Differentiation between drug use and environmental contamination when testing for drugs in hair

The differentiation between systemic exposure and external contamination for certain drug groups has been frequently referred to as one of the limitations of in drug testing in hair. When hair samples are used, three steps are usually employed in order to minimise the possibility of external contamination causing a misinterpretation. The first consists of decontaminating hair samples by washing the hair before analysis, the second is the detection of the relevant metabolites in the hair samples and the third is the use of cut-off levels. Difficulty in the interpretation arises when metabolites are not detected either due to external contamination of the hair or low doses of the drugs used. A wash protocol needs to be practical and ideally remove any drug deposited on the external portion of the hair.We propose an additional step that helps considerably in the interpretation of the results with the aim to establish a consensus: the analysis of the wash residue (W) and its comparison with the levels detected in hair (H). The wash residue is the remainder of a quick wash with methanol which is dried and reconstituted in buffer before analysis. The detection of small quantities of analytes that are not susceptible to external contamination in the wash residue, such as metabolites or drugs such as dihydrocodeine, indicates that the washing procedure is in fact able to remove drugs from the hair shaft. Where the W/H ratio is less then 0.1 or null, it would tend to indicate drug use as opposed to environmental contamination. Where the W/H ratio is above 0.1 but less than 0.5, it is likely to indicate possible use possibly combined with a level of external contamination. A W/H ratio greater than 0.5 is likely to indicate that the source of most of the drug in the wash residue is from external contamination. In this last case, the source of levels detected in the hair is questionable, as it is not possible to be absolutely sure that all external contamination was removed, and so use cannot be confirmed.Two hundred and sixteen hair samples from a population where external contamination could be expected (Police Investigations on drug related cases) and their wash residue were analysed. The W/H ratios of 891 results were evaluated over 13 analytes. Between 74 and 100% of the analytes studied produced W/H ratios less than 0.5, in particular in cannabis (93%) and cocaine (95%), where external contamination is more likely because of the way the drug is used.The data do show that while it is very important to always be aware of alternative explanations for test results, the likelihood of external contamination confounding the interpretation of hair tests can be reduced to manageable proportions.