A hair follicle drug test (HFT) detects past drug use by analyzing the hair shaft for embedded substances. This testing method is utilized for its long detection window, which can span several months, offering a history of consumption rather than just recent use. Drug metabolites, byproducts created when the body processes a substance, become permanently sealed within the hair structure as it grows. Since the test relies on the presence of these substances within the existing hair, whether a newly grown strand will be “drug-free” depends on when the last use occurred and how quickly the hair grows.
How Drug Metabolites Enter the Hair
When a drug is consumed, it is broken down into metabolites. These drugs and their metabolites circulate throughout the body via the bloodstream, reaching the small blood vessels connected to the hair papilla at the base of the follicle. The hair follicle is biologically active, constantly drawing nutrients from the blood supply to fuel hair growth.
As the hair shaft forms, circulating drugs and their metabolites transfer from the blood vessels and become incorporated into the structure of the growing hair. These substances bind to the protein and melanin matrix, trapping them in the keratinized hair strand. The evidence of drug use is sealed inside the hair as it emerges from the scalp. The hair acts as a chronological log of substance use, with the oldest evidence being at the tip and the newest near the root.
Hair Growth Rate and Detection Timelines
The standard timeline for hair drug testing depends directly on the average rate of head hair growth. Head hair grows at a rate of approximately one-half inch (1.3 cm) per month. Testing laboratories rely on this consistent rate to establish a detection window for drug use.
The standard sample collected for a hair drug test is 1.5 inches of hair, cut close to the scalp. This length represents a period of about 90 days, or three months, of drug history. Metabolites must first grow out of the scalp to appear in the sample, which typically takes about 5 to 10 days from the time of consumption. The length of the hair sample analyzed determines the time frame of drug use that can be detected.
The Effect of Shaving on New Hair Growth
Hair that grows after drug use has completely ceased will indeed be free of drug metabolites, as the new hair is formed from a bloodstream that no longer contains the substances. The drug evidence is contained in the existing hair shaft, not stored in the hair follicle itself. However, an immediate test after shaving will not provide a clean result, because the hair must first grow out of the scalp for a sample to be collected.
If head hair is shaved completely, the test administrator cannot collect the standard 1.5-inch sample needed to establish a 90-day history. Testing protocols typically require a minimum hair length of about one-half inch to take a sample. Shaving the head may simply prompt the test administrator to either defer the test until the hair has regrown to a sufficient length, or, more commonly, to move on to an alternative collection method. The act of shaving does not erase the evidence; it simply removes the available medium for the standard test.
Alternative Specimen Collection When Head Hair is Absent
When an individual has insufficient head hair, defined as hair shorter than approximately a half-inch, testing protocols allow for the collection of body hair as an alternative sample. Body hair may be collected from the chest, underarm, leg, or face, with the order of preference often specified by the testing agency. It is generally acceptable to mix hair from different body locations to obtain the necessary sample size, which is collected by weight rather than length.
Body hair grows much slower than head hair and often stops growing at a certain length, meaning it cannot be used to determine a specific timeline of drug use. Because of this slower growth and longer dormancy period, a body hair sample provides a much longer, less specific detection window, often covering up to 12 months of drug history. If a person has no hair available anywhere on the body, the hair test cannot be administered. Testing may then be switched to a method with a shorter detection window, such as urine or saliva.