How Is Hair Tested for THC?
Obtaining a Sample Testing for tetrahydrocannabinol, or THC, is typically conducted using urine or a hair sample. The latter has come to be known as a much more accurate and harder to fool test. The test begins with a chunk of hair cut from the base of the scalp. An adequate sample contains a few dozen hairs and should be about the width of a fork tine. The length of the sample should be approximately 1 1/2 inches, which can adequately test for marijuana use within the previous 90 days. If the patient has no head hair, a sample can be taken from body hair. Analysis A gas chromatograph is used to try and separate cannabinoids due to marijuana use and those produced in the body in reaction to over-the-counter medications. Many of the testing done is proprietary to specific testing companies, but the basic form of the test is often radioimmunoassay or colorometric signals. The former method was developed in the 1950s (its creator won a Nobel prize for the test’s use in determining insulin
