A hair follicle drug test is a method used to detect evidence of past drug use or misuse by analyzing a sample of hair. This type of test is employed in various settings, including employment screening, legal investigations, and sometimes in medical contexts. It offers a longer detection window compared to other common drug testing methods.
The Science Behind Hair Testing
Drug metabolites enter and become embedded in hair primarily through the bloodstream. Once a substance is consumed, it circulates throughout the body. As hair grows, the cells in the hair follicle are nourished by blood capillaries, which carry the drug and its metabolites. These substances then become entrapped within the hair shaft as it forms and grows out from the scalp.
The hair growth cycle plays a significant role in how drugs are incorporated and detected. Hair growth occurs in phases, with the anagen phase being the active growth period where the hair shaft is produced. Drugs and their byproducts are absorbed into this growing hair.
In addition to blood circulation, drugs and metabolites can also enter the hair through sweat glands and oil glands on the scalp, or via diffusion from deep skin compartments. Some drug metabolites also bind to melanin, the pigment that gives hair its color. The combination of these internal incorporation mechanisms allows hair to retain a historical record of substance exposure.
Collection and Laboratory Analysis
Collecting a hair sample for drug testing is a straightforward process typically performed by a trained professional to prevent contamination. A small sample of hair, about 1.5 inches long and roughly the thickness of a pencil, is cut as close to the scalp as possible, usually from the back of the head. This specific length and location are chosen because scalp hair generally grows at an average rate of about 0.5 inches per month, allowing the 1.5-inch sample to represent approximately 90 days of an individual’s history.
If head hair is unavailable or too short, body hair can be collected. Once collected, the hair sample is secured, often in foil, and chain of custody documentation is completed before it is sent to a certified laboratory.
At the lab, analysis typically involves a two-step process to ensure accuracy. An initial immunoassay screening test, such as an ELISA, rapidly screens for drug classes. If this screening yields a positive result, a second, more specific confirmatory test is performed using advanced analytical techniques like Gas Chromatography-Mass Spectrometry (GC/MS) or Liquid Chromatography-Mass Spectrometry/Mass Spectrometry (LC/MS/MS). These methods precisely identify and quantify specific drugs and their metabolites, confirming positive results and ruling out false positives. Results are typically available within a few business days.
Substances Detected and Detection Window
Hair follicle tests can detect a range of common substances and their metabolites. These include cannabis, amphetamines (including methamphetamine and MDMA), cocaine, opiates (such as heroin, codeine, and morphine), and phencyclidine (PCP). Some advanced tests may also identify synthetic opioids, benzodiazepines, and alcohol markers.
The primary advantage of hair follicle testing is its extended detection window, which typically covers a period of up to 90 days. This longer window is possible because drug metabolites become incorporated into the hair shaft as it grows and remain trapped within it.
While the standard detection window for head hair is 90 days, samples taken from body hair, which grows more slowly, may provide a detection window of up to 12 months. It is important to note that hair tests cannot detect very recent drug use, typically within the last 7 to 10 days, because it takes time for the hair containing the metabolites to grow out from the scalp.
Factors Affecting Test Outcomes
Several factors can potentially influence the results of a hair follicle test. External contamination, where drug particles settle on the hair from the environment, is a consideration. However, laboratories employ washing procedures to minimize the impact of such external traces and differentiate them from internally incorporated metabolites. Detecting specific drug breakdown products (metabolites) rather than just the parent drug can help distinguish between external exposure and actual ingestion.
Hair treatments, such as bleaching, dyeing, perming, or chemical straightening, can affect the concentration of drugs and metabolites detected in hair. These chemical processes can damage the hair shaft and potentially reduce the amount of drug present, sometimes leading to lower detected concentrations or even false negatives.
Hair color can also play a role, as certain drugs like cocaine, methamphetamine, and opioids may bind more easily to melanin, the pigment in darker hair. This can lead to higher concentrations of these substances being detected in darker hair types.