A hair follicle test is a forensic tool used to establish a history of alcohol consumption over an extended period. Unlike blood or urine tests, which capture a brief moment in time, hair testing provides a retrospective view of consumption patterns. When alcohol is consumed, the body processes it, and specific metabolic byproducts become trapped within the growing hair shaft. The test analyzes a small hair sample to measure the concentration of these markers, offering insight into alcohol intake over the preceding months. This method is employed in various legal and employment settings to determine if an individual has engaged in repeated or excessive alcohol use.
The Science of Alcohol Incorporation in Hair
The body’s process of breaking down alcohol, or ethanol, creates specific compounds that serve as direct biomarkers for consumption. Laboratories primarily look for two key molecules: Ethyl Glucuronide (EtG) and Ethyl Palmitate (EtPa).
EtG is a water-soluble metabolite formed in the liver during detoxification. This marker is transported through the bloodstream and incorporated into the growing hair shaft, also finding its way into the hair via sweat and sebum.
EtPa, a fat-soluble Fatty Acid Ethyl Ester (FAEE), is formed in the blood and incorporated predominantly through the sebum coating the hair shaft. Analyzing both markers provides a more reliable and comprehensive picture of internal alcohol exposure.
Because EtG is water-soluble, its levels can be reduced by external factors like bleaching and excessive washing. Conversely, EtPa concentrations may be elevated by alcohol-containing hair products, though EtG is generally unaffected by this external contamination.
Standard Detection Windows and Testing Procedures
The hair follicle test provides a detection window based on the average rate of hair growth. Head hair typically grows at a rate of approximately one centimeter per month. Therefore, a standard sample of 1.5 inches (3 to 6 centimeters) represents an approximate 90-day history of use.
The sample is collected by cutting a small bundle of hair, roughly the thickness of a pencil, as close to the scalp as possible, usually from the crown of the head. This ensures the most recently grown hair, representing the 90-day period, is analyzed.
The test is not designed to detect alcohol use in the few days immediately preceding the test, as it takes approximately 7 to 10 days for the biomarkers to incorporate into the emerging hair shaft.
If scalp hair is unavailable, body hair from other areas may be used, though this is less ideal. Body hair growth rates are more variable and slower, meaning the 90-day window is not as precise and may represent a much longer period of time. The test results are intended to show a pattern of consumption over time, not a specific episode of intoxication.
Defining Consumption Levels and Test Thresholds
The hair follicle alcohol test distinguishes between low-level social consumption and chronic, excessive consumption. The Society of Hair Testing (SoHT) established quantitative cut-off points for interpreting results, measured in picograms per milligram (pg/mg) for EtG and nanograms per milligram (ng/mg) for EtPa.
EtG Thresholds
For the EtG marker, a concentration of 7 pg/mg or less in the 3-centimeter hair segment is considered negative, suggesting abstinence or very low-level use. Any concentration above 7 pg/mg suggests repeated alcohol consumption. This low threshold is designed to flag regular, moderate drinking.
The definitive threshold for chronic excessive consumption is 30 pg/mg or greater for EtG. This elevated level signifies sustained heavy drinking over the 90-day period being analyzed. Chronic excessive consumption is defined as consuming an average of 60 grams or more of pure ethanol per day over several months.
EtPa Thresholds
The EtPa marker provides a supporting quantitative measure. A concentration of 0.35 ng/mg (or 350 pg/mg) or greater strongly suggests chronic excessive consumption.
The test is a quantitative measure that places consumption into categories like abstinence, repeated use, or chronic excessive use. While the test is designed to capture sustained heavy use, a single, very heavy drinking episode is unlikely to exceed the 30 pg/mg chronic cutoff when diluted across the full 90-day hair segment.