Hair drug testing is a method used to identify the presence of illicit substances or prescription medications in an individual’s system. This testing procedure often serves various purposes, including pre-employment screening, forensic investigations, and monitoring compliance with substance abuse treatment programs. Unlike some other testing methods, hair tests provide a longer detection window, offering a historical record of substance use.
The Science of Hair Drug Testing
When a person ingests drugs, the substances circulate throughout the body via the bloodstream. As blood flows to the hair follicles, drug metabolites, which are byproducts of the body processing the drugs, become incorporated into the growing hair shaft.
Once incorporated, the drug metabolites remain embedded in the hair as it grows outward from the scalp. This process provides a stable record of drug exposure over time. Since hair grows at a consistent rate, the length of hair collected can correspond to a specific historical period of an individual’s substance use.
Detection Window for Hair Tests
Hair drug tests provide a detection window of up to 90 days. This timeframe is determined by the length of hair collected and the average rate of human hair growth. A common sample size for testing is approximately 1.5 inches of hair, measured from the scalp.
Human head hair grows at an average rate of about 0.5 inches per month. Therefore, a 1.5-inch hair segment represents approximately three months of growth. This consistent growth rate allows laboratories to estimate the period of time over which drug use can be detected.
Factors Influencing Detection
Several factors can influence the detection window and the concentration of drug metabolites found in hair. Individual hair growth rates can vary, with some people experiencing faster or slower growth than the average 0.5 inches per month. Such variations can alter the precise historical period represented by a hair sample.
An individual’s metabolism also plays a role in how quickly drugs are processed and eliminated from the body, affecting the amount of metabolites available for incorporation into hair. The frequency and amount of drug use are factors; chronic or heavy use results in higher concentrations of metabolites in hair compared to single or infrequent use. Hair treatments, such as bleaching, dyeing, or perms, can reduce the concentration of drug metabolites in the hair, though studies vary on the extent of this effect. Some research also suggests minor differences in drug incorporation based on hair type or color, although these effects are typically considered less significant.
Commonly Detected Substances
Standard hair drug tests detect a range of commonly abused substances. These panels include:
- Cannabis metabolites (marijuana)
- Cocaine and its primary metabolites
- Opioids (e.g., heroin, morphine, codeine, oxycodone)
- Amphetamines (e.g., methamphetamine, MDMA)
- Phencyclidine (PCP)
Accuracy and Test Limitations
Hair drug testing is an accurate and reliable method for detecting past drug use, often showing a high correlation with self-reported drug use. However, it has limitations. It cannot detect very recent drug use, within the last 7 to 10 days. This is because hair needs time to grow out of the scalp before any incorporated drug metabolites can be sampled.
While laboratories employ strict cutoff levels to differentiate between active use and potential environmental exposure, passive exposure to certain substances, such as cannabis smoke, can be a consideration. A hair drug test cannot determine if a person is currently impaired or under the influence of a substance at the time the sample is collected. Although rare with proper laboratory procedures and confirmation testing, the possibility of false positives or false negatives cannot be entirely eliminated.