Drug-driving and dribble

By Anna Sandiford 25/11/2010

“Why haven’t we got a testing device that can screen breath samples for drugs in the same way we can test for alcohol?”  As an alcohol/drink-driving calculation expert I get asked this question quite a lot.  The assumption is made that because we can test for alcohol in breath then it must be just as easy to test for drugs.  Alas, no.

Alcohol is detected in breath samples because alcohol is absorbed into the blood and then “evaporates” from the blood into the air in the lungs at the point when the blood+alcohol pass into the alveoli in the lungs – this is the interface where oxygen uptake and carbon dioxide expulsion take place (bear with me, I’m not a medical doctor).  Because there is a relatively constant ratio between alcohol in the blood and alcohol that passes into the air in our lungs, we can relate a blood alcohol concentration to a breath alcohol concentration (allowing for known variations). This blood:breath ratio forms the basis for saying that a breath alcohol concentration of 400 micrograms alcohol per litre  is equivalent to a blood alcohol concentration of 80 milligrams per 100 millitres.   These are the legal adult limits for driving in New Zealand (with 150 and 30, respectively, for youth drivers).

Having a legal blood and breath alcohol limit for driving is possible because alcohol behaves in a relatively predictable way in the body.  Pour alcohol into a body of known size and we can calculate the concentration (again, with allowance for certain variations).  We also know how alcohol breaks down in the body and that it does it at a relatively constant rate.

Other drugs however are a different kettle of fish.  For example, not all drugs are dispersed from the blood into the breath like alcohol.  How do we measure those drugs then if we can’t use breath?  We need to have a non-invasive (no needles) method so that police officers can use the test either at the roadside and/or at the police station. A logical body fluid is therefore saliva.  However, saliva:blood ratios vary between drugs, between people and also within the same individual on different occasions. This makes saliva drug screening very difficult, so how do we account for that?

For the reasons above, some countries don’t use oral fluid screening methods. They rely instead on roadside impairment tests sometimes backed up with examination by a medical doctor (to exclude medical reasons for any impairment identified by police officers) plus a blood test to confirm (or otherwise) the presence of drugs. Some countries do use a roadside drug screening test.  Queensland in Australia uses a preliminary roadside screening test. Victoria also uses a saliva drug screening test.

In June this year, the Home Office in London launched a “competition” to “identify drug detection technologies capable of detecting very low concentrations (nanogram/millilitre) of drugs and their active metabolites in oral fluid that can meet the challenging sensitivity, specificity and reproducibility requirements within a polydrug detection device at the police station.” This is to follow on from the fact that  “Current enforcement methods rely upon the officers’ observation and assessment of the drivers’ impairment. Use of an oral fluid screening device would significantly assist officers and act as a strong deterrent to potential drug drivers.”

This competition was won recently by a joint application between the National Physical Laboratory, King’s College London and MSA Ltd, the latter being the company that developed the technology to detect drugs on individual banknotes.

I look forward to seeing what they develop and how they overcome the problems that have thus far made such police station-based technology so difficult to create.  If it works, you can be pretty sure it’ll end up in use over here!