Drug Testing

This is relevant for testing that is conducted in employment, court cases, social services and driving (roadside testing) and substance dependence.

Drug testing is a multifaceted and complex affair. Tests can include oral fluid, sweat, blood, urine and hair. It is heavily influenced by a wide array of factors making it very difficult, and almost impossible to produce a standardised, accurate and practical testing procedure applicable to every person and to all drugs. This is where the problem lies when dealing with the contentious issue of health and safety at the workplace, on the roadside, in custody cases, and any other situation where drug testing is involved. 

There are many different specimens of the body that can be tested. Below are details of the most common specimens:

Blood: this can only be sampled by a doctor or phlebotomist and is not widely used except in driving offences. Gives an accurate reading of acute impairment, but as most substances are metabolised from their active state rapidly would not be appropriate in most cases.

Oral Fluid: Up to around 10 years ago was only really used for ‘prevalence’ testing (i.e. roughly x % of subjects had used a drug in the last 48 hours). Shorter window of detectability but not as invasive as other methods and can be supervised without issues over supervision.

Urine: Some substances are metabolised and excreted within a couple of days, others are still detectable in chronic users much later. In terms of detection and impairment the most difficult substances are THC (tetrahydrocannabinol) and benzodiazepines which are detectable long after their effects have abated.

The rather unpleasant method of being asked to urinate under supervision or in front of mirror, where someone may or may not be observing you was never popular in drug testing which in drug treatment programmes often sought to confirm that the patient had metabolites for drugs that were prescribed but none for drugs that were not made this type of testing unusual. Virtually every patient who has been in a drug testing programme will have tales of inaccurate test results which can erode the basis of trust between clinician and patient. Random testing appears more effective and in addiction should be used to monitor use as opposed to punish patients for non-compliance.

It is probably still the preferred method of testing for the Courts and employers.

Sweat: Sweat patches are attached to the skin to collect sweat over a long period of time, or used as swipes to detect more recent use. Expensive per sample, sometimes seen at airports and with security agencies.

Hair Testing: The science of hair testing and its popularity has increased greatly over the last few years and is now a preferred method to permit long term investigation of a person’s drug use. Chemical hair analysis may be considered for retrospective purposes when blood and urine are no longer expected to contain a particular contaminant. Hair is assumed to grow at about 1cm per month. It is sampled from the crown if possible although other body hair can be used, different sites have varying growing and resting stages. As the drug metabolite travels round the body tiny particles remain trapped in the hair and as the hair grows a history can be discerned reliably up to 6 months. Very popular with Courts where changes in behaviour can be reliably discerned.

With most of these methods a sample will be retained by/for the subject in case an appeal is required.

For blood, urine and oral fluid specimens, there are two types of test stages; the first and most common simply detects the presence of drugs (either a specific drug or a range of the most common drugs) and is known as a presumptive test. This is usually in the form of an oral swab, blood or urine test. To confirm that the presumptive test is accurate, a second, more specific type of testing is conducted. This uses techniques of Gas/Light Chromatography Mass Spectrometry (GC/LC MS) to state the exact level of drug/metabolite found in the specimen. This is known as a confirmatory test and can only be conducted in a laboratory. Current drug testing practices usually uses both of these types of testing as standard procedure.  It is the presumptive test that would be employed to identify those who have potentially committed an offence (although internal ‘possession’ is not in itself an offence, it is the likely consequences of being ‘under the influence’ that will cause difficulties, the assumption based on the level and usual effect of the substance that it can cause potential impairment) under the specified limits model, and it is this type of testing that is fraught with difficulties: 

Individual Variables Involved in Drug Testing

Pharmacokinetics, defined as ‘what the body does to the drug’, is essentially metabolic processes and breakdown of drugs in the body. These metabolic products are the substances tested for. As can be expected, there are a great number of factors involved that lead to variations from person to person, hence the complexity of forensic drug testing. Factors that influence testing can be summarised as follows, and are applicable to every individual:

  • Metabolism of individual - Generally speaking metabolic processes/reactions break down everything that enters the body. This can occur at a faster or slower rate depending age, body mass, exercise, genetic make-up, and even gender. 
  • Type of drug - Some drugs are broken down faster by the body, whilst some remain in the system for longer. 
  • Purity of drug - Generally speaking, the more pure a drug is the more detectable in the system it is. This is mainly because less pure drugs contain higher levels of cutting agents and not the drug itself.
  • Frequency/pattern of use - the more a drug is taken, either in frequency or actual consumption; the more likely it is to be detected. This can be related to the section below on specific variables: tolerance. 
  • Diet - There is evidence to suggest certain foods can affect detectability of drugs, or alter metabolism which has its own effects (see above).

Specific Variables


Tolerance is also relevant to certain drugs and with the issue of impairment [1]. Drugs such as opiates, benzodiazepines, and cannabis (others including alcohol are also in this list) are more prone to producing tolerant effects than other drugs. This means that, people who take these drugs frequently, over a long period of time, no longer have the same effects, and would probably not be considered ‘impaired’. This particularly affects people who are on prescription medication, including pain killing medication such as codeine, anxiolytic medication such as Diazepam (Valium), and opiate substitution medication such as methadone. Those who have a high tolerance to a particular drug, and have a high pharmaceutical/forensic level of a drug in their system but are not in any way mentally or physically impaired. 

False Positives

Another issue that arises through the Helpline at Release and Expert Witness work is of unintended positives in forensic testing, which are not caused by technical issues. Most cases we deal with are in relation to urine and hair testing, with the principle also being relevant for oral fluid testing. We have had a number of cases where passive consumption and ingestion of substances similar to illicit drugs (such as poppy seeds showing up as opiates) can produce a positive reading.

[1] Drummer OH (2006). Drug Testing in Oral Fluid. Clinical Biochemistry Review 27:147-159 Review of developments (over last 10 years) on application of testing for drugs in oral fluid.