Hair follicle drug testing detects past substance use. This test analyzes a hair sample for illicit substances and their metabolites. It provides a historical record of drug exposure, offering a longer detection window compared to urine or blood tests that indicate very recent use or current impairment.
How Drugs Enter Hair
Drugs and their metabolites enter hair strands through a biological process involving the bloodstream. After ingestion, a substance circulates throughout the body via the circulatory system. As hair grows, the hair follicle receives substances from surrounding blood vessels. These substances become incorporated into the hair shaft as it forms and grows from the scalp. Once embedded, the drug compounds are trapped within the hair’s keratinized structure.
Detection Timeframes and Variables
Hair follicle tests offer a detection window of up to 90 days for a standard 1.5-inch hair sample. This timeframe is based on the average rate of human head hair growth, which is approximately 0.5 inches per month. Therefore, a 1.5-inch segment of hair represents roughly three months of growth.
Several factors influence this detection period. An individual’s hair growth rate can cause variations; slower growth might extend the detection window for a given hair length, while faster growth could shorten it. The type of drug and its potency also play a role, as different substances incorporate into hair at varying rates and concentrations.
The frequency and amount of drug use impact detectability. Chronic or heavy use leads to higher concentrations of substances in the hair, making them more easily identifiable and for a longer duration compared to a single instance of use. An individual’s metabolism, which dictates how quickly the body processes and eliminates substances, also influences the amount of drug incorporated into the hair. Hair treatments like bleaching or perming may cause some degradation of drug metabolites within the hair shaft, potentially reducing concentrations. However, these treatments rarely eliminate the presence of drugs entirely, and laboratories can detect evidence of such treatments.
What Substances Hair Tests Detect
Hair follicle drug tests identify a range of commonly used substances. These include:
Opioids such as heroin, morphine, and codeine.
Cocaine and its primary metabolite, benzoylecgonine.
Amphetamines, including methamphetamine and MDMA (ecstasy).
Cannabis metabolites, specifically THC-COOH, indicating marijuana use.
Phencyclidine (PCP).
Specialized tests can be ordered to screen for a broader spectrum of compounds, depending on specific testing requirements.
Important Considerations for Hair Testing
Hair samples for drug testing are collected from the scalp, closest to the root, to ensure accurate representation of recent drug exposure. A 1.5-inch segment of hair is usually sufficient for analysis. When scalp hair is insufficient, body hair from areas such as the chest, armpit, or legs may be used. Body hair grows at a different rate than scalp hair, which can extend the detection window to 12 months for a similar sample size.
External contamination, such as exposure to secondhand smoke or drug residue on surfaces, is a concern. However, laboratories employ wash procedures and look for drug metabolites, which are only produced within the body, to distinguish between external exposure and actual ingestion. Hair testing primarily indicates past drug use and does not provide information about current impairment.