A hair follicle test detects drug metabolites, providing a history of substance use over an extended period. This analysis is highly accurate and is favored over urine testing for its longer detection window, typically covering the previous 90 days. Despite its reliability, the test is not infallible, and inaccurate results, including false positives and false negatives, can occur. Understanding the mechanisms of drug incorporation and the potential for external interference is necessary to evaluate the validity of any test outcome.
The Science of Drug Incorporation
Hair testing relies on the body’s physiological processes following substance ingestion. After consumption, the drug is metabolized by the liver, and the drug and its metabolites circulate through the bloodstream. These compounds are delivered to the hair follicle via capillaries surrounding the hair root, the site of active growth. As the hair shaft forms, the drug compounds become chemically embedded within the hair’s internal structure, the keratin cortex. Hair grows out of the scalp at about one-half inch per month. Laboratories typically collect a 1.5-inch segment closest to the scalp, providing a retrospective window of three months of exposure. The presence of the drug’s metabolite—the breakdown product created by the body—is the definitive indicator that the substance was ingested, not just encountered externally.
External Factors Leading to False Positives
A major concern in hair testing is the possibility of a false positive result caused by environmental exposure, which occurs without the person ingesting the substance. This external contamination happens when drug residue, such as smoke particles or fine powder, settles onto the surface of the hair shaft. Substances like cocaine, methamphetamine, and cannabis are particularly prone to environmental deposition from secondhand smoke or contact with contaminated surfaces.
To mitigate this risk, certified laboratories subject the hair sample to extensive chemical washing procedures before analysis. The goal of this decontamination process is to remove any drug compounds adhering to the outside of the hair cuticle. However, no washing protocol is guaranteed to eliminate all surface contamination, and the wash procedure itself can inadvertently drive some external contaminants into the hair matrix.
The distinction between true ingestion and contamination is primarily made by analyzing for the presence of drug metabolites, which are only produced by the body after the drug has been processed internally. For example, testing for the metabolite THCCOOH is crucial to confirm cannabis ingestion, as THC itself can easily adhere to hair from secondhand smoke. If a drug is detected but the corresponding metabolite is absent or below a certain threshold, the result may be deemed inconclusive or indicative of external exposure.
Procedural and Biological Sources of Error
The accuracy of a hair test can also be compromised by factors related to the donor’s biology or errors in the laboratory’s process.
Biological Variables
One significant biological variable is the use of cosmetic hair treatments. Chemical processes like bleaching, dyeing, and perming structurally alter the hair, which may cause a reduction in the concentration of drug compounds retained within the hair matrix. Research suggests these treatments can decrease the detectable amount of drug metabolites by 30% to 80%, potentially leading to a false negative result. Furthermore, the amount of melanin, the pigment that determines hair color, can influence drug retention. Certain drugs, such as opioids and cocaine, tend to bind more strongly to melanin, meaning a person with darker hair may naturally retain higher concentrations than a person with lighter hair, even with the same exposure.
Procedural Errors
Procedural errors introduce another layer of potential inaccuracy, beginning with the chain of custody. Improper sample collection, mislabeling, or failure to maintain secure storage can compromise the integrity of the specimen. Once in the lab, initial screening tests use methods like immunoassay, which are fast but can produce false positives due to cross-reactivity with common medications like ibuprofen or certain antidepressants. Any positive screening result must be confirmed using a highly specific technique, such as Gas Chromatography-Mass Spectrometry (GC/MS), which precisely identifies the molecular structure of the compounds present. Failure to perform this mandated confirmatory testing or instrument malfunction represents a serious analytical error.
Confirming or Contesting a Result
If a positive hair test result is believed to be incorrect, established steps exist to formally contest the finding. The primary action is requesting a retest of the original sample. In regulated testing scenarios, a split sample is collected initially, and this second portion can be sent to a different certified laboratory for independent analysis.
The tested individual usually has a limited timeframe, often 72 hours from notification, to make this request through the Medical Review Officer (MRO). The MRO is a licensed physician trained to interpret drug test results and evaluate them within the context of the individual’s medical history. When contesting, the MRO should be provided with documentation of any prescription or over-the-counter drugs consumed, as well as evidence of potential environmental exposure. The MRO determines if a legitimate medical explanation exists for the positive result before it is reported as verified.