Hair follicle testing (HFT) is a method used to screen for past substance use by analyzing a small sample of hair. This method differs significantly from blood or urine testing because it offers a much longer window of detection for drug use. HFT is a common tool used in employment screening and legal contexts to establish a history of drug exposure rather than immediate impairment. The test works by detecting the specific drug molecules and their metabolites that become trapped within the hair shaft.
The Process of Drug Incorporation into Hair
When a substance is consumed, the parent drug and its metabolites circulate through the bloodstream. Each hair follicle is nourished by a small blood vessel. As blood flows past the hair papilla, the drug compounds diffuse from the circulation into the newly forming hair cells. These chemicals become permanently embedded in the hair’s inner core, known as the keratin matrix.
Once the hair grows out from the scalp, the drug evidence is sealed within the hair shaft structure. Because hair is metabolically inactive once it exits the scalp, the drug evidence remains stable. This process creates a chronological record of substance use. It generally takes about five to ten days for the hair containing metabolites to grow past the scalp line and become available for collection.
Standardized Detection Window for Head Hair
The standard detection window for a hair drug test is typically 90 days, or three months, based on the average rate of hair growth. Human head hair grows at a relatively consistent rate of approximately 0.5 inches (1.3 to 1.5 centimeters) per month. To cover the three-month historical period, testing laboratories require a sample that is 1.5 inches long, cut as close to the scalp as possible.
The testing protocol for this standard window often excludes the first 0.5 inches of hair closest to the scalp. This segment, representing the most recent seven to ten days of use, is typically disregarded because drugs take time to incorporate and grow out of the follicle. By analyzing the 1.5-inch segment starting just beyond the scalp, the test focuses on the 90-day period where the drug evidence is fully incorporated. This standardization provides a reliable, non-invasive method for assessing chronic or repeated substance use over a defined timeline.
Variables That Affect Test Results
The standardized 90-day window is a strong guideline, but individual physiological differences and external factors can alter the test results. Hair growth rate varies among individuals; those with slower growth will have a 1.5-inch sample representing a longer period, while faster growth shortens the detection window. Similarly, hair color plays a role because certain drug metabolites, such as those for cocaine, amphetamines, and opioids, tend to bind more readily to melanin, the pigment found in darker hair. This difference can lead to higher measured concentrations in dark hair compared to light hair, even when the dose consumed was the same.
Chemical hair treatments, including bleaching, dyeing, perming, and relaxing, can significantly impact the drug concentration detected in the hair shaft. These processes can chemically degrade the hair structure and wash out a portion of the incorporated drug metabolites, sometimes reducing the measured levels by 40 to 80 percent. This degradation can potentially lead to a false negative result, especially for lower levels of use.
Environmental exposure is a factor that testing laboratories must carefully address to avoid false positives. Being in a space where drugs are smoked, such as from crack cocaine or marijuana, can cause airborne drug molecules to passively adhere to the outside of the hair shaft. Laboratories use specialized washing procedures on the hair sample before analysis to distinguish between external contamination and the internally incorporated drug metabolites.
Types of Substances Screened and Sample Requirements
Standard hair drug testing panels screen for the five most common classes of substances, often referred to as the five-panel test.
Five-Panel Substances
- Marijuana metabolites
- Cocaine
- Opiates
- Amphetamines (including methamphetamine)
- Phencyclidine (PCP)
Many labs now offer expanded panels that test for a wider range of substances, such as synthetic opioids, ecstasy (MDMA), and benzodiazepines.
For collection, the required sample is a lock of hair roughly the thickness of a pencil, which is about 200 individual strands. The hair must be cut as close to the scalp as possible to ensure the most accurate chronological record is captured. If head hair is too short or unavailable, body hair from the chest, armpit, or leg may be collected instead. However, body hair grows much slower and has a less consistent growth cycle than head hair, resulting in a detection window that can be longer, sometimes extending up to a year, but with a less precise timeline of use.