Hair follicle testing is a method detecting substances in an individual’s system. This type of testing is often employed in various contexts, such as employment screenings, legal proceedings, and medical evaluations, to determine past substance use. By analyzing a small sample of hair, these tests can provide a historical record of drug exposure.
The Standard Detection Window
A hair follicle test provides a detection window of up to 90 days for drug use. This timeframe is considered standard for samples taken from the scalp. The duration of this window is directly related to the average growth rate of head hair. Hair grows about 0.5 inches (1.25 cm) per month, and a standard 1.5-inch hair sample is collected for testing. This length allows laboratories to assess substance use over the preceding three months.
How Hair Follicle Tests Work
Hair follicle tests identify drugs or their metabolites, which are breakdown products formed as the body processes substances. When a substance is consumed, it enters the bloodstream and circulates throughout the body. As the drug is metabolized, both the drug itself and its metabolites are incorporated into the hair follicle through the blood vessels that nourish the growing hair.
As hair grows, these substances become encased within the hair shaft, creating a chronological record of drug exposure. Each segment of hair then contains information about the substances present in the body during its growth. Laboratories analyze these segments to determine the presence and the approximate timeframe of substance use.
Factors Influencing Detection
While the 90-day detection window is standard, several factors can influence the results of a hair follicle test. Individual hair growth rates can vary, with some people’s hair growing faster or slower than the average 0.5 inches per month, which might alter the exact historical period covered. The specific type of substance used also plays a role, as some drugs and their metabolites may bind more readily to hair than others. For instance, drugs like cocaine, methamphetamine, and opioids may bind more easily to melanin, the pigment in dark hair.
The frequency and dosage of substance use can also affect the concentration detected in hair. External factors, such as environmental exposure to drugs (e.g., secondhand smoke) or the use of certain hair treatments like bleaching, dyeing, or perming, can impact test results. Reputable laboratories employ procedures, including washing hair samples before testing, to account for external contamination and ensure accurate analysis.
Accuracy and Test Limitations
Hair follicle tests are highly accurate for detecting past substance use, providing a longer detection window compared to other methods like urine or saliva tests. They are useful for identifying patterns of chronic use over time. The testing process involves a two-step analysis, starting with an initial screening followed by a confirmatory test, such as gas chromatography-mass spectrometry (GC-MS), to minimize the risk of false positives.
Despite their reliability, hair follicle tests have limitations. They cannot determine the exact date of drug use due to variations in hair growth rates among individuals. These tests also cannot indicate immediate impairment. Hair tests may not detect very recent drug use (within 7 to 10 days) because it takes time for drug metabolites to be incorporated into the growing hair shaft. While effective for historical use, they are not suited for detecting acute or one-time use at low levels.