Building a history of substance use: How hair testing helps

Hair tests are a reliable, fast and cost-effective way of proving drug or alcohol use.

They’re highly accurate, with a long detection window and almost impossible to cheat - and that’s not all. With the right techniques, a hair drug test can also give rich information about the nature of, and conditions for, substance use by an individual.

What additional information can hair tests provide, beyond the bare fact of whether a person used drugs or alcohol?

1. Patterns of substance use

When a person stops using drugs or alcohol, the levels of the substance in their hair drop very quickly. For this reason, it’s possible to pinpoint when a donor significantly increased or decreased their consumption levels. Laboratories typically test for changes in consumption across month-long periods.

Importantly, laboratories can only detect these changes one week after they occur. That’s because hair takes approximately seven days to grow out of the scalp. Before this point, hair samples cannot be taken.

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Traces of drugs or alcohol can also remain present in hair for up to four months after a person stopped using a substance. This is because hair exists in either a growing or resting phase. ‘Resting’ hairs are dead but still attached to the scalp – and can therefore test positive for substances that are not present in the growing hairs around them.

2. Concentration of substances

Contrary to reports by some hair test laboratories, it is not possible to judge how much of a drug an individual used.

Partly, this is because our bodies metabolise target substances at different rates. Hairs taken from one donor might exhibit very low levels of a substance and another a very high concentration – even if both individuals used the same amount of the drug at the same time. Moreover, drugs vary in their purity, in turn depositing greater or lesser concentrations of the substance in hair.

Laboratories are unable to conduct reliable tests on drug concentration in hairs – making it impossible to offer a standardised consensus on how the concentration of drugs in hair is evidence of how much a person used. These tests would be unethical – requiring test subjects to consume a controlled amount of an illicit substance. For this reason, hair tests can only indicate how much an individual’s consumption of a drug changed relative to their own historical use of the drug.

This evidence can be combined with the laboratory’s historical test results to offer some useful context for a drug test result. For example: between 2012 and 2017, Cansford Labs have tested more than 100,000 hair samples for cocaine use. We’re therefore able to tell whether an individual’s hair sample contains a concentration of cocaine that ranks in the bottom, bottom-middle, top-middle or top 25% of all samples we’ve ever tested. It is the responsibility of the person commissioning the test to make their own judgement from this information.

Such obstacles don’t exist for testing for alcohol. Laboratories have performed successful tests to understand how different volumes of alcohol affect the concentration of alcohol in the hair. The Society of Hair-Testing agrees that hair samples with less than 7 picograms per milligram of alcohol suggest the donor has abstained from using alcohol. Readings of more than 30 pg/mg indicate that the donor uses alcohol at chronic levels. As such, hair tests offer a reliable indication of how much alcohol a person consumed in a given time period.

3. Types of substance use

It is possible for hairs to test positive for cocaine or cannabis even if the donor has not used the drugs themselves. Hairs can be contaminated on their external surfaces when a person is near another using a drug – because the molecules of the substance are in the air, and thus end up deposited on the hair of people nearby.

The testing laboratory should prepare hair samples to prevent this from affecting the donor’s test result. Hairs are washed before being tested to remove external contamination. Washed hairs that still contain metabolites of a drug are evidence that the donor’s body has processed the drug – and therefore, that the donor used the drug themselves. In this way, hair tests can be used to prove whether a person used drugs or was only exposed to them.

Understanding the limits

Hair testing for drugs has rich potential for helping family lawyers and social workers understand the circumstances and nature of a person’s substance use – including patterns around use, the amount of alcohol they drank and whether or not they took substances themselves.

There is however, an important ‘but’. The individual commissioning the hair test must tell their laboratory which of the three types of contextual information above they need from their hair test. Professionals must understand the limitations around these three insights, to make successful evidence-based decisions – which could have impactful real-world consequences for the individuals and families involved.