What affects a hair drug or alcohol test result

When you receive your laboratory report it is important to understand that the following factors will have influenced the result. These should all be taken into consideration as part of an expert report ; 

  1. Previous Drug Use – it can take up to 6 months for a heavy user of drugs to show a clean result in a hair drugs test, (even in a segmented hair drugs test) due to something called sleeping/resting hair. This is where up to 15% of the hair is in the penultimate hair growth phase. DNA Legal see many cases where clients may have stopped taking drugs in December and show positive in March. If the expert has not taken into account previous drug use and the report is concluding a positive result for current drugs use, it may well be that the hair is positive for drugs but from the time previously used. This can also be the case if a segmented test is done. 
     
  2. Chemical Treatments And Hair Dyes – clients may choose to chemically treat their hair which can remove between 40 – 80% of the drugs from their hair during each chemical treatment (depending on the drug type) which opens up the hair shaft to possible passive contamination. If hair is bleached it may remove some drugs but allow other drugs to be absorbed by the hair. In addition hair dyes sometimes contain substances such as hemp and other products that can lead to potential false positive results. 
     
  3. Hair Colour and Texture – there are published studies that show that drug metabolites attach to the melanin in hair. If you have two clients that take the same quantity of drugs a client with light blonde hair will most likely show a lower level of drug compared to one with dark brown hair. Controlled studies in the USA showed levels in blondes to be on average 3.1 compared to levels in african americans of 14.9 – for the same dose of the drug. 
     
  4. Area of Hair Collection – the SoHT guidelines are based on collection of hair from the crown or nape of the head. If the client does not have hair in this location or another area of the head is tested then results can vary by up to 20% meaning a result could be 20% higher or lower than if taken from the correct area of the head. 
     
  5. Hair Growth Rates – the average hair growth rate is 1cm per month, however this means that most people will not be average. The range can be from as little as 0.5cm per month up to 2cm per month. It is important to remember that reports that state a time period covering certain months are based on averages. A 3cm segment could cover as little as six weeks and as much as six months. 
     
  6. Passive exposure and contamination - Not every drugs test is the same. Many firms test just a small number of metabolites for each drug group and some do not test any. This means it is very difficult to identify if the drugs have been consumed by the client or appear as a result of passive exposure.

    In general the more detailed the test and the larger the number of metabolites tested the more accurate the result.

    To further support this, DNA Legal carry out an extended wash process to ensure all traces of contamination are removed from the hair. As a unique service we even test the wash to determine the level of drugs found, once they are close to zero we know the result of the hair test is not due to contamination.

    This takes 3 times as long as just testing the hair and is not common practice in UK labs, but it is something we feel is essential to avoid false positive results.

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