Setting the record straight: why cut-off levels are still not the answer in testing

Many testing companies doggedly continue to advocate for Society of Hair Testing (SoHT) cut-off levels as an alleged basis for accuracy and fairness in drug and alcohol testing. FTS remains opposed to their use.

SoHT cut-off levels provide guidance on the minimum concentrations at which a substance can be deemed “positive” for use. They were established over 20 years ago, at the infancy of hair strand testing technology.

Although these cut-off levels are only optional guidelines, they have been elevated to an almost pseudo-legal requirement by many companies, who also erroneously argue that cut-offs make reports easier to interpret and defend in court.

This is despite case law explicitly outlining how drug and alcohol evidence should be instructed, presented and explained. This includes the requirement to report all findings, as well as to explain their meaning.

Many also mistakenly claim SoHT cut-off levels help distinguish routine use from one-off use or environmental/passive exposure.

The fact is that SoHT cut-offs can distort test results by overriding and disregarding other pertinent contextual information, as well as leading to the omission of relevant information from reports. Together, this can lead to misinterpretations that have serious consequences for children and parents in the family courts. This is precisely why FTS never uses them.

A flawed endorsement

A few labs are now citing a new study, which investigated how second-hand cannabis smoke exposure can lead to detectable levels of cannabinoids in hair. They claim the study underlines the importance of using SoHT cut-off levels.

In fact, this research highlights precisely the opposite: that cut-offs can falsely penalise innocent people who have only been exposed environmentally, making them appear as active users.

For the study, four individuals were placed in a “medium” sized car once a week for four consecutive weeks, during which time a cigarette containing either “high CBD/low THC” or just “high THC” was artificially smoked. The windows of the car were closed and the participants remained in the car for 15 minutes after the cigarette had extinguished.

Following the high CBD/low THC exposure, eight of the sixteen samples (50%) analysed were above the SoHT cut-off of 50 pg/mg for CBD. Following the high THC exposure, two of the samples (12.5%) were above the SoHT cut-off of 50 pg/mg for THC.

The study’s take-home message was that, even following this quite infrequent and low-level exposure, some hair samples would have been interpreted as consistent with cannabis or CBD use if strictly adhering to SoHT cut-offs.

The report authors themselves acknowledge that even following these infrequent and relatively low level exposures, CBD levels “were frequently above the SoHT cut-off for drug use,” and, given the false positives also observed following exposure to THC, that this “suggest(s) that the interpretation of hair results should be conducted with utmost caution, possibly refraining from the application of parent drug-based threshold only” (i.e. cut-offs).

The continued defence of SoHT cut-off levels as the primary tool for distinguishing active use from passive exposure is thus undermined by the very research these companies are quoting.

Why expert interpretation matters

When it comes to care proceedings, this kind of misleading reporting could contribute to a child being left in an unsafe care environment or unjustly removed from a safe one.

In either case, the study only serves to reinforce the fact that there is strong potential for a miscarriage of justice when using SoHT cut-offs.

These dangers are made even more real when they are accompanied by overly simplistic reports or certificates of analysis, where terms like “not detected,” “positive,” and “negative”, or when someone’s usage is explained with terms “low”, “medium” or “high”, but not contextualised or properly explained.

This yes/no approach easily risks misinterpretation by non-specialists, which is why UK 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.

Indeed, the paper’s online summary itself states that “cut-off concentrations for drugs in hair to assess drug use should be questioned.”

A commitment to proper reporting

While SoHT cut-offs were designed to aid interpretation, they should not be considered gospel.

Instead, each individual case should be assessed on its own merits, with all relevant contextual information considered to allow passive contamination to be differentiated from active use.

This is especially important given SoHT cut-off levels also fail to account for the radically different results that darker hair (particularly Asian and Afro-Caribbean hair within the subset of darker hair) show, leading to potential hair colour and racial bias.

FTS gives you expert interpretation

FTS rejects the idea that a single guideline level can justly account for individual variables like age, metabolism, hair type and styling, gender, and lifestyle.

Our reports include a detailed client questionnaire that factors in important contextual information about the person being tested, such as their hair colour, lifestyle and hair product use.

FTS never applies SoHT cut-off levels to our results. We report all findings, as the law requires, and provide expert interpretation to deliver results that genuinely reflect a pattern of behaviour. This way, we can do right by families.

Instruct Us

To instruct our expert services, contact us on 01924 480 272 or email expert@forensic-testing.co.uk.