Hair follicle drug testing is a prevalent method for detecting substance use. It offers an extended detection window compared to other testing techniques, providing a retrospective view into an individual’s drug history that goes beyond immediate or recent use. This makes it a valuable tool in various contexts.
How Hair Follicle Testing Works
Hair follicle testing works because drug metabolites circulating in the bloodstream become incorporated into the growing hair shaft. Once consumed, a substance enters the bloodstream and travels to the hair follicles. As hair cells form and grow, they absorb these molecules, embedding them within the hair’s structure. This process creates a chemical record of substance use.
Head hair typically grows about one-half inch (1 to 1.5 cm) per month. A standard sample for testing consists of 1.5 inches of hair cut close to the scalp, providing a detection window of approximately 90 days. During collection, 90 to 120 strands are cut from the scalp, often from the crown, to minimize visible impact. The collected sample is then sent to a laboratory for analysis.
Deciphering the Test Report
A hair follicle test report typically details the specific drugs tested and their measured concentrations. These concentrations are usually expressed in picograms per milligram (pg/mg) or nanograms per milligram (ng/mg). A picogram is one trillionth of a gram, and a nanogram is one billionth of a gram, with 1000 picograms equaling 1 nanogram.
Interpreting the report involves comparing detected substance concentrations to established cut-off levels. Laboratories use these thresholds to distinguish between incidental exposure or environmental contamination and actual drug use. A positive result means a substance’s concentration is at or above the specified cut-off level. A negative result indicates no drugs or drug metabolites were detected above these thresholds.
The testing process involves two stages: an initial screening test and a confirmation test. The initial screen, often an immunoassay, quickly identifies specimens that may contain drugs above a preliminary cut-off level. If positive, a portion of the original sample undergoes a more specific confirmation test, such as gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS/MS). Confirmation testing provides a detailed analysis, identifying specific drug metabolites and ensuring accuracy by ruling out false positives.
Common Substances and Detection Windows
Hair follicle tests commonly screen for several drug classes, including marijuana, amphetamines (methamphetamine and MDMA), cocaine, opioids (heroin, codeine, morphine, oxycodone), and phencyclidine (PCP). The extended detection window, typically up to 90 days of past substance use, is a primary advantage. This is significantly longer than urine or oral fluid tests, which generally detect drugs for only a few days.
The 90-day detection period is based on head hair’s average growth rate, where a 1.5-inch sample represents approximately three months of growth. While 90 days is standard, the exact window can vary based on an individual’s hair growth rate and the amount and frequency of drug use. Drugs may appear in hair within 7 to 10 days after exposure and remain detectable for that hair segment’s lifespan.
Factors Affecting Hair Test Outcomes
Several variables can influence hair follicle drug test outcomes. External contamination, such as exposure to secondhand smoke or drug residues in the environment, can theoretically leave traces on the hair shaft. Laboratories use specialized washing procedures to remove external contaminants and differentiate between environmental exposure and actual ingestion. They analyze drug metabolites, which are byproducts formed in the body after drug consumption. Higher drug levels in wash residue than in the hair itself may suggest external contamination.
Hair treatments, including bleaching, dyeing, perming, and straightening, can affect drug concentrations. These chemical processes can damage the hair shaft, potentially reducing detectable drug levels. Cosmetic treatments can decrease drug content, with bleaching often causing greater reductions than dyeing. However, such treatments rarely change a positive result to a negative one, especially for consistent users, as drugs are incorporated into the hair’s internal structure.
Head hair is preferred for its consistent growth rate and ability to provide a chronological history of drug use. However, body hair can be used if head hair is unavailable. Body hair, collected from areas like the chest, legs, or armpits, has a longer resting phase in its growth cycle. This means body hair can offer a detection window of up to 12 months, but it cannot be segmented for month-by-month analysis due to variable growth rates.