Rethinking Cut-offs: Thoughts on The Society of Hair Testing Meeting

FTS recently attended the 29th Annual Meeting of the Society of Hair Testing (SoHT), held in London June 2025.

The SoHT is an internationally recognised organisation working to advance research, methodology and practice in hair testing for forensic and clinical toxicology.

The event brings together global forensic toxicologists to discuss developments and challenges in hair analysis.

While the conference showcased important developments in analytical techniques and interpretative frameworks, FTS remains concerned about the hair testing industry’s continued reliance on SoHT cut-off levels in reporting.

This continued reliance on long-standing SoHT guidance raises concerns for both scientific robustness and fairness in family law matters, issues we have highlighted previously.

The Problem with Cut-offs

Throughout the conference, the issue of cut-off levels, or the minimum concentrations at which a substance is deemed “positive” for use, emerged as a recurring theme.

Presenters acknowledged the variability in hair growth rates, hair colour and use of cosmetic treatment, all of which affect incorporation of drugs or alcohol in the hair.

Yet, despite such factors, SoHT guidelines continue to promote fixed cut-off levels and ways of reporting that fail to account for these.

FTS has long argued that the hair strand testing sector should move beyond binary interpretations based on cut-offs, and to report and explain all findings.

The presence of a drug or its metabolite above a laboratory’s validated limit of quantification (LOQ) should be reported transparently, regardless of whether it falls above or below a cut-off.

This approach aligns with ISO/IEC 17025 standards and supports more nuanced interpretations, as is required by the courts.

The Cut-off Paradox

One of the most revealing presentations came from a company who analysed 750 family law cases to assess differences between self-reported drug use and laboratory findings.

Their data showed that 13% of cannabis users and 7% of cocaine users received “not detected” results, despite admitting to drug use. These so-called false negatives were attributed to cosmetic hair treatments and poor drug incorporation, yet no empirical evidence was provided to support these claims.

More troubling was the company’s handling of cocaine metabolite data. They acknowledged that a metabolite concentration above 0.05 ng/mg is forensically significant, even when the parent cocaine analyte has a level below the SoHT cut-off of 0.5 ng/mL. However, they still classified such cases as negative if the cocaine level fell below the SoHT cut-off, in support of rigid adherence to cut-offs.

This contradiction of recognising the presence of a drug metabolite but then reporting “no use” is scientifically indefensible and ethically problematic, particularly in legal proceedings where such reports influence child custody decisions.

What was particularly revealing is that despite these findings, the company concluded, somewhat incongruously, by advocating for the continued application of fixed cut-off values in forensic casework.

This stance appears inconsistent with the data presented, which demonstrated the potential for cut-off values to obscure true positive cases, particularly in low-level users and in instances where metabolites provide clear evidence of recent use.

FTS believes this highlights a fundamental flaw in the current SoHT framework: it prioritises adherence to simple cut-offs over analytical rigour.

If a metabolite is detected and the client has declared use, the result should not be dismissed simply because it falls below the SoHT guideline cut-offs. This is not just a scientific issue, it’s a matter of fairness and accuracy in forensic toxicology reporting.

FTS’ own research retrospectively applying cut-off values to over 3,000 of our findings highlights how misleading they can be.

A Call for Transparent Reporting

Another presentation at the conference further underscored the inconsistencies in how laboratories apply SoHT guidelines.

An analysis of anonymised reports revealed widespread variation in interpretative language and thresholds, even among labs citing the same consensus document.

Terms like “not detected,” “positive,” and “negative” are used without context, increasing the risk of misinterpretation by non-experts such as judges, lawyers, and social workers.

As we argue here, the law is clear on the need for hair strand testing to be seen as expert opinion evidence, and for all findings to be reported and explained.

FTS supports the call for laboratories to report all values above their LOQ and to clearly state the limitations of their methods.

We also support recommendations to avoid vague terminology and instead use standardised, unambiguous language that reflects the complexity of forensic toxicology.

Evidence Over Shortcuts

The conference also featured compelling data on THC-COOH, a metabolite of cannabis. A study demonstrated a strong correlation between THC-COOH levels and self-reported cannabis use, reinforcing its value as a biomarker.

The study’s findings showed that THC-COOH concentrations were not significantly affected by hair colour or cosmetic treatment, an argument often used to justify dismissing low-level results.

This reinforces FTS’s position that forensic interpretation should be grounded in empirical evidence, not the SoHT’s outdated guideline cut-off levels.

If a validated method detects a drug or metabolite, that information should be reported and contextualised, not discarded because it doesn’t meet a suggested cut-off. That is indeed what the caselaw requires.

Toward a More Responsible Sector

The final session of SoHT 2025 acknowledged the need to critically review and harmonise cut-off values, and to develop interpretative frameworks that reflect analytical uncertainty. FTS welcomes this recognition but urges the Society to go further.

We call for:

• Transparent reporting of all results above LOQ, regardless of SoHT guideline cut-off levels.
• Contextual interpretation that considers declarations about use and lifestyle made by the client, as well as metabolite presence and method limitations.
• Standardised language that avoids misleading terms like “positive” or “negative”.

As forensic toxicologists, we have a responsibility to ensure that our findings are scientifically sound, legally defensible, are clearly communicated and understandable to the courts.

The continued reliance on SoHT cut-off levels, particularly when they contradict analytical evidence, risks undermining public trust and judicial fairness.

FTS remains committed to advancing a more transparent, evidence-based approach to hair testing.

We urge our colleagues and the SoHT to join us in rethinking the use of cut-offs in forensic toxicology.

To benefit from FTS, Instruct an Expert services, contact us on 01924 480 272 or email expert@forensic-testing.co.uk