Hair Strand Testing for Cocaine
Long awaited guidance has been provided from the Judgment in H (A Child: Hair Strand Testing) [2017] relating to the validity of hair strand testing for cocaine.

The case related to a baby removed at birth from her mother, with a history of drug use, because of a positive hair strand result.  At the Final Hearing mother denied drug use.  The hair strand tests taken over a 2 year period showed low-level cocaine use for some of the time.  It considered the reliability of testing for cocaine where results are in the ‘low range’, and where contamination, for example through social contact with users, may have been an issue.   It was argued that as abstinence could not be evidenced there was a risk of the child being around future drug use as had been the case in the past.  In addition it was argued that the results showed mother had been untruthful and called her honesty into question.

The difficulties in interpreting results accurately from reports in their current form caused Peter Jackson J to include in his judgment guidance intended to improve the way in which such evidence is presented and understood.

Peter Jackson J identified the topics in issue as including:
  1. The significance, if any, of the variability of the results as between the different laboratories.  
  2. The nature and significance of industry guidelines.
  3. The significance of findings of cocaine or its metabolites below cut-off levels.
  4. The significance of the comparison between wash samples and test samples.
The reader must take care to understand what is being read, and not jump to a conclusion about drug or alcohol use without understanding the significance of the data and its place in the overall evidence.  The judgment concludes that the variability of findings from hair strand testing does not call in to question the underlying science but underlines the need to treat numerical data with proper caution.

These tests can provide important information, but in order for that to be of real use, the expert must (a) describe the process, (b) record the results, and (c) explain their possible significance, all in a way that can be clearly understood by those likely to rely on the information.  If these important requirements are not met, there is a risk that the results will acquire a pseudo-certainty, particularly because (unlike most other forms of information in this field) they appear as numbers.