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Testing times

Dr Dan Hegarty of Express Medicals has now been involved in workplace drug & alcohol testing for 17 years.

I am very aware of how confused many employers are when faced with choices about which type of drug testing to select and under which circumstances. A recent case has brought this into focus again.

A healthy man presented himself for two different types of workplace testing within 45 minutes. One sample was collected as part of a governmental agency’s testing regime and the other as a pre-employment test.

The first sample used oral fluid (saliva) and the second method involved a collection of urine. In both instances the collection was done under formal chain-of- custody protocols and the samples were analysed in appropriately accredited analytical laboratories.

There were no declared prescribed, over-the-counter or illicit drugs. The oral fluid test resulted in a PASS result. No drugs of abuse were detected. However the urine test resulted in a FAIL result. Cocaine and cannabis were detected.

The individual concerned appealed the result of the urine test and the employer was understandably confused as to why two tests on the same day could produce two different outcomes.

Window of detection

There are a number of explanations. However, the simplest message is that, in general, the positive pick-up rate (number of fail results) for cocaine will be lower when using formal laboratory-based oral fluid (saliva) testing as opposed to formal laboratory-based urine testing.

One factor to realise is that cocaine is only detectable for a short time in oral fluid (saliva).

This window of detection is just 12-24 hours. It is significantly longer when urine is analysed for the presence of cocaine and so the likelihood of positive results for cocaine is enhanced by virtue of the longer detection window.

As regards the cannabis results, urine is also generally better for detecting cannabis in workplace testing regimes. Firstly, cannabis disappears quickly from oral fluid (saliva). Secondly, significant amounts of cannabis sample are lost from many oral fluid (saliva) collection devices and this loss can be of the order of 50%.

This means that false negative results for cannabis (ie: the test fails to detect the cannabis present in samples) are significantly more likely when using formal laboratory-based oral fluid (saliva) testing as opposed to formal laboratory-based urine testing.

Complexities

Thus a seemingly simple and straightforward case of two professionally collected samples, both of which underwent formal laboratory analysis, highlights the complexities that have to be considered when establishing a workplace testing programme.

In the case cited above, the cannabis and cocaine were missed when saliva was collected and analysed. This is termed a false negative result.

Furthermore, the likelihood of such a false negative result would have been even greater had an instant test kit been used (ie: a PoCT / point-of-care or point-of- collection test kit). This is because the ability of PoCTs to detect drugs of abuse is less reliable than laboratory-based tests.

When establishing a workplace testing and/or screening regime it is essential to seek out high quality, informed advice in respect of both a company’s drug and alcohol policy and the ongoing issue of which methods of testing to opt for.

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