The hair follicle drug test is a common screening method used by employers for pre-employment, random, or post-accident testing. This analysis detects evidence of drug use over a prolonged period, distinguishing it from tests that only look for recent use. A common question for those facing this test is whether cutting their hair will affect the result. Understanding the science of how the hair test works is necessary to determine if such preparations are effective.
Understanding How Hair Testing Works
The physiological mechanism of the hair test relies on drug metabolites becoming incorporated into the hair shaft. After a substance is consumed, the parent drug and its metabolites circulate through the bloodstream, eventually reaching the hair follicle beneath the scalp. These compounds then become permanently embedded within the hair strand as it grows outward from the root.
Hair grows at an average rate of about 0.5 inches per month. This predictable growth rate allows laboratories to establish a historical record of drug use. The standard analysis examines the segment of hair closest to the scalp, typically providing a detection window of approximately 90 days. Once the drug metabolites are trapped within the hair’s keratin matrix, they remain there indefinitely, acting as a timeline of past substance use.
The Role of Hair Length in Testing
The test is focused on the most recent growth, not the overall length of the hair. Laboratories typically require a sample of hair that is 1.5 inches in length, measured from the root end. This 1.5-inch segment corresponds to the standard 90-day detection window based on the average growth rate of scalp hair.
Cutting the ends of the hair, even significantly, will not remove the history contained in the required segment near the scalp. The collection process involves cutting a small bundle of hair, roughly the thickness of a pencil, as close to the scalp as possible. If a person cuts their hair shorter than 1.5 inches, they only reduce the total available sample size, but the remaining hair still holds the history for the time it was growing.
If the hair is cut to a length that is too short for a full 90-day analysis, the test will simply cover a shorter period. For instance, a 1-inch sample can only reflect approximately a 60-day history. Cutting the hair to an extremely short length, such as under 0.5 inches, may prevent the collection of a scalp hair sample entirely.
Does Shaving or Removing Hair Prevent Testing?
Shaving or completely removing all scalp hair before a test does not prevent the process; it only forces the collector to seek alternative samples. If the head hair is less than about 0.5 inches long, technicians are instructed to look for body hair. Body hair may be collected from the armpit, chest, leg, or face, with the collector noting the source of the sample.
The use of body hair fundamentally changes the detection window because it grows much slower and has a different growth cycle than scalp hair. A body hair sample provides a much longer, often indefinite, window of detection, sometimes covering up to 12 months. However, the highly variable growth rate makes it impossible for the laboratory to segment the sample to determine specific dates of use. Removing all hair from the body often raises immediate suspicion and changes the sample type, potentially extending the period under review.
Factors That Do Not Impact Test Results
Many common external treatments and cleaning methods do not reliably eliminate drug metabolites from the hair shaft. Standard washing, specialized shampoos, or hair sprays have little effect on the embedded drug compounds. Accredited laboratories address external contamination, such as environmental smoke, by performing a thorough chemical wash before analysis.
Chemical treatments like bleaching, dyeing, or perming can damage the hair shaft and may reduce the concentration of drug metabolites. Bleaching, in particular, can decrease drug levels by 40% to over 60% due to the chemicals damaging the hair structure. However, because the effect is inconsistent and laboratories are aware of this potential manipulation, these treatments are not a reliable way to alter a positive result.