While a single instance of substance use can be incorporated into hair, detecting it through a hair follicle test is challenging. These tests are designed to provide a historical overview of substance use, making a single, isolated event difficult to pinpoint.
How Substances Enter Hair
When a substance is consumed, it enters the bloodstream and circulates throughout the body. As the body processes these substances, they are broken down into metabolites. Both the parent substance and its metabolites then reach the hair follicles, which are located just beneath the skin’s surface.
Substances primarily enter the hair follicle through surrounding capillaries and secretions from sweat and sebaceous glands. Once absorbed by actively growing hair cells, they become encased within the hair shaft as it forms and emerges from the scalp. Some substances, like nicotine, cocaine, and opioids, also bind to melanin, the pigment that gives hair its color.
Hair Follicle Testing Explained
A hair follicle drug test analyzes a small hair sample to identify substances or their metabolites over an extended period. These tests screen for illicit drugs, prescription medications, and nicotine.
For scalp hair, a 1.5-inch sample is collected, usually from close to the scalp. This segment provides a detection window of approximately 90 days, as head hair grows at about one-half inch per month.
The testing process involves an initial screening, followed by a more precise confirmation test for any presumptive positive results. Laboratories use specific “cutoff levels,” measured in picograms per milligram of hair, to determine if a detected substance’s concentration is high enough to be reported as positive.
Detecting Single Exposure
Detecting a single instance of substance use through a hair follicle test is challenging because the concentration from a single exposure might be below the test’s established cutoff levels. Hair tests are more effective at identifying patterns of chronic or repeated use.
Several factors influence the likelihood of detection. The amount of substance used in a single exposure and an individual’s unique metabolism play a role in how much of the substance or its metabolites are incorporated into the hair. Hair growth rates can vary among individuals, which impacts the timeline of detection. Certain drugs also bind more readily to hair, particularly to melanin, affecting their detectability.
External factors can also influence results. While laboratories attempt to wash samples to remove environmental contamination, some residue might remain. Hair treatments like bleaching, dyeing, or perming can also alter the concentration of substances in the hair, potentially affecting test outcomes. Ultimately, a negative test result does not necessarily mean no substance was present, but rather that any detected amount was below the laboratory’s reporting cutoff level.