Blog - Hair testing for drugs of abuse

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Workplace drug testing is most commonly based on oral fluid or urine samples in which the presence of drug (or metabolites) above permitted levels can be determined by on-site devices (usually immunoassay). Laboratory confirmation tests may then accurately measure  concentrations of specific drug identities. These approaches are generally able to measure recent consumption of drugs – usually in the past few days, although the carboxylic acid metabolite of the psychoactive component of cannabis (THC; tetrahydrocannabinol) may persist for much longer. In some cases, there is a need to determine the longer-term use of illicit drugs.

Hair testing for drugs is a strategy that allows the measurement of drugs that were consumed weeks to months prior to sample collection. It is sometimes mistakenly called ‘hair follicle testing’. However, the hair follicle itself is not tested, but rather the hair shaft. The confusion may be because of the use of hair follicles for DNA testing, where the hair follicle, pulled intact from the scalp, includes a small amount of cellular tissue from which DNA can be extracted.

Head hair grows at an average rate of one centimetre per month. This means that the hair up to one centimetre from the scalp grew in the past month. Hair 3 to 4 centimetres from the scalp grew four months ago. Drugs that were consumed during those time intervals will have been deposited into the structure of the hair shaft. By cutting the available hair into segments corresponding to the growth period and analysing each segment allows the history of drug consumption to be examined.

There are limitations to the use of hair tests for drug analysis.

  1. Hair must be available for sampling corresponding to the period of interest. Where there is little head hair, due to baldness or shaving, this may not be possible. While hair from other locations (facial hair, armpit etc) may be substituted, differences in growth rate may reduce the value of the test.
  2. Hair is washed prior to analysis to ensure that no externally deposited drug (such as from ambient cannabis smoke) contributes to the analysis. Washing may itself alter drug test results, so accredited laboratories with reliable and reproducible procedures, verified by quality assured methods, should be used.
  3. There is no uniform standard concentration of drugs in hair with which results may be compared (similar to AS 4760:2006 for oral fluid and AS / NZS 4308:2008 for urine). Rather, there are in-house detection and quantitation limits for each laboratory, and in 2007 the Australasian Faculty of Occupational and Environmental Medicine published a policy paper on hair analysis

Although there are specific issues to be considered when using hair drug tests, it is an approach becoming more widely used in matters such as compliance testing of those on parole, for Family Court orders, to identify cases of drug facilitated sexual assault, for non-invasive testing of children exposed to illegal drug laboratories, and many others. The range of drugs that can be tested is expanding, with at least 60 drugs in most laboratory listings. All in all, hair drug analysis has an increasing role in drug surveillance and we can expect more request for this service.

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