Why we think cut-off levels are inappropriate for the family courts

Why we think cut-off levels are inappropriate for the family courts

Recent debate on the limitations of The Society of Hair Testing (SoHT) cut-off levels has prompted concerning remarks from industry members who support their use.

Forensic Testing Service (FTS) do not apply SoHT cut-offs to hair strand test results for children or adults. In our experience it is not appropriate, scientifically accurate or honest to filter evidence in this way, particularly for use in such sensitive matters as childcare proceedings. What follows is the first in a series of articles explaining our position.

Watch ‘Why cut-off levels in hair strand testing fail’ with Paul Hunter, Technical Director and Sarah Branson, Barrister at Coram Chambers

Cut-off levels should not be used as part of hair testing evidence in civil legal proceedings

High Court guidance from 2017 states that in childcare proceedings, all hair drug test results should be reported, regardless of cut-off levels. This should be the standard for all cases where evidence is provided in courts.

In 2012, when cut-off concentrations for specific drugs were first included in the SoHT’s guidelines, they were stated as being the recommended levels to “enable identification of chronic drug use”. Although the concentrations of the cut-offs have not changed since then, the most recent 2023 guidance from the SoHT has removed the word “chronic” and now simply states that the “cut-off is the value that enables identification of drug users”. It is our view, and that of the High Court, that this is an inappropriate oversimplification of the complex matter of interpretation of drug concentrations within hair fibres.

Whilst the SoHT cut-off concentration levels provide useful guidelines to aid interpretation, numerous factors influence the measured drug concentration within an individual’s hair, and as such each case is unique. It therefore makes little sense to apply a set threshold concentration level as the single determining factor to decide if someone is a drug user. Indeed, we believe that to do so is both scientifically and ethically unjustifiable.

Cut-off levels and limits of detection: a combination of terms

Other laboratories frequently combine SoHT cut-off levels with the ‘limits of detection’ in the way in which they report on results of hair strand testing. However, there is an important distinction between the two terms, which can significantly influence how the results should be both interpreted and reported.

A ‘cut-off level’ is an empirically defined benchmark for interpretation which can be set independently of whether a drug has been detected or not. On the other hand, the ‘limit of detection’ is the concentration at which a drug can be reliably detected but not quantified and represents the inherent sensitivity of the analytical method used.

However, many laboratories do not highlight this distinction in the way that they report results, with drug concentrations that are above the limit of detection but below the cut-off level reported as “Not Detected” or “Negative”, giving the impression to the reader that the drug was not present. This issue was highlighted in the case of Re:H, where drug levels below the cut-off were reported as “Not Detected”, only for the full results to be court ordered. When this happened, it was revealed levels were in fact detected. FTS agrees with the courts that no company should knowingly withhold potentially relevant findings from court proceedings.

Cut-off accuracy

FTS has collected data that shows how misleading SoHT cut-off levels can be. From over 3,000 declared frequent users of cannabis, some 85% of the cohort were found to have a level of delta-9-THC (the principal psychoactive constituent of cannabis) below the SoHT cut-off. Among 1,000 declared users of cocaine, 41% had cocaine levels below the SoHT cut-off level.

As the graph above illustrates, the majority of cannabis users who had declared consistent use had THC levels below the 50pg/mg SoHT cut-off in every hair section. In addition, as THC is somewhat atypical in that, despite consistent levels of use, the concentration increases over time, it is quite common to find situations where the older hair sections are above the cut-off threshold and the younger hair sections are below the threshold. When applying cut-offs, this would be interpreted as being consistent with decreasing use, when in reality the frequency of cannabis use has remained unchanged.

This shows that applying a cut-off in cannabis testing undoubtedly leads to misreporting results as “negative” or “decreasing use”, when neither is accurate. The data highlights the flawed confidence attributed by other laboratories in the hair testing sector to the accuracy of the SoHT cut-offs.

When examining individual cases, there are many known factors that can significantly alter the levels of drugs detected in hair fibres. For example, it is known that the melanin content of hair can drastically impact the propensity of certain drugs to incorporate into hair fibres. We have previously highlighted our concerns about the potential for SoHT cut-offs to discriminate against individuals with darker hair, specifically those from Afro-Caribbean, African or Asian backgrounds.

In addition, cut-offs are applied to each section equally, disregarding the hair’s age or unique analyte properties that can cause substance accumulation/deterioration over time, even though it is known that longer hair becomes more porous and damaged over time. Some metabolites, such as benzoylecgonine, can increase, due to in-situ formation, while drugs that are not incorporated via the bloodstream, such as THC, tend to increase in concentration the further the hair gets from the scalp.

In short, FTS does not support the use of cut-offs without acknowledging their intended purpose and limitations. The SoHT guidance on cut-offs, together with the reports that adhere to them, are something we should leave in the past.

Our commitment to accurate, transparent interpretation

FTS was set up 15 years ago with the sole purpose of supporting drug and alcohol testing and expert interpretation for family court proceedings. Our goal is to achieve the best possible result for children and families. It is increasingly clear that outdated SoHT cut-off levels do not support this goal.

If you have any questions about our methods or would like to instruct us on a case, please contact the FTS Customer Support Team on 01924 480272.