Hair follicle drug tests detect past substance use, often employed in employment screening, legal proceedings, and medical assessments. This method analyzes a hair sample to identify specific substances or their metabolites. Unlike other testing methods, hair analysis provides a longer historical record of drug exposure, reliably assessing an individual’s substance consumption over an extended period.
The Standard Detection Period
A hair follicle drug test typically provides a detection window of up to 90 days, or three months. This timeframe is established because laboratories usually analyze a 1.5-inch segment of hair collected from the scalp. Head hair generally grows at about 0.5 inches per month, so a 1.5-inch sample represents approximately three months of growth, offering a historical snapshot of drug use.
It takes 7 to 10 days for drugs or their metabolites to incorporate into the hair shaft and grow out beyond the scalp, making recent use undetectable by this method. While scalp hair is preferred, body hair can be used if head hair is too short or unavailable. Body hair grows slower, so a body hair sample can reveal drug use for an even longer duration, sometimes up to 12 months.
How Drugs Enter and Remain in Hair
When a substance is consumed, it enters the bloodstream. The circulatory system then distributes these substances and their metabolites throughout the body. Hair follicles are nourished by tiny blood vessels.
As hair grows, drugs and their metabolites circulating in the bloodstream are absorbed into the hair follicle. These compounds become trapped within the hair shaft, forming a lasting record of drug use. Some drug metabolites also bind to melanin, the pigment responsible for hair color, integrating them into the hair structure. This process allows hair to serve as a record of an individual’s substance exposure.
Factors Influencing the Detection Window
Several variables can influence the detection window and accuracy of hair follicle drug tests. Individual hair growth rates, which vary due to genetics, age, and sex, might marginally affect how far back a 1.5-inch sample can accurately reflect. The specific drug consumed also plays a role, as some substances integrate into the hair matrix more readily. The quantity and frequency of drug use impact the concentration of detectable substances, with heavier or more chronic use leading to higher levels.
External factors, such as hair treatments like bleaching or dyeing, can potentially reduce drug metabolite concentration. However, these treatments typically do not eliminate substances entirely, and the 90-day detection window for scalp hair remains standard. Environmental exposure to drugs, like secondhand smoke, might also introduce substances to the hair, but laboratories employ washing procedures to minimize false positives from external contamination.
Commonly Screened Substances
Hair follicle drug tests detect a range of substances. Standard panels typically screen for five major drug classes: marijuana, amphetamines (like methamphetamine and MDMA), cocaine, opioids (including heroin, codeine, and morphine), and phencyclidine (PCP).
Beyond illicit substances, hair tests can also identify prescription medication misuse, including sedatives, stimulants, or barbiturates. The specific substances screened vary by test purpose, such as employment, legal requirements, or medical monitoring.