Page 1716 - Week 05 - Wednesday, 5 May 2010
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drug in a person’s oral fluid—because it simply does not exist—it is impossible to implement the random roadside testing scheme that Mr Hanson envisages through this legislation. As there will be no random roadside aspect to the legislation because high volume testing cannot be undertaken through limitations on technology, there will be no cultural effect. No behaviours in our community will change as a result of the legislation drafted on this model.
Another reason that has been presented to me as a result of my department’s understanding of this particular issue is that any drug testing regime based on a prescribed concentration approach to roadside screening tests is impractical because it assumes, quite wrongly, that it is easy to detect the level of a prescribed drug in a person’s oral fluid and to set a legislative limit for oral fluid or blood concentration that statistically correlates to the actual risk of driving impairment at that drug concentration level.
For alcohol, there is a clear statistical correlation between the level of alcohol detected through breath analysis and the level of alcohol in the blood. By contrast, the relationship between drug levels in oral fluid and drug levels in blood is far less clear. The quantity of a drug present in a person’s oral fluid can be substantially affected by factors that are unrelated to the quantity of drug actually ingested, such as the person’s oral fluid production rates or the person’s hydration levels.
Significantly, some drugs that are found in oral fluid are not actually secreted in saliva at all. An example is THC, which is the primary psychoactive substance found in cannabis. Although THC is not secreted in saliva, it can be detected in a person’s oral fluid for a short time after ingestion because it binds to mucosal cells in the person’s mouth and throat, and those cells act to “contaminate” the person’s oral fluid. The method of ingestion affects the rate at which THC is bound to those cells. For this reason, oral fluid analysis does not provide a reliable indication of the quantity consumed, although it is a good means of detecting recent cannabis use.
As I mentioned earlier, another significant problem that the department advised me of is that, with a prescribed concentration approach to drug-driving laws, there is comparatively little statistical information on the relationship between the quantities ingested, the resulting concentration in oral fluid or blood and any associated risk of driving impairment.
The data that we do have, which are primarily derived from blood taken from road accident victims, clearly show that taking certain illegal drugs before driving increases the driver’s accident risk. How that risk level changes, either with dosage or as drugs are metabolised and withdrawn, the relationship between blood or oral fluid concentration and impairing effect needs more research.
It is known that some drugs in the stimulant class—amphetamine-based drugs—have their greatest impairing effects as they wear off rather than when they are at their highest blood concentration. For drugs in this category, a system based on prescribed concentration could be especially dangerous, as users may delay driving until the drug starts to wear off, at which point the level of driving impairment may be at its greatest.
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