There are reasons that many cannabis users  fear hair tests as a method of checking for the presence of THC or its metabolite, THC-COOH, in the body. Testing hair, blood, urine and saliva are often popular methods of testing for drug and/or alcohol use in DUI, workplace, child-custody and other cases in which drug use is important.

But there may be another big reason to fear hair tests, according to a recent article: Hair tests may be a bit too liberal in their recognition of the presence of cannabis, which we will explain below as part of the good news and bad news.

What Does a Hair Test Detect?

When a hair is subjected to a drug test, especially one to test of marijuana use, the test is designed to check for not only the active ingredient THC, but it also supposedly can detect the presence of THC-COOH, a metabolite of THC, and THCA-A, a chemical that is more passive.

The challenge with hair tests is that while it may not necessarily detect very recent use of marijuana (if you are a first-time user), if you have any history of use a hair test could detect the presence of THC or its derivatives in your system for several months after your last joint.

The Structure of a Hair

When you look at a hair and its environment, that alone could lend some clarity as to the hair test and its extreme sensitivity to cannabis.

A hair follicle starts inside a pore of skin, which has an external surface and blood capillaries internally. As the hair grows and leaves the layer of skin, it comes into contact with outside environment like the air. At any given time, that hair could come in contact with skin, blood, sweat and air, not to mention water or heat. You can see this in the image shown here, courtesy of Nature magazine.

What this means in its most simple form is that the hair could come into contact with cannabis in several different ways, and not necessarily just from smoking. Virtually any contact with cannabis can show up in a hair sample.

The Good News and Bad News

With the way a hair test is conducted, there are some things to consider when subjecting yourself to a hair test for marijuana use. There are some good news and bad news about these tests.

First, the reality: Hair can collect THC, THC-COOH and THCA-A from other places besides just inhaled smoke. In that way, hair tests can be very sensitive to the presence of cannabis.

What does this mean?

Well, the bad news first: This means that you could test positive for marijuana use even if you don’t ingest or inhale it – you could test positive from second-hand smoke from another user or handling cannabis (such as putting it into pot brownies, even if you don’t eat one). And any positive test can have negative consequences for you, and chances are the onus to prove a false positive will be on you, not on the testing lab.

But here is the good news: Because of the sensitivity of these hair tests, they are unreliable in terms of showing actual consumption of cannabis. If you got a false positive, you can challenge a result by requesting a more accurate consumption test (blood or urine) to invalidate a positive hair test.

The bottom line here is that you do not have to fear a hair test. The science shows that these tests can present far more false positives than other consumption-based tests such as blood or urine, and thus can empower you to ensure that you don’t fail on account of someone else’s use of cannabis.



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