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.