Drug Tests: How Do They Test for THC?

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When an employer sends you for a drug test, the tests indicate the levels of THC-COOH, THC's metabolite that results from chemical reactions within the body.

Although THC is the psychoactive substance in marijuana, urine tests trace THC's metabolite, THC-COOH. Other tests are designed specifically to test THC levels; however, the amount of THC in the system decreases rapidly over a short period of time. Drug tests, which include marijuana identification, are required by some employers. Although prone to incorrect results, the enzyme multiplied immunoassay technique (EMIT) is most commonly used.

With lab instructions in hand, give a blood or urine sample. Both samples should be held in sterile containers to prevent contamination. Laboratory personnel will use EMIT to test for the presence of THC-COOH. EMIT uses antibodies that can only react to one drug. The cutoff for a positive test is 50 nanograms of THC metabolites per milliliter. Beyond the limit to test positive, EMIT cannot determine how much or how long ago a person used marijuana.

All federal jobs and some employers require a follow-up test to a positive EMIT result. The retest is completed using gas chromatography/mass spectrometry (GC MS), which has a lower cut-off point of 15 nanograms of metabolites per milliliter. Smoking marijuana will cause a positive GC MS result typically for two to three weeks after last usage.

Oral swabs may be used to test for marijuana; however THC and THC-COOH do not remain in saliva long. This type of testing is typically used after workplace accidents to determine if the employee was under the influence of marijuana at the time of the accident.

Some employers require a hair follicle test for drugs including marijuana. This test also tests for THC-COOH, which passively diffuses from the blood stream to the hair follicle. Hair follicle testing is also an inaccurate way of testing for the presence of marijuana. The THC-COOH levels that remain in the hair do not stick the way other drugs, such as cocaine, do. Exposure to high humidity will break down levels further and occasional smokers may not register due to the lack of sensitivity of the test. Hair follicle drug testing has been called racist and ageist because the tests are more sensitive to minorities and those with darker hair colors and will rarely register the amount of THC-COOH in grey hair because it holds the metabolite less than normally-pigmented hair.

Although there are a variety of ways and reasons to test for the use of marijuana, the testing methods often prove inaccurate, creating both false-positive and false-negative results. For this and cost-effective reasons, the EMIT followed by the GC MS technique is preferred.



The National Organization for the Reform of Marijana Laws: The ABCs of Marijana and Drug Testing - http://norml.org/index.cfm?Group_ID=6991

Biology Online: Enzyme-Multiplied Immunoassay Technique - http://www.biology-online.org/dictionary/Enzyme-multiplied_immunoassay_technique