Hair analysis for cocaine continues to be a valuable tool in forensic and clinical toxicology.

Recently, LeBeau and Montgomery (1) informed readers that the FBI Laboratory decided to suspend cocaine analysis in hair except children’s hair. This decision was explained by newer research findings, according to which the criteria of the Substance Abuse and Mental Health Services Administration (SAMSHA) for a hair sample positive for cocaine (2) can also be met in contaminated hair specimen (3,4). These criteria, which are generally accepted for forensic purposes, consist of a cocaine concentration ≥ 500 pg/mg and either a benzoylecgonine concentration ≥ 50 pg/mg plus a benzoylecgonine-to-cocaine ratio ≥ 0.05 or, alternatively, a concentration ≥ 50 pg/mg of the non-hydrolytic cocaine metabolites norcocaine or cocaethylene. In principle, cocaethylene could be expected only if cocaine is consumed in combination with alcohol. The harmful effects of cocaine use on physical and mental health, social life, and criminal behavior are undisputed. Hair analysis for cocaine proved to be an essential and very helpful tool to obtain evidence about the exposure of this drug in cases of driving ability examination, workplace testing, drug trafficking, criminal liability, child custody, or drug use during pregnancy. The advantage of the sample material hair in comparison to blood, urine, sweat, or oral fluid is the extended time window, allowing the retrospective detection of chronic consumption up to years. Generally, exposure can mean consumption or external contamination. The debate about the possibility of distinguishing between them is not new (5,6) and has led to criteria based on metabolite concentrations on one hand (2) and specific washing procedures on the other (7). Indeed, the very thorough experimental investigations by Ropero-Miller and Stout (4) show that external contamination of hair with street cocaine under certain conditions can lead to deposition of cocaine, benzoylecgonine, norcocaine, and cocaethylene in concentrations and concentration ratios that fulfill the aforementioned criteria and cannot be discriminated from incorporation after drug use even by extensive decontamination and washing procedures of the specimen. Benzoylecgonine as a hydrolysis product of cocaine is present in various concentrations in street cocaine and, in addition to that, is slowly formed from cocaine within the hair samples or during sample preparation. Norcocaine is a minor natural concomitant in coca leaves (8) but is formed mainly by treatment with KMnO4 during illicit cocaine manufacturing (9). Cocaethylene arises from transesterification of cocaine in a newer clandestine purification procedure of crude cocaine base with ethanol (10). According to the results of Ropero-Miller and Stout (4), the usual content of both non-hydrolytic metabolites in street cocaine from these sources is rather low (not detected to 0.33% and not detected to 0.012%) but may rise up to 8.7% and 2.3%, respectively, in exceptional powder specimens that were selected for their contamination experiments. In summary, until now there has been no unambiguous analytical proof for cocaine consumption as the reason of a cocaine positive hair sample, and it remains a question of probability whether an external contamination could lead to a positive result. This probability is relatively high for persons who are professionally or illegally in a steady close contact to the drug, such as narcotic officers and subjects involved in illicit cocaine manufacturing or distribution. However, it is not very probable that the contaminating drug contains high concentrations of norcocaine and cocaethylene, and it is quite unlikely that an innocent citizen in his daily environment might contaminate his hair to such an extent leading to a cocaine-positive result according to the criteria. A possible approach to estimate the likelihood of a hair sample belonging to a cocaine user or be a contaminated sample of a non-drug user is the statistical discrimination analysis described by Hoelzle et al. (11) with additional inclusion of ecgonine methyl ester, ecgonine, and anhydroecgonine methyl ester. First of all, the detection of cocaine in hair means that the person had contact with cocaine. This is confirmed by the detection of benzoylecgonine, norcocaine, and cocaethylene. If the SAMSHA criteria is fulfilled, there is a high probability but no final proof that the drug was consumed. The newer results show once more that in hair analysis, criteria must not schematically and uncritically be applied but should always be questioned in context of the case history and other findings. In case of doubt, additional evidence is required. This could be provided, for example, by additional investigation of non-head hair, which is much less prone to external contamination.