A hair follicle test detects substances in an individual’s system, primarily to identify patterns of substance use over an extended period. This testing is often employed for employment screening, legal and forensic investigations, and medical assessments. It offers a longer detection window compared to other drug testing methods.
How Hair Follicle Tests Detect Substances
Substances enter the body and circulate through the bloodstream. As blood nourishes the hair follicles, drugs and their metabolites (byproducts of drug breakdown) become incorporated into the growing hair shaft. The hair itself is composed of keratin, and as it forms, it encapsulates a historical record of substances present in the body. This process allows for the detection of substances long after their immediate effects have worn off.
Substances Commonly Identified
Hair follicle tests identify common substances and their metabolites. These include opioids (e.g., morphine, codeine, oxycodone), cocaine and its metabolites (e.g., benzoylecgonine), and amphetamines (e.g., methamphetamine, MDMA, MDA). Phencyclidine (PCP) is also identified. Marijuana use is detected via its metabolite, THC-COOH.
Understanding the Detection Window
A hair follicle test provides a detection window of up to 90 days. This timeframe is based on average head hair growth of approximately 0.5 inches per month. A standard 1.5-inch hair sample represents about three months of growth. This allows the test to reflect a history of substance use rather than just recent consumption or current impairment. Hair tests cannot pinpoint the exact date of use due to variations in individual hair growth rates.
Factors Affecting Test Outcomes
Several factors influence hair follicle test results. Hair treatments (e.g., bleaching, dyeing, perming, chemical straightening) affect drug metabolite concentration. These treatments damage the hair shaft, reducing detectable substance levels. Individual variations in metabolism and hair growth also play a role in how substances are incorporated and retained. Frequency and quantity of substance use also impact detectable metabolite concentration.
What Hair Follicle Tests Do Not Show
Hair follicle tests have limitations in what they can reveal. They do not detect very recent substance use, as it takes 7 to 10 days for hair containing drug metabolites to grow out of the scalp. Therefore, these tests are not suitable for identifying acute or immediate impairment. While specialized hair tests for alcohol exist, standard hair follicle drug tests do not detect alcohol consumption. Over-the-counter medications or environmental exposure are not a concern for false positives, as these tests primarily identify drug metabolites produced by the body after ingestion.