Why Context Matters More Than Cut-Offs in Forensic Drug Testing

Cut-off levels and screening thresholds are widely used in drug and alcohol testing. When large numbers of samples must be processed efficiently, such as for workplace testing, they can be a useful tool to support analysis. However, when testing is needed for family law, social services or court-ordered cases, reports that use cut-off levels are not only insufficient but often harmful.

In complex legal cases dealing with children and families, forensic drug and alcohol testing must do more than produce a positive or negative result. Accredited testing providers must provide evidence that is thorough, meaningful and legally defensible. As outlined in caselaw over the past 16 years, this requires expert interpretation, as well as for all findings and their significance to be explained. This simply cannot be achieved through the misguided reliance on the accuracy of cut-off levels used by most companies operating within the industry as the basis for their interpretations and recommendations.

Forensic Testing Service (FTS) has long taken the position that when it comes to decisions about families, the reporting of drug and alcohol tests should be based on evidence-led forensic investigations rather than simplified reporting thresholds.

What Are Cut-Off Levels in Drug and Alcohol Testing?

Cut-off levels are predefined concentration thresholds used to classify test results as either positive or negative. When it comes to hair strand testing, most providers use cut-off levels suggested by the Society of Hair Testing (SoHT). These guidelines, which are not legally binding, were developed over 20 years ago, when hair strand testing was in its infancy, and have changed little since.

When reports are produced using the SoHT cut-off levels, results that are above the cut-off levels are reported as positive and consistent with drug use, whereas results that are at a concentration below the cut-off level are reported as negative or even not detected. This inaccurate terminology and way of reporting not only withholds potentially relevant findings from the courts but also provides a misleading depiction of the true nature of an individual’s drug exposure. SoHT cut-off levels are often used by companies who also produce certificates of analysis or simple reports, where terms like “not detected,” “positive,” and “negative”, or where someone’s usage is explained with terms “low”, “medium” or “high”, but is not contextualised.

This approach can easily lead to misinterpretation by non-specialists, a problem that the courts have recognised. Caselaw requires all findings to be fully reported and their significance explained.
For FTS, the key to reliable testing is not a single, pre-determined cut-off level, but expert interpretation of all results.

Drug testing and alcohol testing evidence in the family courts, as noted in successive rulings, is required to achieve a standard of proof known as the “balance of probabilities”. In both High Court and Court of Appeal rulings, hair strand testing has been recognised as needing interpretation by experts expressing an opinion. The expert is required to describe the process, record the results, explain their possible significance in a clear and understandable way, and to fully and faithfully report all findings.

Cut-off levels, which were not designed to answer complex forensic or legal questions, cannot achieve this. They do not account for individual biological variation or contextual factors that may influence results. It has been established that there is good reason to look at the wider evidential picture. For example, peer reviewed studies have demonstrated that, when administered an identical, controlled dose of a drug, individuals with dark brown and black hair (which contain more melanin) can incorporate up to 10 times more drug than individuals with blonde or ginger hair. Levels found in black hair from people from Asian and Afro-Caribbean backgrounds can often be significantly higher, meaning they are at disproportionately greater risk of having their child removed based on results alone.

The Dangers of Relying on SoHT Cut-Off levels

Depending on SoHT cut-off levels in drug and alcohol testing can oversimplify what is, in reality, a complex interaction between biology, behaviour and environment. Results that sit close to a cut-off level may be particularly vulnerable to misinterpretation, especially if they are treated as definitive indicators of behaviour.

A reported positive result does not automatically indicate frequent or ongoing substance use. Equally, a negative result does not necessarily demonstrate abstinence over a longer period. Cut-off levels do not distinguish between isolated use, historical use or patterns of change over time.

To demonstrate the potential for misleading results, FTS applied SoHT cut-off levels to some of our own historical data. We took 3,000 of our previous cases where we had recorded outcomes from declared usage and other forms of testing (such as nail testing), then applied SoHT cut-off levels to them. The results were troubling:

• 12% hair samples in cases not using heroin came back Positive
• 18% hair samples in cases not using cocaine came back Positive
• 22% hair samples from chronic heroin users came back Negative
• 20% hair samples from chronic cocaine users came back Negative
• 60% hair samples from chronic cannabis users came back Negative

In family law proceedings, where the court must make potentially life-changing safeguarding decisions, the high frequency of incorrect interpretations formed as a result of application of cut-off values is simply not acceptable.

Why Expert Opinion Is Essential in Drug and Alcohol Test Interpretation

Context, together with expert opinion evidence, allows test results to be interpreted accurately and fairly. Without these, even analytically sound results may be misunderstood or over relied upon.

Key contextual information includes client disclosure and case history, declared use of prescribed medications and illicit substances, and potential environmental exposure. Biological factors such as hair colour, hair colouring or thermal treatments, as well as natural variations in growth rate also influence interpretation.

Expert interpretation brings these elements together, ensuring that results are considered as part of a wider evidential picture rather than in isolation.

Screening vs Forensic Investigation

Screening tests and forensic investigations serve different purposes. Screening is designed to identify the possible presence of substances quickly and efficiently. It is not intended to provide detailed insight into patterns of use or to support complex legal decision making.

In contrast, forensic drug and alcohol testing for family court proceedings must meet the burden of proof required by the courts and be able to withstand scrutiny. This requires more than the black and white approach offered by use of a cut-off level. It requires defensible methodology, transparent reporting and expert interpretation.

This is why it is so important to instruct court-approved drug tests that properly answers the questions being asked and that supports clear and informed decision-making.

How FTS Approaches Evidence-Based Interpretation

Forensic Testing Service approaches drug and alcohol testing as an evidence-led process rather than a screening exercise. UKAS-accredited methods underpin analytical reliability, but given many accredited companies use SoHT cut-off levels, accreditation alone is not the endpoint.

Our interpretation is led by scientific expertise, not rigid and outdated thresholds. FTS has consistently advocated for moving beyond reliance on cut-offs, recognising that thresholds are tools rather than conclusions. We believe every individual case should be assessed on its own merits, with all relevant contextual information considered alongside our scientific analysis. This approach allows for things like passive contamination to be differentiated from active use, supports legal defensibility and ensures that results are meaningful and personalised to the context of each individual case.

Scientific Expertise and Sector Leadership

FTS’s approach is grounded in scientific expertise and longstanding experience in forensic toxicology. Our toxicologist-led interpretation allows for complex results to be explained clearly and proportionately, supporting courts and instructing professionals without overstating what the evidence can show. The aim is not to reach predetermined outcomes, but to assist the court in understanding the evidence before it and to make decisions that are in the best interest of the child.

Why This Matters in Family Law and Child Protection

In family law and care proceedings, the consequences of misinterpretation can be significant. Decisions about contact, supervision or the removal of a child from a parent should never rest on cut-off levels or yes/no results alone.

Proper contextualised reporting and accuracy are essential. Expert interpretation helps ensure that testing informs decision making rather than distorting it. It also protects against undue reliance on single results that may not reflect the full picture.

Drug and alcohol testing for family law cases require a careful balance between safeguarding, fairness and accuracy. Forensic testing and expert opinion evidence supports that balance.

Key Takeaways

• Cut-off levels are not appropriate for family court settings
• Nuanced approaches and expert interpretation of all results are essential
• Forensic drug and alcohol testing must be fully and faithfully explained

Need to instruct a hair strand test?

If you require guidance on appropriate drug or alcohol testing for a family law or court matter, our experienced case managers can advise on the most suitable testing approach and interpretation framework.

You can request a quote, make an instruction or speak to a member of the Forensic Testing Service team to ensure your testing meets the burden of proof required by the courts. Contact us by email: expert@forensic-testing.co.uk or call us on +44 (0)1924 480272.

Frequently Asked Questions

What are cut-off levels in drug testing?
Cut-off levels are predefined concentration thresholds used to classify test results as either positive or negative. In hair strand testing, most providers use cut-off levels suggested by the Society of Hair Testing (SoHT). Results above the cut-off are generally reported as positive, while results below are reported as negative or “not detected.”

Why aren’t SoHT cut-off levels reliable for family court cases?
SoHT guidelines were developed over 20 years ago when hair strand testing was in its infancy and have changed little since. They don’t account for individual biological variation, hair colour differences, or contextual factors that could be relevant to findings. Our analysis of 3,000 historical cases where we had recorded outcomes showed that applying SoHT cut-offs led to 12-18% false positives in non-users and 20-60% false negatives in chronic users.

How does hair colour affect drug test results?
Hair colour significantly impacts drug incorporation. Peer-reviewed studies show that individuals with dark brown or black hair (which contains more melanin) can incorporate up to 10 times more drug than those with blonde or ginger hair when given identical doses. Research has also found that people from Asian and Afro-Caribbean backgrounds may face disproportionately higher readings, placing them at a disproportionately higher risk of having their child removed based on results alone.

What’s the difference between screening tests and forensic drug testing?
Screening tests are designed to quickly identify the possible presence of substances and provide yes/no answers. Where a legally defined threshold has been established, such as in the analysis of blood alcohol concentration to determine if someone has been driving under the influence of alcohol, their use is entirely appropriate. This is because their purpose is to determine whether or not the person has broken the law. However, forensic drug testing for family courts must meet a burden of proof known as the “balance of probabilities,” requiring expert interpretation, transparent reporting, and consideration of all contextual factors rather than simple positive/negative results.

What does “expert interpretation” mean in drug and alcohol testing?
Expert interpretation and expert opinion evidence involves considering test results alongside contextual information including client disclosure, case history, prescribed medications, potential environmental exposure, biological factors (hair colour, treatments), and natural variations in hair growth rate. This ensures results are understood as part of a wider evidential picture rather than in isolation. Expert reports and witnesses will consider the client’s complete background, including their declared history of substance use, their living situation, such as if they live with or are in regular close proximity to another substance user, and any other relevant factors that could influence test results.

By doing so, the expert can provide the courts with an informed, holistic opinion on the likelihood of personal consumption versus environmental exposure. Without this contextual analysis, the court is left with an incomplete picture, making it impossible to reach a conclusion “on the balance of probabilities”, the standard of proof required in care proceedings.