A hair follicle drug test (HFT) is a screening method that analyzes a hair sample to detect the presence of drug use over an extended period. The test relies on the biological process of drug incorporation into the growing hair shaft. While the scalp is the preferred collection site, the primary concern for many people is whether the test can still be performed if head hair is minimal or completely absent. The answer is definitively yes, and understanding the science behind the test reveals why sample collection is possible even for those who are bald.
The Science of Drug Incorporation into Hair
Drug metabolites enter the hair shaft after a substance is consumed and circulates throughout the body. Once a drug is absorbed, it travels through the bloodstream, reaching the hair follicle at the root of the hair. The tiny blood vessels connected to the follicle deliver the drug and its byproducts, called metabolites, into the hair matrix, where the hair is formed.
As the hair grows out from the follicle, these drug markers become permanently encased within the keratinized structure of the hair strand. This mechanism essentially creates a biological record of substance use over time. Hair grows at a predictable rate, typically about one-half inch per month, which allows the laboratory to estimate the timeline of drug exposure based on where the metabolites are found along the hair shaft.
Testing When Head Hair is Absent
Being bald, having a shaved head, or simply having hair that is too short does not automatically prevent a hair drug test from being administered. The standard collection procedure requires a sample of approximately 100 to 120 strands of hair, or a 100-milligram sample, cut as close to the scalp as possible. This is usually taken from the crown of the head.
If the individual cannot provide the minimum required length of head hair, which is generally 1.5 inches for the standard detection window, the collector will look for an alternative site. The goal is to obtain a sufficient quantity of hair rather than a specific length, although length is important for determining the detection window. The possibility of testing always remains, even if the primary collection site is unavailable.
Using Body Hair for Collection
If head hair is insufficient or non-existent, the collector will use body hair as an alternative sample source. Common sites for body hair collection include the chest, underarm, legs, arms, and face, such as a beard or mustache. Unlike head hair, which is typically cut to a specific length, body hair is collected by weight to ensure the lab has enough material for analysis.
The analysis of body hair introduces a different detection timeframe because body hair has a slower and less predictable growth cycle than head hair. Head hair is often segmented to provide a month-by-month analysis of drug use, but body hair cannot be segmented due to its variable growth and longer resting phases. Consequently, a body hair sample typically provides a general overview of drug use over a much longer period, sometimes up to 12 months, rather than a precise 90-day history.
The Standard Detection Window
The typical hair follicle test using scalp hair focuses on detecting drug use over the past 90 days. This timeframe is based on the average growth rate of head hair, where a 1.5-inch segment cut closest to the scalp represents about three months of history. The test is highly effective for revealing a pattern of historical use.
It is important to note that the test cannot detect very recent drug use, typically within the last 7 to 10 days. This is because the hair containing the drug metabolites must first grow out beyond the surface of the scalp to be included in the collected sample. The hair test is therefore used to identify chronic or repeated use, rather than current intoxication.