Hair follicle drug testing is a common method for detecting substance use over an extended period. This approach analyzes hair samples for the presence of drugs and their metabolites, which are byproducts formed when the body processes a substance. Hair testing is frequently employed in various settings, including employment screening and legal contexts.
How Hair Follicle Drug Testing Works
Drugs and their metabolites enter the hair through a biological process after consumption. Once ingested, a substance circulates throughout the body via the bloodstream. Each hair follicle contains blood vessels that nourish the growing hair cells. As hair develops, drugs and their metabolites from the bloodstream become incorporated into the hair shaft.
These substances also enter the hair through sweat and oil glands on the scalp, where they become trapped as the hair grows.
To perform a hair follicle drug test, a small sample of hair, typically about 1.5 inches long and the thickness of a pencil, is cut close to the scalp. This sample usually consists of 90 to 120 strands of hair. The collected hair is then sent to a laboratory for analysis, where it undergoes a two-step testing process to ensure accuracy. Initial screening tests, such as ELISA, identify the presence of drug classes, and any positive results are then confirmed using more precise methods like gas chromatography-mass spectrometry (GC-MS).
Commonly Detected Substances
Hair follicle tests are capable of detecting various categories of drugs and their metabolites. These tests can identify illicit substances as well as the misuse of prescription medications. The specific drugs included in a hair test panel can vary, but several common categories are screened.
Commonly detected substances include:
Opioids: A class including prescription pain relievers and illicit substances (e.g., morphine, codeine, oxycodone, hydrocodone, hydromorphone, 6-acetylmorphine, a heroin metabolite).
Cocaine: And its metabolites, such as benzoylecgonine.
Amphetamines: Stimulant drugs like methamphetamine, MDMA (ecstasy), and MDEA.
Cannabis: Detected via its primary metabolite THC-COOH.
Phencyclidine (PCP): A dissociative anesthetic.
Detection Timelines
Hair follicle drug tests offer a significantly longer detection window compared to other testing methods like urine or blood tests. For head hair, the typical detection period is approximately 90 days, or three months. This timeframe is based on the average rate of head hair growth, which is about half an inch (or 1 to 1.5 centimeters) per month.
While drugs generally become detectable in hair about 7 to 10 days after exposure, they remain embedded in the hair shaft as it grows. This means that a hair test cannot detect very recent drug use, but it excels at revealing patterns of chronic or past substance use. If head hair is unavailable or too short, body hair can be used for testing, providing an even longer detection window, potentially up to 12 months, due to its slower growth rate.
Factors Influencing Test Results
Several factors can influence the results and interpretation of hair follicle drug tests. Individual variations in metabolism can affect how drugs are processed and incorporated into the hair. Hair cosmetic treatments, such as bleaching, dyeing, perming, and straightening, can impact the concentration of drugs and their metabolites. These chemical treatments may damage the hair shaft and reduce the amount of drug present, with bleaching potentially causing higher decreases than dyeing.
External contamination, where drugs from the environment, like secondhand smoke, can deposit on the hair, is another consideration. While laboratories typically wash hair samples to remove external contaminants, it can sometimes be difficult to distinguish between environmental exposure and actual drug ingestion. Some studies suggest that certain drugs, like cocaine, may bind more easily to melanin in darker hair, potentially leading to higher concentrations in testing results for individuals with darker hair. Additionally, certain prescription medications or even common foods like poppy seeds can sometimes lead to false positive results, highlighting the importance of confirmatory testing and disclosing all medications.