A hair follicle test analyzes the hair shaft for drug compounds and their metabolites to determine a history of substance use over an extended period. Unlike blood or urine tests, which primarily detect recent use within a few hours or days, the hair test captures a long-term pattern of use. It is frequently employed in legal, forensic, and employment contexts where a sustained history of abstinence or use is being investigated.
The Science of Substance Incorporation
Substances consumed are processed by the body, yielding the parent drug and its metabolites. These compounds enter the bloodstream and circulate throughout the body. The blood supplies the hair papilla, the structure at the base of the hair follicle where new hair cells are formed.
As the hair grows, circulating substances and metabolites from the blood are incorporated directly into the hair shaft’s inner structure, the keratin matrix. This traps the drug compounds within the hair, creating a chemical record of exposure. Substances can also enter the hair externally through sweat and sebum (skin oil) that coat the hair strand as it emerges from the skin.
Sample Collection Procedures
A collector obtains a lock of hair, which must be cut as close to the scalp as possible to ensure the root end is preserved. The preferred location for collection is typically the posterior vertex, or the crown area at the back of the head. Hair from other body areas can be used if head hair is unavailable.
The required sample amount is usually 90 to 120 strands of hair (approximately 100 milligrams), roughly the thickness of a soda straw. The collected hair sample is standardized to a length of 1.5 inches from the root end, which is the segment analyzed by the laboratory. Once cut, the sample is secured in foil with the root ends clearly marked and placed into a specialized envelope.
A strict chain of custody must be maintained until the sample reaches the laboratory for analysis. This process involves the donor verifying their identity and the collector documenting the collection date and securing the sample with tamper-evident seals. Observed collection procedures minimize the chance of sample adulteration or substitution, safeguarding the forensic defensibility of the results.
Laboratory Analysis and Interpretation
Laboratory preparation begins with a decontamination wash to remove any externally deposited substances, such as drug residue from secondhand smoke or incidental contact. After washing, the hair is chemically dissolved and prepared for the two-step analytical process.
The first step is a preliminary screening test, often an immunoassay (e.g., ELISA), which quickly identifies drug compounds above a set cutoff level. Any presumptive positive result must then proceed to a second, more specific confirmation test. Confirmation is typically performed using advanced techniques like Gas Chromatography/Mass Spectrometry (GC/MS) or Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS).
These chromatographic mass spectrometric methods separate the chemical components of the hair extract and identify the molecular structure of the drug and its metabolites. This verifies the identity of the substance with high specificity, distinguishing between structurally similar compounds and ensuring the positive result is not a false reading. Only after confirmation using these forensic-standard instruments is a sample officially reported as positive.
The standard 1.5-inch length of hair tested corresponds to a detection window of approximately 90 days, based on the average human head hair growth rate of about one-half inch per month. A lag time of about seven to ten days exists before the hair containing the substance grows above the scalp where it can be collected. A positive result indicates that the concentration of the substance and its metabolites in the hair exceeds a standardized cutoff level established by regulatory guidelines.