In this article, I try to explain what this test actually means and how to avoid making mistakes, as well as how to protect oneself against possible excesses or unjustified interpretations.
What is it used for, and when is it requested?
The test is requested to identify certain substances—usually drugs or medications—for legal purposes.
Hair is preferred to other biological fluids because it preserves evidence of exposure to these substances over a long period of time, almost like a recording of the passage of these substances from the blood into the hair (that is, exposure from within, after the substances have been taken through various routes—through the nose, lungs, injection, transdermal absorption, etc.).
What hair length is useful?
When a sample is requested, the required length should be specified, so that the person can prepare for the collection without making mistakes.
Some people, in an attempt to eliminate as many traces as possible, show up with their hair shaved or cut very short, and then allow it to grow just enough to be considered “acceptable” and suitable for sampling.
However, this can be a mistake.
If a person essentially shows up “shaved,” it is somewhat like saying that they cannot urinate when asked to provide a urine sample: the test effectively becomes “invalid,” and therefore, if the person assessing the situation were to take a strict approach—without necessarily being unfair—the result could be interpreted as presumptively positive because the person altered the conditions of the test or refused to provide the sample to be examined.
The same applies if the request specifies a particular hair length and time period: freshly cut hair may be unsuitable.
A couple of examples.
A person is asked to account for drug use during a period surrounding a road accident, or any other punishable act, in order to determine whether they were using a substance during that period, without being able to establish whether they were using it specifically on that occasion, but rather whether they were using it generally during that period.
If, knowing that they will have to appear after a certain amount of time for this purpose, the person cuts their hair beforehand, they thereby erase the “past” traces, and the recording effectively starts again from the days immediately preceding the haircut.
So, for example, suppose the hair is subsequently “clean” with regard to the most recent month. There would then be no information about the preceding period. If that was precisely the period the test was intended to cover, cutting the hair could be viewed as a deliberate attempt to erase evidence, resulting in a measurement that can no longer be reproduced.
Clearly, for these measurements to be reliable and unambiguous, they should be carried out promptly and with detailed information provided in advance. In practice, this is often not the case.
For example, if the person was not informed about the required length of the sample or about the purpose of the test, the fact that they provide hair that has recently regrown—and therefore contains information only about a recent period rather than several months earlier—should not necessarily be regarded as “suspicious” or deliberate.
What does it mean when the hair test is positive?
A hair test does not in any way allow a diagnosis of drug addiction or “pathological” use to be made. It simply detects exposure, which at most can be quantified.
In other words, it can be established that during a certain period, from one point in time to another, the person was exposed more or less regularly, and more or less heavily, to a particular substance.
If the hair is examined in different segments, a more precise chronology can be reconstructed, although at the cost of reduced sensitivity for detecting low-level use.
A positive hair test cannot in any way establish whether a person was intoxicated or impaired at a specific moment, just as a urine test cannot.
What should you do if the hair test is positive and you want to demonstrate that it is erroneous?
First of all, the test should automatically be repeated in order to minimize errors related to contextual factors or errors in the testing kit.
There can, however, be false positives—that is, positive results because the test reacts to substances other than the one that was theoretically the target of the test, whether chemically related to it or not.
Obviously, medications being taken are one possible cause of false positives.
Not all such reactions are known, so new information about previously unrecognized false positives is sometimes published over time.
It is important to know which testing kit was used, in addition to knowing which type of test was performed, which is nevertheless generally standardized.
Depending on the kit used, it is possible to better assess which false positives might be expected.
Some medications, such as opioid painkillers, can produce a positive reaction. This is “false” if the intention was to detect opioids of another type—for example, heroin—but, unless proven otherwise, it is a true positive if the intention was to detect a broader group of chemically similar opioids.
For example, if a person is abusing oxycodone or buprenorphine, the test may not allow the source or the manner of use to be distinguished. It will simply indicate that the person has taken that substance.
Clearly, to dispel doubts of this kind, a person should ideally document from the outset everything they are taking. However, many people choose not to do so because they fear negative consequences.
For example, they may be afraid of revealing that they are people with opioid addiction who are undergoing treatment, even if the treatment is successful and there is nothing to hide. Alternatively, they may worry that disclosing this information could be interpreted as an attempt to confuse the issue when, in fact, there is nothing to conceal.
Another reason is that, even when a substance is derived from a medication and taken for therapeutic purposes, a number of substances may still appear, at least on an initial test, on the list of substances that are legally problematic or prohibited.
Statements and hair testing
Many people feel obliged—or think it is a good idea—to make spontaneous statements about what they use or have used, in anticipation of subsequently having to provide a hair sample.
The best course of action is to consult your lawyer or adviser first.
For example, if a person states that they do not use any substances and have never used any, and the test subsequently reveals otherwise, this may create a negative impression, which they may then have to try to correct or clarify.
If, on the other hand, a test comes back positive but the person had already provided their own account of what they had used, this can make it more difficult for someone subsequently seeking to accuse them of something.
In other words, someone who has disclosed having used a substance, knowing that this would subsequently be detected, is not, unless there is evidence to the contrary, necessarily a person with a drug addiction—other types of evidence would be required to establish that.
By contrast, the fact that someone has denied use, and perhaps continues to deny it even in the face of a repeated test showing otherwise, may suggest that there is an addiction problem.
It is therefore important to understand that the test cannot tell us everything, but what it does tell us must have a plausible interpretation and cannot simply be “discarded” unless it can be demonstrated that the test was performed inadequately.
“Passive” exposure and passive smoking
Passive contamination is possible. However, substances originating from within the body become incorporated into hair differently from substances deposited externally.
The sample-preparation process therefore makes this type of interference generally irrelevant.
Internal contamination resulting from passive exposure can theoretically be hypothesized. However, by analogy with urine testing, the quantities tend to be low.
Therefore, when a test has a threshold or cutoff level, it is unlikely that passive exposure would make a hair test positive for a specific substance present only in minimal quantities, such as THC.
Even in experiments specifically designed to investigate this issue, the levels detected are minimal under highly unrealistic conditions, such as exposure to smoke inside a sealed vehicle for an hour.