A hair follicle test detects a history of substance use by analyzing drug compounds or their metabolites within the hair shaft. Unlike urine or saliva tests, which provide a snapshot of recent use, hair analysis offers a longer-term retrospective view of substance exposure. Understanding the specific requirements for the hair sample, including the amount needed and the collection process, ensures accuracy during screening.
Required Hair Sample Specifications
The physical requirements for a hair sample ensure the test covers the standard detection period. For typical screening, the laboratory requires a sample mass of 100 milligrams, which is a bundle of hair about the thickness of a standard pencil. This volume is usually composed of 90 to 120 individual strands.
The collection process involves cutting the hair as close to the scalp as possible, ensuring the sample contains the newest growth nearest the root. Laboratories typically only test the 1.5 inches of growth closest to the head. This standardized length provides the window needed to look back over a three-month period.
Testing Window and Alternative Collection Sites
The standard 1.5-inch hair length is used because head hair grows at an average rate of approximately one-half inch per month. This consistent growth rate allows the 90-day testing window to be accurately established by analyzing drug compounds along the hair strand. The test cannot detect use from the immediate past, as it takes about seven to ten days for a drug to show up in the hair that has grown above the scalp.
If an individual has insufficient head hair or if their hair is too short to meet the 1.5-inch requirement, body hair becomes an acceptable alternative collection site. Acceptable sites include the chest, underarm, leg, or facial hair. Body hair grows at a much slower and more variable rate than head hair, reflecting a much longer, though less precise, detection window, potentially spanning up to a year. Head hair and body hair should not be mixed in a single sample, and the root end must always be identified for accurate time-based analysis.
How Drugs Become Trapped in Hair
Drug incorporation into the hair shaft begins after a substance is ingested. The substance circulates throughout the body via the bloodstream, and metabolites are produced as the body breaks it down. The hair follicle, fed by blood vessels, absorbs these compounds from the circulating blood.
As new hair cells form in the hair matrix, the drug compounds and their metabolites become structurally embedded within the keratin core of the growing hair shaft. Once trapped, these substances remain fixed in the hair as it pushes outward from the scalp. The hair is then sent to a lab, where it is washed to remove surface contamination and dissolved to extract the compounds for analysis, typically using techniques like gas chromatography/mass spectrometry (GC/MS).
External Factors Affecting Test Results
Cosmetic treatments or environmental factors can affect the accuracy of a hair test. Hair treatments like bleaching, dyeing, or perming involve chemicals that can damage the hair shaft and may reduce the concentration of metabolites detected. Laboratories have established procedures to account for and mitigate these effects, and such treatments rarely result in a false negative.
Environmental contamination is another external factor, such as exposure to secondhand smoke or physical contact with drug residue. To address this, testing facilities perform a rigorous wash procedure on the hair sample before analysis to remove any surface contamination. The laboratory can then compare the amount of drug found on the surface wash with the amount embedded within the hair matrix to differentiate between passive exposure and actual ingestion.