A hair drug test detects illicit substances or their metabolites within a person’s hair strands. This method offers a longer detection window for past drug use compared to other common tests like urine or blood tests, providing a historical record of substance exposure.
How Drugs Enter Hair
Drugs and their metabolic byproducts enter the hair after consumption. Once ingested, a substance circulates throughout the body via the bloodstream. As blood flows through the scalp, drug metabolites are absorbed into the hair follicle, the root structure where hair growth originates. These substances then become incorporated into the hair shaft as it grows.
The hair shaft acts like a chronological record, trapping these substances within its keratin structure. External factors can also contribute to drug presence in hair; sweat and sebum can contain drug residues that deposit onto the surface. Laboratories typically employ specific washing procedures to differentiate between internally incorporated drugs and external contamination.
The Laboratory Analysis Process
Once collected, a hair sample undergoes a multi-step laboratory process to identify drug metabolites. The initial step involves an external wash using solvents. This washing procedure is important for removing surface contaminants, such as environmental drug particles or hair product residues, ensuring only substances incorporated within the hair shaft are analyzed.
Following the wash, the hair sample is prepared for analysis by cutting it into smaller segments and pulverizing it into a fine powder. This physical preparation helps break down the hair matrix, making internally trapped drug metabolites accessible for extraction. The prepared sample then proceeds to a screening test, often an immunoassay, which quickly identifies common drug classes.
If the immunoassay screen yields a positive result, a confirmatory test is performed to precisely identify and quantify the specific drug metabolites. Common techniques include gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS). These methods separate individual components within the sample and identify them based on their unique molecular signatures, providing definitive evidence of drug presence and eliminating false positives.
Detection Timeline and Substances Identified
Hair drug tests typically detect drug use over the past 90 days. This detection window is determined by the average rate of human head hair growth, which is approximately 0.5 inches (1.3 cm) per month. For a standard 90-day detection period, laboratories usually require a hair sample about 1.5 inches (3.8 cm) in length, taken from the root end closest to the scalp.
The test can identify various classes of substances. Commonly detected drugs include:
Amphetamines (such as methamphetamine and ecstasy)
Cocaine and its metabolites
Opioids (like heroin, morphine, and codeine)
Marijuana metabolites (THC-COOH)
Phencyclidine (PCP)
Factors Affecting Test Outcomes
Several factors can influence hair drug test results, though laboratories employ measures to mitigate their impact. Hair treatments, such as bleaching, dyeing, perming, or relaxing, can potentially alter the hair’s structure and may, in some cases, reduce the concentration of drug metabolites within the hair shaft. The extent of this impact varies and depends on the specific treatment and drug.
External contamination, where drug particles come into contact with the hair from the environment, is addressed by the initial washing step during laboratory analysis. This process removes surface residues, ensuring only substances absorbed into the hair from the bloodstream are considered. Individual variations in hair growth rates or metabolism can also subtly affect the exact detection window. However, standardized sample collection and analysis protocols aim to provide consistent and reliable results.