Hair drug testing – fit for forensic use?

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Hair testing – is it a method that can be used in the court room or should it be consigned to research laboratories until it is a flawless technique?

Following my blog last week on testing for sports and social drugs, I got pointed towards a link in the comments, an article describing criticism of the use of drug testing in hair.  A US Civil Service Commission investigating the firing of six police officers following positive hair tests for cocaine reported:

“the present state of hair testing for drugs of abuse … does not meet the standard of reliability necessary to be routinely used”.

Do I agree?  Well, no not really.  I can see why some people are negative towards hair tests but I am a strong proponent of hair as an additional matrix in forensic testing.  The use of hair for medico-legal testing is not new, having been used for at least 20 years to look at drug use.  I have been to court on many occasions to present hair testing evidence over the last 6 or 7 years.  Ten years ago the Society of Hair Testing set out their ‘Recommendations for Hair Testing in Forensic Cases‘, designed to provide a consistent approach between labs.

To break this argument down I approach hair testing as two separate areas: 1) finding drugs in a hair sample and 2) interpreting that result.


Detection of drugs in hair

This area is the more straight forward to argue of the two.  Methods have been well studied and validated for detecting drugs in hair samples.  The extraction of the drugs and the tools for definitive identification (mass spec techniques) are not new or novel.   As long as a definitive identification tool is used it should be possible to say what has been detected and if the is method validated as a quantitative method it can also give a value of how much is there.  Methods have been published for most drugs of abuse, medicinal drugs and new emerging drugs using GC- and LC-MSn techniques.

The methods have been shown to be reliable in proficiency tests (as run by the Society of Hair Testing amongst others).  These show that different labs detect the same drugs at similar levels.



I see this as the biggest issue with hair testing.  The detection outlined above tells us what is there. How the drug entered the hair is a lot less certain. We can perform measurements to help whether it is likely to be from external contamination.  This is done by washing the hair sample and analysing those washes.  However I showed in the paper on meth lab children it is still possible to test positive even if it is very unlikely you used a drug without having evidence of external contamination.  Other factors such as passive exposure come into play.

The report mentioned above states:

‘A reported positive test result is not necessarily conclusive of ingestion’

I do agree with this it doesn’t necessarily prove use. It does prove that the drug is within the hair sample tested though.   Is it the job of the detecting scientist to provide an explanation as to how it got there?  In sports drug testing there is strict liability on the athlete, which means they have to account for any drugs detected.  In criminal proceedings the prosecution have to prove their case rather than the defendant.  In many cases the expert scientist will provide evidence evaluation that will report ‘the chance of this event occurring is more likely given X explanation rather than Y’.  I have not seen this approach used with hair testing.  It could be ‘it is more likely that this evidence is consistent with the defendant knowingly using the drug rather than being exposed through being in a room with someone smoking the drug’ or some such.  This is standard hypothesis testing and is used, for example, in DNA reporting.

The excuses used by the cops do not seem to be very convincing though. Would brushing powder off a seat lead to a positive result? Unlikely. Storing drugs in the same pocket as their cookies?  I would suggest their evidence handling procedures need to be looked at!

Further caution is required when interpreting the amount present.  Dose response relationships are not well studied and often when they have been studied do not give very strong correlation. I didn’t like the way the Maynard inquest was told the levels in his hair were those of someone who used daily – how was that determined?  As far as I can see papers have been published were the authors have analysed a load of hair samples then simply stated that the bottom quarter of levels relate to low use, the middle half relates to moderate use and the highest values relate to heavy use.  Again the meth lab children throws this method out as the highest methamphetamine level found at ESR was from a child.   I would like to see proper evaluation of the drugs being reported before this type of evidence is used in court.  This is very difficult given ethical permissions required.


In summary

I think there is enough literature on the subject and enough evaluation has been completed to use hair to detect drugs, but with some level of caution in interpretation.  I would go as far as to say that qualitative evaluation is probably as much as you can give.  I am still to be convinced quantitative data is reliable.  Tell the court what was found but don’t try and interpret how much.

Perhaps it is time for the Society of Hair Testing to reissue their guidelines?  Technology and knowledge have moved on a great deal in the last ten years.  Guidance on the issues listed above would be greatly appreciated by practitioners and the legal system.

I do think hair has a role in detection of drug use in various ways and should still be used as such.


Have a read of the ‘Hair testing for sport and social drugs’ article now: click!


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About Dr Tom Bassindale

Dr Tom Bassindale is a forensic scientist, and the founder of We Are Forensic. He is currently the subject lead for chemistry and forensic science at Sheffield Hallam University. He's managed hundreds of forensic toxicology cases, and is an experienced court witness. He has specialist expertise in forensic toxicology and drug testing in sport. And yes... he watches CSI.