Practice Reflection
Hair strand testing comes up regularly in the cases I assess. It is one of those pieces of evidence that everyone treats as definitive, but in practice it is more nuanced than most people realise. I have seen positive results where the parent was genuinely abstinent, and negative results where every other piece of evidence pointed to ongoing use. Understanding what these tests can and cannot tell us is important for anyone involved in proceedings where substance misuse is a concern.
How hair strand testing works
The basic science is straightforward. When a person consumes alcohol or drugs, metabolites enter the bloodstream and are incorporated into the hair shaft as it grows. Head hair grows at roughly one centimetre per month. A laboratory can cut a strand of hair into segments and test each one, giving a month-by-month picture of substance use over the length of hair available. A standard sample of three centimetres gives a three-month window. Longer hair gives a longer window, though the further back you go, the less precise the picture becomes.
For alcohol, the test measures two markers: ethyl glucuronide (EtG) and fatty acid ethyl esters (FAEE). EtG is water-soluble, which matters because it can be affected by hair washing, colouring, and other treatments. FAEE is lipophilic, meaning it sits in the oily layer of the hair and is less affected by external treatments but more susceptible to contamination from alcohol-containing hair products. The laboratory will usually test for both and interpret the results together.
For drugs, the test detects metabolites specific to each substance. Cannabis, cocaine, opiates, amphetamines, and benzodiazepines can all be detected through hair analysis. The detection thresholds are set by the Society of Hair Testing, and results are typically reported as either above or below the relevant cut-off, sometimes with a quantitative level as well.
What comes up in practice
In my experience, the complications with hair strand testing fall into several recurring patterns.
The first is hair treatments. I have assessed cases where a parent's hair was bleached, dyed, or chemically straightened between the period of alleged use and the date of testing. Bleaching in particular can strip metabolites from the hair shaft, producing a lower reading than the actual level of consumption would warrant. This does not mean the test is useless, but it means the result needs to be interpreted with caution. When I see a negative or low-level result in combination with evidence that the person has recently changed their hair, I note both facts and explain the limitation to the court. A negative test is not the same as proof of abstinence if the hair has been chemically treated.
The second issue is the length of available hair. Some people keep their hair very short. Others shave their heads entirely in the period before testing, which means there is no head hair available to test. Body hair can be used instead, from the arms, legs, or chest, but body hair grows at a different and less predictable rate than head hair. It cannot be segmented into neat monthly periods. A body hair sample gives a broader overview, typically covering several months, but without the chronological precision that head hair provides. When someone who knows they are facing a hair strand test arrives at the appointment with a freshly shaved head, the court is entitled to draw its own inferences about the timing.
The third complication is environmental contamination. This argument comes up regularly, particularly with cocaine. The defence is that the person was not using the substance themselves but was present in an environment where others were using it, and the metabolites entered the hair externally. Laboratories address this by washing the hair sample before testing and by looking for the presence of specific metabolites that are only produced by the body's internal processing of the substance. A positive result for benzoylecgonine, the primary metabolite of cocaine, is harder to explain through environmental contamination than a positive result for cocaine itself. But the argument persists, and assessors need to understand the science well enough to evaluate it.
What a positive result does and does not prove
A positive hair strand test tells me that a person's body metabolised a particular substance during the detection window. It does not tell me how much they consumed, how often, whether they were impaired, or whether their substance use affected their parenting. Those are all separate questions, and they are the questions that actually matter for the assessment.
I have assessed parents with positive results for alcohol whose drinking was within social norms and posed no risk to their children whatsoever. I have also assessed parents with positive results at chronic excessive levels whose alcohol use was fundamentally incompatible with safe parenting. The number on the laboratory report is a starting point, not a conclusion. What matters is the pattern, the context, the person's insight into their own use, and the impact on the child.
When I receive a positive result, I explore it in interview. What does the person say about their use during that period? Does their account align with the test results, or does it contradict them? If there is a discrepancy, how do they explain it? Honesty matters here. A parent who says "yes, I was drinking heavily during that period, and this is what I've done about it" gives me far more to work with than a parent who denies everything in the face of a positive result. Denial in the face of clear evidence is itself a risk factor, because it suggests a lack of insight that makes change less likely.
What a negative result does and does not prove
A negative result is similarly limited. It tells me that no metabolites above the detection threshold were found in the sample tested. It does not prove that the person was completely abstinent. Detection thresholds exist for a reason; occasional or very low-level use may fall below the cut-off. Hair treatments can reduce metabolite levels. And the test only covers the period represented by the hair sample. A person could have been using heavily before the window and stopped just long enough for the tested section to come back clear.
This is why I never rely on a hair strand test in isolation. I look at the test result alongside everything else: self-reported substance use history, information from the GP, substance misuse service records, observations during home visits, the accounts of family members and professionals who know the person, and the child's own experience and presentation. A negative test result that is consistent with everything else in the evidence base gives me reasonable confidence. A negative test result that sits at odds with multiple other sources of information raises questions rather than answering them.
How I use the results in assessment
In my risk assessments, hair strand test results are one piece of evidence among many. They are important, but they are never the whole picture. The court needs to know whether a parent's substance use poses a risk to their child, and answering that question requires me to look at the pattern of use over time, the parent's willingness to engage with treatment, the support network available, and the specific vulnerabilities of the child involved.
A parent who has a history of problematic drinking, a positive hair strand test showing chronic excessive use, no engagement with alcohol services, and a young baby in their care presents a very different risk profile from a parent with the same test result who has been attending a programme, has a supportive partner, and whose older child is thriving developmentally. The test result is the same in both cases. The analysis is entirely different.
I explain all of this in my reports because the court deserves to understand the strengths and limitations of the evidence it is relying on. A hair strand test is a useful forensic tool. It is not a crystal ball, and treating it as one leads to poor decision-making in both directions.
If substance misuse is a concern in your case and you want to understand how it fits into the broader assessment process, my guide on risk assessment in family court explains the framework I use.