Hair follicle drug testing detects substance use over an extended period, commonly applied in employment screenings, legal proceedings, and medical contexts due to its long detection window. Understanding how to interpret these test results is essential. This article explores the mechanics of hair follicle tests, how to decipher their outcomes, factors that can influence accuracy, and what these results ultimately cannot reveal.
How Hair Follicle Tests Work
Hair follicle tests operate on the principle that drugs and their metabolites, which are breakdown products of drugs, circulate in the bloodstream after substance consumption. As hair grows, these substances are incorporated into the hair shaft from the blood vessels supplying the hair follicle, as well as from sweat and oil glands on the scalp. This process creates a historical record of drug exposure within the hair strand.
A standard hair sample is typically collected from the scalp, close to the root, measuring approximately 1.5 inches. Given that head hair grows at about half an inch per month, this 1.5-inch segment provides a detection window of approximately 90 days. While drug use may not appear in the hair until 7 to 10 days after exposure, once incorporated, substances remain detectable. Hair from other body areas, which grows more slowly, can offer a longer detection window, up to 12 months.
Deciphering Test Outcomes
Interpreting hair follicle test results involves understanding negative and positive outcomes, cutoff levels, and quantitative measurements. A negative result means no drugs or drug metabolites were detected above the established cutoff levels. This suggests no substance use, use outside the detection window, or consumption too low to be identified. Conversely, a positive result indicates a drug or its metabolite was found in the sample at a concentration exceeding the predetermined cutoff.
Cutoff levels are thresholds used by laboratories to differentiate between exposure that signifies actual use and incidental or environmental contamination. Laboratories employ a two-tiered testing process: an initial screening test (often immunoassay), followed by a confirmatory test using precise methods like gas chromatography-mass spectrometry (GC-MS). The confirmatory test, performed if the initial screen is positive, verifies the substance’s presence and often has slightly lower cutoff levels. These cutoff levels vary depending on the specific drug being tested and the type of sample. For hair tests, concentrations are reported in picograms (pg) per milligram (mg) or nanograms (ng) per milligram (mg).
When a test is positive, quantitative levels, expressed as specific drug concentrations (e.g., ng/mg), provide additional detail. These levels indicate the amount of drug or metabolite present in the hair sample. Higher quantitative levels generally suggest greater exposure or more frequent use, but they do not precisely quantify the amount or frequency of drug intake. Hair follicle tests commonly detect substances such as:
Marijuana
Amphetamines (including methamphetamine and MDMA)
Cocaine
Phencyclidine (PCP)
Opioids (like codeine, morphine, and heroin)
Factors Affecting Accuracy
Several factors can influence drug metabolite concentration in a hair follicle test, though laboratories employ protocols to mitigate their impact. Hair treatments, such as dyeing, bleaching, perming, or straightening, can potentially reduce drug concentrations in hair. While these chemical processes may cause some drug loss, it is typically not substantial enough to turn a positive result into a negative one, as drug metabolites are embedded within the hair shaft. Bleaching, for instance, can significantly decrease drug content.
Environmental exposure to drugs, such as passive marijuana smoke or cocaine dust, can lead to external hair contamination. To address this, laboratories implement wash procedures prior to analysis to remove external contaminants. Detecting drug metabolites, produced by the body after ingestion, rather than just the parent drug, helps distinguish actual use from external contamination. A ratio comparing drug levels in wash residue to those in the hair sample can also assist in this differentiation.
Hair color and type can also play a role, with some studies suggesting drugs like cocaine, methamphetamine, and opioids may bind more readily to melanin, the pigment in dark hair. This can lead to higher concentrations of substances in darker hair compared to lighter hair, even with the same exposure. However, these variations are generally not significant enough to alter a positive to a negative result.
What Results Cannot Reveal
While hair follicle tests offer a long-term detection window, it is important to understand their limitations. A hair follicle test cannot pinpoint the exact date or time of drug use within the 90-day window. Hair growth rates vary among individuals, making precise timing impossible. The test also cannot determine the exact frequency or quantity of drug use.
Additionally, hair follicle tests cannot determine if an individual was impaired at the time of testing or drug use. Drug metabolites in hair indicate past exposure, not current intoxication. Results should be interpreted within a broader context, considering all available information. A single quantitative level on a positive test does not indicate use severity.