Hair testing for drugs of abuse: evaluation of external cocaine contamination and risk of false positives.

In some laboratories hair testing may be the main method for the evaluation of individual's drug history, however, compelling evidence supports the possibility that the presence of a small amount of drug in hair can derive from external contamination. The aim of the present study is to verify if a single external contamination with a small amount of cocaine will last sufficiently long to make a contaminated subject indistinguishable from active users, and if normal washing practices together with the decontamination procedures are sufficient to completely remove the external contamination. The results obtained using the decontamination methods suggested in literature demonstrate that significant concentrations of cocaine (>1 ng/mg) and moderate quantities of benzoylecgonine (generally <0.5 ng/mg) are still detectable up to 10 weeks after contamination. These results question the reliability of hair testing. In fact, even using the most sophisticated decontamination procedures it is not possible to distinguish a drug-contaminated subject from an active user. Thus, while a negative result excludes both chronic use and "contact" with drugs, a positive result cannot and must not be interpreted as a sure sign of drug addiction, but should be further confirmed by urine analysis.