Hair follicle tests have become a common method for detecting past drug use. These tests analyze hair strands to provide a historical record of substance exposure. Unlike other drug testing methods, hair follicle tests offer a longer detection window, making them a useful tool for various applications.
How Hair Follicle Tests Work
Drugs enter the bloodstream after consumption and circulate throughout the body. As hair grows, the cells in the hair follicle absorb these substances, including the parent drug compounds and their metabolites, which are byproducts formed when the body processes the drug. These drug compounds then become embedded within the hair shaft, effectively creating a chronological record of drug use.
During a hair follicle test, a small sample of hair, typically about 1.5 inches in length, is cut close to the scalp. The sample is sent to a certified laboratory for analysis. Although commonly referred to as “hair follicle tests,” the actual hair follicle is not tested; instead, the hair strand itself is analyzed. Laboratories typically use a two-step process: an initial screening test followed by a confirmatory test if drugs are detected.
Standard Detection Window
Hair follicle tests are notable for their extended detection window, typically up to 90 days. This timeframe is directly related to the average rate of human head hair growth. On average, human head hair grows approximately 0.5 inches per month.
Therefore, a 1.5-inch hair sample, collected from the scalp, represents roughly three months of hair growth. While drugs may not appear in the hair until 7 to 10 days after exposure, once incorporated, they remain detectable for the duration of the hair’s growth.
Factors Affecting Detection Times
The standard 90-day detection window can be influenced by several factors. The type of drug and its potency play a role, as different substances metabolize and incorporate into hair at varying rates. Individual metabolic rates also impact how quickly drugs are processed and deposited into the hair structure.
Hair characteristics, such as color and growth rate, can affect detection. The frequency and amount of drug use are also significant; chronic or heavy use results in higher concentrations of drug metabolites in the hair, making them detectable for longer periods.
Cosmetic hair treatments, such as bleaching or dyeing, can reduce drug concentrations in hair, but tests are designed to account for these alterations, and such treatments do not eliminate all detectable metabolites. Laboratories also have protocols to distinguish between drug compounds incorporated through ingestion and those from external environmental contamination, such as exposure to secondhand smoke.
Drugs Detectable by Hair Tests
Hair follicle tests are designed to detect a range of commonly used substances. The specific drugs screened can vary depending on the test panel requested.
Common categories of drugs typically screened for include amphetamines, such as methamphetamine and MDMA. Cocaine and its metabolites are also frequently detectable. Opiates, including morphine, codeine, and heroin, are another class of drugs that hair tests can identify.
Phencyclidine (PCP) is also commonly included in standard panels. Additionally, marijuana (THC metabolites) is detectable in hair samples. Some tests can also screen for other prescription drugs, such as benzodiazepines and barbiturates, if specifically requested.