A hair follicle drug test is a forensic method used to determine patterns of substance use over an extended period, offering a detection window significantly longer than that of urine or oral fluid tests. The test works by analyzing a hair sample for drug compounds and their metabolites, which are byproducts created when the body processes a substance. These compounds enter the bloodstream and become permanently incorporated into the hair shaft as it grows, providing a chemical history of internal exposure.
The Standard Reporting Format
A hair test result is presented as a detailed laboratory report that systematically lists the specific drug classes and their corresponding analytes that were tested. A standard panel typically includes metabolites for substances like cocaine, amphetamines, opioids, and marijuana, each with its own quantitative result. The report distinguishes between the initial screening result, often performed using an immunoassay (EIA or ELISA), and the confirmation result.
If the initial screen is presumptive positive, the sample undergoes a second, more specific analysis, usually by gas chromatography-mass spectrometry (GC/MS) or liquid chromatography-tandem mass spectrometry (LC/MS/MS). Quantitative values are expressed in ultra-low concentrations, most often picograms per milligram (pg/mg) or nanograms per milligram (ng/mg) of hair. The report shows the measured concentration alongside a defined “cutoff level” used for interpretation. A final result for each substance is reported as either ‘Detected’ (concentration exceeded the cutoff) or ‘Not Detected’ (concentration was below the established cutoff).
Interpreting Positive and Negative Findings
A definitive “positive” finding requires confirmation by the highly specific mass spectrometry method, not just the initial screen. This result means the concentration of the drug metabolite met or exceeded the laboratory’s established cutoff level after confirmation. The cutoff level is scientifically determined to reduce the possibility of a false positive, ensuring the result reflects actual ingestion.
The term “non-negative” is sometimes used for a sample that failed the initial screening but lacks confirmation. A “negative” result indicates the drug or its metabolite was either not detected or was present below the established cutoff level. A Medical Review Officer (MRO)—a licensed physician—reviews any confirmed positive results to determine if a legitimate medical explanation, such as a valid prescription, exists. The MRO ensures the final reported result is accurate and medically sound before release.
Understanding the Detection Window
The hair follicle test is designed to assess drug use over approximately 90 days preceding the collection date. This timeframe is based on the average growth rate of head hair, which is about half an inch (1.27 cm) per month. Laboratories standardize the test by analyzing the most proximal 1.5 inches (3.81 cm) of hair, corresponding to the past three months of growth.
Hair does not begin to grow out of the scalp until about 7 to 10 days after use, creating a small gap where recent drug use cannot be detected. The test confirms use occurred within the three-month window but cannot pinpoint the exact date of consumption due to variations in individual hair growth rates. Standard reports provide a single finding for the entire 90-day period, although some specialized tests use segmented analysis.
Factors Influencing Result Accuracy
Several external variables can influence the concentration of drug metabolites found in a hair sample, which is considered during result interpretation. Cosmetic treatments like bleaching, dyeing, perming, and chemical straightening can chemically alter the hair shaft. This may reduce the concentration of drug compounds, potentially leading to a false negative result. Bleaching, in particular, can significantly reduce drug levels by damaging the hair’s internal structure and removing melanin, where many drug metabolites bind.
External contamination occurs when hair is exposed to drug residue from the environment, such as second-hand smoke or handling drugs. To mitigate this risk, laboratories subject all hair samples to a rigorous washing procedure before analysis to remove surface contamination. Testing focuses on the presence of the drug’s metabolite, a substance only produced by the body after ingestion, which helps distinguish between environmental exposure and actual substance use.