for the use of cannabis in medicine, sport, and law. Cannabis use is highly detectable and can be detected by urinalysis, hair analysis, as well as saliva tests for days or weeks.
Unlike alcohol, for which impairment can be reasonably measured using a breathalyser (and confirmed with a blood alcohol content measurement), valid detection for cannabis is time-consuming, and tests cannot determine an approximate degree of impairment. The lack of suitable tests and agreed-upon intoxication levels is an issue in the legality of cannabis debate, especially regarding intoxicated driving.
The concentrations obtained from such analyses can often be helpful in distinguishing active use from passive exposure, elapsed time since use, and extent or duration of use.
The Duquenois-Levine test is commonly used as a screening test in the field, but it cannot definitively confirm the presence of cannabis, as a large range of substances have been shown to give false positives.
Most cannabinoids are lipophilic (fat soluble) compounds that easily store in fat, thus yielding a long elimination half-life relative to other recreational drugs. The THC molecule, and related compounds, are usually detectable in drug tests from 3 days up to 10 days according to Redwood Laboratories; heavy users can produce positive tests for up to 10 months after ceasing cannabis use. The length of time varies greatly according to metabolism, quantity, and frequency of use.
Marijuana use can be detected up to 2–5 days after exposure for infrequent users; For heavy users: 1–15 days; For chronic users and/or users with high body fat: 1–30 days Under the typical 50 ng/mL cutoff for THC in the United States, an occasional or on-off user would be very unlikely to test positive beyond 3–4 days since the last use, and a chronic user would be unlikely to test positive much beyond 7 days. Using a more sensitive cutoff of 20 ng/mL (less common but still used by some labs), the most likely maximum times are 7 days and 21 days, respectively. In extraordinary circumstances of extended marijuana use, detection times of more than 30 days are possible in some individuals at the 20 ng/mL cutoff.  However, one must remember that every individual is different, and detection times can vary due to metabolism or other factors. It also depends on whether actual THC or THC metabolites are being tested for, the latter having a much longer detection time than the former. THC (found in marijuana) may only be detectable in saliva/oral fluid for 2–24 hours in most cases.
Cannabis use is included in the "10-panel urine screen", as well as the "SAMHSA-5", the five drugs tested for in standard NIDA approved drug tests.
False positives have been known to be triggered by consuming hemp-seed bars and other products, although the more detailed, more expensive gas chromatography-mass spectrometer (GCMS) test can tell the difference.
In 2011, researchers at John Jay College of Criminal Justice reported that dietary zinc supplements can mask the presence of THC and other drugs in urine. Similar claims have been made in web forums on that topic.
Common known pharmaceutical drugs which cause false positives in instant THC dip tests include: Dronabinol, Efavirenz, Ibuprofen, Ketoprofen, Naproxen, Piroxicam, Promethazine, Proton pump inhibitors, Sulindac, Tolmetin 
The Duquenois–Levine test is a simple chemical color reaction test initially developed in the 1930s by Pierre Duquénois.
To administer the test, a police officer simply has to break a seal on a tiny micropipette of chemicals, and insert a particle of the suspected substance; if the chemicals turn purple, this indicates the possibility of marijuana. But the color variations can be subtle, and readings can vary by examiner.
It was adopted in the 1950s by the United Nations as the preferred test for cannabis.
Cannabis use is detectable with hair tests and is generally included in the standard hair test. Hair tests generally take the most recent 1.5 inches of growth and use those for testing. That provides a detection period of approximately 90 days. If an individual's hair is shorter than 1.5 inches, this detection period will be shorter. If the hair sample is taken from the body, the detection window of hair cannabis testing will be longer since body hair grows slower than head hair and distorts detection timeframe. Hair drug testing measures the marijuana parent metabolite embedded inside the hairshaft and eliminates external contamination as a source of a positive result. The detection window of hair drug testing for cannabis can be as low as 1 pg/mg.
Cannabis is detectable by saliva testing. Just like blood testing, saliva testing detects the presence of parent drugs and not their inactive metabolites. This results in a shorter window of detection for cannabis by saliva testing. Delta 9 THC is the parent compound. If saliva sample is tested in a lab, the detection level can be as low as 0.5 ng/mL (up to 72 hours after intake) and if an onsite instant saliva drug test is used, the cut off level is generally 50 ng/mL (up to 12 hours after intake). Per National Institute on Drug Abuse saliva drug testing provides a reasonable alternative to other drug testing methods.
Cannabis is detectable in the blood for approximately 2–3 days after use, with heavy/frequent use detectable in the blood for approximately two weeks. Because they are invasive and difficult to administer, blood tests are used less frequently. They are typically used in investigations of accidents, injuries and DUIs.
Urine contains predominantly THC-COOH, while hair, oral fluid and sweat contain primarily THC. Blood may contain both substances, with the relative amounts dependent on the recency and extent of usage.
Though very unlikely to be used, and more unlikely in court, Electroencephalography or EEG shows somewhat more persistent alpha waves of slightly lower frequency than usual. Cannabinoids produce a "marked depression of motor activity" via activation of neuronal cannabinoid receptors belonging to the CB1 subtype.
Usage in law
While there are blood, urine and hair tests that can track marijuana’s active ingredient in the body, the fact remains that marijuana lingers around for too long a period in order for one of these tests to determine the actual intake time. Also, the variation between different metabolisms makes an objective cannabis intoxication test very difficult.
Cannabis testing is common in intoxicated driving investigations, and also as in probation guidelines.
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