Despite having been extensively discussed over the last decade, the differentiation between systemic exposure and external contamination still continues to be one of the limitations of hair testing for drugs. For this reason, we consider it worthwhile to re-state some basic principles in this short review. Various studies investigating a diversity of wash protocols, most using artificially contaminated hair with cocaine, have been valuable in evaluating wash efficacy and in understanding the incorporation of drugs in hair. However, assessments of wash efficacy made with real hair samples, as opposed to artificially contaminated samples, provide a different perspective, and demonstrate how rarely external contamination affects the interpretation of results. Data from a large number of hair samples from crack cocaine users, confirmed the usefulness of our protocol to remove most of the externally deposited cocaine. The data showed that hair levels of cocaine and benzoylecgonine in crack cocaine users were overall high with ratio of benzoylecgonine to cocaine in all samples above 0.1. The wash residue concentrations of cocaine ranged from not detected to 21ng/mg with a median of 0.5ng/mg. Cocaine was detected in the wash residue in 105 out of 138 samples. The wash to hair cocaine ratio ranged from not detected to 0.36 with a median of 0.02. The wash to hair cocaine ratios were below 0.07 in 133 cases. The five cases that produced wash to hair ratios above 0.1, one sample was at 0.11, three at 0.13 and one at 0.36, possibly because these cases were at the lower end of cocaine levels, however, we could not rule out that the hair was contaminated. Whilst it is not possible to differentiate between the drug extracted from the hair and the drug attached to the outside of the hair, we can compare levels of drug in the wash residue with levels detected in the hair sample. In addition, further diagnostic criteria must be applied to minimise potential misdiagnosis of external contamination. When drugs are detected in hair, individuals have clearly been in an environment where drugs are present, but it is only on rare occasions that it is unclear whether this is the result of drug use or of external contamination, and, in those cases, the results of testing need to be interpreted in the light of corroborating evidence from clinical data or social context.
Keywords: Drugs; External contamination; Hair testing.
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