Hair drug testing is a highly accurate, non-invasive method for screening for substance use over an extended period. This form of analysis is often employed in legal, employment, or forensic settings due to its reliability and the difficulty of tampering with the sample. This article explores the biological mechanisms that make the hair test so robust and examines the common methods people use to try and evade detection.
The Science of Hair Drug Testing
Once a substance is ingested, it enters the bloodstream and circulates throughout the body. Drugs and their resulting metabolites are transferred to the hair follicle through surrounding blood capillaries and sweat or oil glands. These compounds become physically trapped within the hair as it forms and hardens.
The hair is primarily composed of a protein called keratin, and drug metabolites become permanently encased within this structure as the hair grows out from the scalp. The test analyzes the hair shaft, which is the dead, visible portion of the hair, not the living follicle beneath the skin. Metabolites are chemically bound to the hair matrix, particularly to melanin, the pigment that gives hair its color.
The binding to melanin means individuals with darker, more pigmented hair may show higher concentrations of certain basic drugs, such as cocaine and amphetamine. Since the drugs are trapped inside the hard keratin structure, they remain in the hair as it continues to grow. This biological process establishes a permanent, time-stamped record of substance use.
Understanding the Detection Window
The detection period for a standard hair test is based on the average rate of hair growth, which is approximately 0.5 inches (1 centimeter) per month. Laboratories generally collect a hair sample measuring the 1.5 inches closest to the scalp. This length corresponds to the standard 90-day, or three-month, detection window used for most workplace and legal screenings.
It takes seven to ten days for a hair strand containing drug metabolites to grow from the follicle and emerge above the scalp. Therefore, a hair test cannot detect very recent drug use. Body hair grows more slowly and can reflect a much longer, though less precise, detection window, sometimes up to 12 months.
Common Methods Used to Attempt to Pass
Since metabolites are locked within the hair shaft, individuals often attempt to use external treatments to remove or mask the internal evidence. This commonly involves specialized “detox” shampoos, based on the theory that they can penetrate the keratin and wash the metabolites out.
The effectiveness of these cleansing products is highly variable and usually insufficient to remove the internally encased metabolites below laboratory cutoff levels. While some products may reduce drug concentrations, they rarely eliminate them entirely, especially for chronic users. The deep chemical bond between the metabolite and the hair matrix is difficult to break with surface-level washing.
Chemical treatments like bleaching, dyeing, or perming are also frequently used to compromise the test. These harsh processes cause structural damage to the hair shaft, which may lead to a reduction in drug concentration as metabolites leach out. For instance, bleaching hair has been shown to reduce the concentration of certain substances, such as Tetrahydrocannabinol (THC), by an average of 14% to 34%.
However, this reduction is often not enough to guarantee a negative result. Laboratories can visually detect the signs of such treatments, which may raise suspicion and require a retest or additional analysis. Furthermore, if head hair is unavailable due to shaving or extreme shortness, the collector is instructed to take a sample of body hair from areas like the chest, underarm, or leg, which provides an alternative, albeit longer, detection history.
Limitations and Reliability of Hair Follicle Testing
The high reliability of hair drug testing stems from a rigorous, two-step laboratory procedure. The first step is an initial screening, typically an immunoassay, which is a fast and cost-effective method to detect the presence of drug classes. This preliminary test is highly sensitive but can sometimes produce a false positive result due to cross-reactivity with similar chemical structures, such as certain medications.
Any positive screening sample immediately proceeds to the second, definitive step: confirmation testing. This is performed using advanced analytical techniques like Gas Chromatography/Mass Spectrometry (GC/MS) or Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS). These methods precisely identify and quantify the specific drug and its metabolite by separating compounds based on their chemical properties and molecular structure.
This confirmation process provides an exact chemical fingerprint of the substance, ruling out false positives and confirming the presence of the ingested metabolite. Laboratories also use decontamination protocols to wash the hair sample before analysis, which helps to minimize the risk of a positive result from external contamination. The permanency of metabolite incorporation, combined with the stringent confirmation process, makes manipulating the outcome very difficult.