About Drug Tests
Hair is a good medium for drug testing, because a lot of what we ingest deposits residue in our hair. This residue does not coat the outside of the hair. The hair root actually picks up the substance from the blood stream and deposits it on the inside of the shaft. The residue remains on the hair shaft indefinitely, and leaves a pattern as the hair grows out, similar to rings in a tree. It is possible to tell not only what was ingested, but also--based on where it appears on the hair shaft-- when it was ingested. The longer the hair, the more residue there is. Employers are allowed to test hair for drugs and alcohol.
Hair Drug and Alcohol Tests
Drugs, like marijuana and heroin, actually leave particles of the drug inside the hair shaft. Alcohol does not leave alcohol in the shaft, but it does leave chemical markers that indicate alcohol use. In both instances, the amount of the drugs, or chemical markers, in the hair shaft corresponds with the amount a test subject has consumed. On average, hair grows at a rate of a 1/2 inch every 30 days. Laboratories collect between a 1/2 inch and 1 1/2 inches for testing. Each 1/2 inch corresponds to roughly 30 days of potential drug use and employment drug tests go back 90 days. The tests won't show use within the last seven days, however, because those hairs have not grown out yet. The lab will measure the length of the hair from the scalp, or root bulb, out because the part closest to the scalp has the most recent information. If the test subject does not have enough scalp hair, the testing lab may collect hair from other parts of his body. If the lab is unable to collect a good sample of body hair, it will extract hair follicles.
The Hair-Testing Process
The laboratory collects a sample of approximately 50 hairs and washes the hairs to remove any dirt or external drug deposits. Cleaning the hair guards against false positives from factors like second-hand smoke residue. The lab then strips any melanin from the hair and screens the sample using enzyme-linked immuno sorbent assay (Elisa). If the Elisa is positive, some labs may run a second test using gas chromatography/mass spectometry (GC/MS) to confirm the results. The use of the second test protects against a false positive.
