Hair follicle testing assesses an individual’s substance use history over an extended period, offering a distinct advantage over blood or urine tests that only capture recent consumption. This method analyzes a hair sample for specific biomarkers trapped in the hair shaft as it grows. For alcohol, the test determines a pattern of heavy or chronic excessive use, rather than confirming recent intoxication. The analysis focuses on metabolites produced after alcohol consumption, providing a retrospective view of drinking habits.
The Specific Alcohol Markers Used
Laboratories analyze two main classes of direct biomarkers to detect alcohol consumption in hair: Ethyl Glucuronide (EtG) and Fatty Acid Ethyl Esters (FAEEs). Both markers are produced only when ethanol, the alcohol found in beverages, has been consumed. The presence of these metabolites in the hair serves as objective evidence of alcohol ingestion.
Ethyl Glucuronide (EtG) is a water-soluble metabolite of ethanol, primarily formed in the liver. EtG is incorporated into the hair shaft mainly through the bloodstream via the hair root and through sweat. Since its formation is directly linked to the body’s processing of alcohol, EtG is considered a robust indicator of consumption.
Fatty Acid Ethyl Esters (FAEEs) are a group of fat-soluble compounds, such as ethyl palmitate, formed when ethanol reacts with fatty acids. These markers enter the hair mostly through the sebum, the oily substance secreted by glands on the scalp. Laboratories often test for both EtG and FAEEs to increase interpretation accuracy, as external factors affect each marker differently.
Standard Detection Window and Methodology
The standard hair follicle test provides insight into alcohol consumption over approximately 90 days. This timeframe is based on the average growth rate of human head hair, which is about one centimeter per month. To cover the 90-day window, a hair sample approximately 1.5 inches (3.9 cm) in length is cut close to the scalp.
This proximal segment of hair, closest to the root, represents the most recent three months of growth and is the section analyzed. If a person’s scalp hair is too short or unavailable, body hair may be collected instead. However, body hair growth rates are more variable and slower than head hair, meaning the results represent a longer and less precise time window. Hair is not typically segmented for alcohol testing, providing an overview of the entire period rather than month-by-month analysis.
Consumption Levels and Testing Thresholds
A hair test is designed to distinguish between low-level social drinking and a pattern of chronic excessive alcohol consumption. The interpretation relies on established quantitative thresholds for the concentration of the EtG and FAEE markers, measured in picograms per milligram (pg/mg). Chronic excessive consumption is defined as an average intake of 60 grams or more of pure ethanol per day over several months, equivalent to four to six standard drinks daily.
For the EtG marker, the Society of Hair Testing (SoHT) and international guidelines established multiple cut-off levels. A concentration of EtG at or below 5 pg/mg is consistent with abstinence or very rare, low-level consumption. A result greater than 5 pg/mg but less than 30 pg/mg suggests repeated alcohol consumption, indicating regular but not necessarily excessive use.
The concentration threshold associated with chronic excessive use is the high positive level: EtG at or above 30 pg/mg. This concentration suggests chronic excessive alcohol consumption throughout the 90-day testing period. For FAEEs, the corresponding threshold for chronic excessive consumption is set at 350 pg/mg or higher.
Individual metabolism and hair properties influence how much of the biomarker is incorporated, meaning results are not a perfect measure of absolute volume. Some studies show that consuming as little as 21.5 grams of alcohol per day, which is below the chronic excessive threshold, could result in an EtG level above 30 pg/mg for some individuals. The test results reflect the cumulative volume necessary to sustain marker levels above the chronic use threshold over the three-month detection window.
Variables That Influence Hair Test Outcomes
Several factors unrelated to drinking habits can alter the concentration of alcohol biomarkers in a hair sample. Cosmetic treatments, such as bleaching, dyeing, and perming, can significantly reduce marker levels. This effect is more pronounced for water-soluble EtG, potentially leading to a false-negative result or an underestimation of consumption.
External environmental exposure also influences the test results, particularly for FAEEs. Alcohol-containing products, such as certain hairsprays, lotions, or even hand sanitizers, can be absorbed by the hair and artificially increase FAEE levels. EtG is less susceptible to external contamination and is considered a more reliable marker in these situations.
Individual biological variations, such as hair color, also play a role in the final outcome. The presence of melanin, the pigment that determines hair color, influences how substances are incorporated and retained in the hair shaft. Laboratories must take a detailed history of a donor’s cosmetic and product use to ensure accurate interpretation of the final biomarker concentrations.