Hair forensics could yield false positives for cocaine use

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One of the most useful and standard diagnostic tools for recent and long-term drug abuse has been hair analysis, wherein the forensic experts identify and quantify the drugs along the length of the hair shaft from scalp hair. However, there is a chance of external contamination of a non-user’s hair, if they were present in the same room as the junkies which could implicate them. To avoid this, testers wash hair samples to get rid of any potential external contaminants.

Testing a person’s locks for evidence of drug abuse has several advantages over urine and blood analyses. Sampling is simple and non-invasive. And a person’s hair provides a record of use over a long period, whereas body fluids can only provide a short-term picture.

Recently, some experts questioned whether the current method of washing away external contaminants from samples might affect test results. A team of researchers have now confirmed in a study published in the journal Analytical Chemistry that for cocaine detection, a pretreatment step can cause the drug on the outside of a hair shaft to wash into it and potentially lead to falsely identifying someone as a drug user.

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A close-up of cocaine distribution in washed hair strands that were either contaminated with the drug (left two columns) or came from a user shows that telling the difference is difficult. Credit: American Chemical Society

In this study, lead researcher Eva Cuypers and colleagues followed standard procedures to wash off cocaine from non-users’ hair. By examining the cross-sections of these samples, they found that the drug had migrated into the hair shafts thus suggesting that current methods to decontaminate hair can have the opposite effect.

The researchers conclude that this new insight could have implications for future hair analyses in forensics.

This study was funded by the Netherlands Organization for Scientific Research (NWO) and Fonds Wetenschappelijk Onderzoek.

Credit: American Chemical Society