A hair follicle drug test analyzes a small sample of hair to detect the presence of substances used over a significant period. Unlike urine or saliva tests, which primarily detect very recent substance use, this test provides a historical record of consumption. The testing procedure typically involves collecting about 100 milligrams of hair, cut close to the scalp. It is frequently utilized in professional settings for pre-employment screening, random employee testing, and various legal or forensic contexts. This method offers a long-term detection window.
How Drug Metabolites Enter Hair
After a substance is consumed, the body processes it into drug metabolites. These compounds circulate throughout the body via the bloodstream, which feeds the hair papilla at the base of the hair follicle. As a hair strand forms and grows beneath the scalp, the drug and its metabolites from the blood are incorporated directly into the structure of the hair shaft.
Compounds can also enter the hair from the surrounding skin. Substances and their metabolites present in sweat and sebum (skin oil) are deposited onto and absorbed into the hair strand as it emerges from the skin’s surface. This mechanism traps the drug compounds within the hard keratin matrix of the hair, creating a permanent record of past substance use.
The Standard Panel of Detectable Substances
The most common screening method is the standard five-panel hair test, which looks for evidence of the most frequently misused classes of substances. Laboratories typically analyze for compounds and their corresponding metabolites, which confirm the substance was ingested and processed by the body. These five categories form the foundation of most hair follicle drug testing protocols.
Cocaine
Testing looks for the parent drug and its primary metabolite, benzoylecgonine. The presence of this metabolite helps distinguish active use from simple external exposure.
Amphetamines
This category includes methamphetamine, amphetamine itself, and substituted amphetamines such as methylenedioxymethamphetamine (MDMA), commonly known as ecstasy.
Opiates
The standard panel focuses on naturally occurring compounds like morphine and codeine. It also screens for 6-monoacetylmorphine (6-MAM), the unique metabolite of heroin, confirming its use.
Phencyclidine (PCP)
Detection is usually based on the parent drug compound rather than a specific metabolite.
Marijuana
The test specifically targets the inactive metabolite Carboxy-THC (THC-COOH). Focusing on this metabolite helps differentiate active use from passive exposure to secondhand smoke.
Laboratories can offer expanded panels that include other substances like oxycodones, benzodiazepines, barbiturates, and synthetic opioids like fentanyl, depending on the requirements of the requesting entity.
Factors Influencing Detection Time
Hair follicle tests are valued for their broad detection window, which is directly related to the average human hair growth rate. Head hair typically grows at a rate of approximately 0.5 inches (1.3 centimeters) per month. For a standard test, a laboratory analyzes the 1.5 inches of hair closest to the scalp, which corresponds to roughly 90 days of substance use history.
Substances do not appear in the hair immediately following consumption. A lag time of approximately seven to ten days exists before the hair containing the embedded compounds grows above the scalp where it can be collected. This means a hair test is ineffective at detecting very recent substance use that occurred within the past week.
The 90-day window is an approximation, as individual hair growth rates can vary. If head hair is unavailable, body hair can be collected as an alternative. Body hair grows more slowly and has a longer resting phase, meaning a sample can reflect a longer, less precise history of use, sometimes extending the detection window up to a year.
Limitations and Accuracy of Hair Follicle Testing
Hair follicle testing has specific limitations that can affect results. One major challenge is the potential for external contamination, where drug residue from the environment, such as secondhand smoke, can adhere to the hair shaft. To address this, laboratories employ extensive washing procedures on the sample before analysis and look for the presence of metabolites, which are only formed inside the body.
The use of cosmetic treatments like bleaching, dyeing, or perming can also influence the concentration of substances in the hair. These chemical processes can damage the hair shaft, which may cause a reduction in the concentration of drug compounds, although they rarely eliminate them entirely. Simple washing or the use of specialized shampoos is generally ineffective at removing the compounds incorporated into the hair’s internal structure.
Furthermore, the natural color of a person’s hair can affect how certain drugs bind to the hair shaft. Some substances, including cocaine and various opiates, have a higher affinity for melanin, the pigment that gives hair its color. This phenomenon can result in higher concentrations of the drug being detected in darker hair compared to lighter hair, even with the same level of substance use.