A new police power to calculate your past intoxication level is scientifically invalid

Police testing for drunk driving will be able to presume a driver’s past intoxication level based on breath or blood samples under a new amendment to the Criminal Code. Drivers who are tested several hours after the time of driving could still be found to have operated a vehicle while over the legal limit, even if their blood alcohol content (BAC) is under the threshold at the time of the test. The method that will be used to make these presumptions, known as retrograde extrapolation, is scientifically unreliable for a number of reasons. This amendment is not only full of holes, it is also constitutionally invalid and eventually, it will be torn to shreds by defence lawyers. In the meantime, however, it could lead to numerous miscarriages of justice.

The provision was included in Bill C-46 which gained royal assent in June and will come into effect on October 17.

Section 320.31(4) of the bill states:

…if the first of the samples of breath was taken, or the sample of blood was taken, more than two hours after the person ceased to operate the conveyance and the person’s blood alcohol concentration was equal to or exceeded 20 mg of alcohol in 100 mL of blood, the person’s blood alcohol concentration within those two hours is conclusively presumed to be the concentration established in accordance with subsection (1) or (2), as the case may be, plus an additional 5 mg of alcohol in 100 mL of blood for every interval of 30 minutes in excess of those two hours.

So if a driver provides a breath or blood sample equal to or higher than 0.02, or 20 mg of alcohol in 100 ml of blood, more than two hours after they stopped driving, then police will be able to go back and add 10 mg for every hour that passed between the time of driving and two hours before the test.

This new power would most likely be applied in impaired driving cases involving a crash or fatality where blood or breath evidence can often be several hours old. Say you are a driver involved in a crash and you are taken to a hospital. A police officer meets you there and takes a sample of your blood maybe four hours after the time of the crash. A test shows you have a BAC of 0.07. Under current rules, this would be below the 0.08 limit set by the Criminal Code. Using the new formula, however, you could be convicted of an offence because officers could add 0.01 to your result for each hour in excess of two hours since the accident – in this case, two hours. So your BAC would be “conclusively presumed” to have been 0.09 at the time of the accident – above the legal limit.

The fact is retrograde extrapolation is subject to too many variables to be reliable.

The rate of elimination of alcohol from our bodies varies from person to person

How quickly our bodies absorb and eliminate alcohol differs from person to person. Forensic scientist Kurt Dubowksi PhD, in his 1985 paper, Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects,  stated that constant-rate elimination is, “an inappropriate description for the elimination of alcohol in humans, at least in a significant portion of the population”.

He goes on to say: “Both the absorption and elimination rates of alcohol are significantly subject to the effects of food and, to a much smaller extent, the effects of type and dilution of the ingested alcoholic beverage.”

Blood alcohol concentrations rise and fall at different rates depending on whether someone has eaten and also whether they were drinking beer or whisky, for example. A one-size-fits-all approach to alcohol elimination will not be able to tell you whether or not someone was actually over 0.08 at the time of driving. To more reliably calculate BAC through extrapolation you need to take factors such as which drinks a person consumed, the alcohol concentration of each drink and if and when they consumed food into account. Police will probably not obtain this evidence so it becomes a moot point.

It is impossible to know if a person’s body has reached the elimination stage

Of course, if you are simply adding 10 mg of alcohol per hour you are not taking into account whether someone is still absorbing alcohol or whether they are sobering up unless you take multiple blood samples over a period of time. According to Dubowski, our bodies absorb alcohol after consumption until BAC reaches its peak. After this, we enter what is called the elimination phase where the concentration starts to drop. It is often impossible to be able to tell whether someone is at the absorption phase or elimination phase based on a single test. However, the Bill C-46 amendments require only one blood sample or two breath tests, with an interval of at just 15 minutes required between each one, be taken. Determining whether someone has reached the elimination phase would require detaining them for an extended period of time and putting them through multiple tests.

A multitude of factors, including food, sex and body weight, affect how quickly someone will reach their peak BAC, according to Dubowski. Without being able to determine if someone has reached this stage, police are not able to tell if someone has entered the elimination phase and, therefore, it is unsafe to apply retrograde extrapolation.

Alcohol is not necessarily eliminated at a constant rate

According to Dubowski, a person’s BAC fluctuates during the elimination phase. Applying a simple, constant rate formula of 10 mg per hour, therefore, will not give an accurate determination of someone’s past BAC.

“For some purposes, a trend-line curve is markedly inappropriate or useless,” Dubowski said.

“That is especially true for attempts to engage in retrograde or forward extrapolation of blood or breath alcohol concentrations beyond observed values,” he added.

Reasons Dubowski cites for a trend-line curve being unsuitable to calculate a person’s BAC in the past include: the lack of knowledge about whether or not someone is at the elimination phase at the time of the test; a lack of knowledge about the characteristics of the rate of alcohol elimination in an individual’s blood; and “unpredictable irregularities” in the curve.

How courts have interpreted retrograde extrapolation

Courts have already had to deal with the issue of retrograde extrapolation in impaired driving cases. In this case in Ontario, the Crown relied on the opinion of a toxicologist to presume that the defendant’s BAC was above the legal limit at the time of a collision, several hours before he had provided breath samples.

According to the toxicologist, there needed to be an allowance for up to two hours for a “plateau” which represented the period of time in which there is no significant change in the BAC due to, “the rate of absorption of alcohol into the body being approximately equal to the rate of elimination of alcohol from the body”. This may be why the two-hour allowance was stipulated in the Bill C-46 amendment.

Although the toxicologist was of the opinion that the defendant was over the limit at the time of driving, the evidence was invalidated due to an error in calculating the time of the crash. The Crown urged the judge to apply the rate of elimination himself but he raised doubts about whether retrograde extrapolation should be taken as fact and not expert opinion.

“In my view,” the judge said. “The safer course is to leave such calculations to those who are trained to perform them.” The defendant was acquitted of the charge. This gives a clear indication that courts may be reluctant to take on the responsibility of calculating previous intoxication using retrograde extrapolation.

The new provisions will be unconstitutional

We are of the opinion that there is a strong case that when these new rules come into effect they will be found unconstitutional. We debated whether to present our concerns to Parliament when the Bill C-46 was being proposed, however, we instead chose to focus on challenging other provisions in the legislation, specifically those relating to marijuana-impaired driving and random roadside breath testing. Unfortunately, there is a limit to how much you are able to cover when testifying to a parliamentary committee.

We believe this amendment represents an infringement on the Charter right to be free from unreasonable search and seizure. Retrograde extrapolation is based on false assumptions that could lead to innocent people being found guilty.

Rest assured, as soon as one of these cases comes across our desk, we plan to file a constitutional challenge.