Drinking & Driving – Proving blood alcohol concentration

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Drinking & Driving – Proving blood alcohol concentration

When a person gives a breath sample into an approved instrument by a qualified technician, a determination still needs to be made of what the person’s Blood Alcohol Level (BAC) was at the time of the offence. That requires evidence of two things:

  • The person’s BAC at the time of giving the breath samples, and
  • Based on the person’s BAC at the time of giving the breath samples, the person’s BAC at the time of the offence.

Presumption of accuracy

To work out the person’s BAC at the time of giving the breath samples, the prosecutor can rely on a Certificate of a Qualified Technician, which states what the results were of the analysis of the breath samples, and is evidence of its contents. This is commonly referred to as the presumption of accuracy. It is still open for the defence to call evidence showing why the results are not accurate, leaving it for the court to weigh the evidence.

If there is no Certificate, or the Certificate is flawed, the prosecutor can always call the qualified technician to give evidence about the accuracy of the results. The prosecutor may still call the qualified technician if there is a Certificate in order to counter the defence’s evidence.

Typically, the Certificate will round the BAC results down to a hundredth of a percentage (e.g. 0.116 percent is truncated to 0.11 percent).

Presumption of identity

To work out the person’s BAC at the time of the offence, the prosecutor generally needs to show the following:

  • The breath samples were taken as soon as practicable,
  • The first breath sample was taken within two hours of the offence, and
  • A second breath sample was taken 15 minutes or more after the first sample.

If the three criteria are met, then the lower of the two results is presumed to be the person’s BAC at the time of the offence. This is commonly referred to as the presumption of identity.

The presumption can be rebutted two ways, depending on whether the defence is challenging the accuracy of the results. If defence is challenging the accuracy of the results, they need call evidence that shows:

  • the approved instrument was malfunctioning or not being operated properly,
  • the malfunction or improper operation resulted in a reading of a BAC in excess of 0.08 percent, and
  • the person’s BAC would not have been in excess of 0.08 at the time of the offence.

The last criteria is typically met by calling reliable evidence of how much the person had to drink prior to the offence, and expert evidence of what their BAC would have been at the time of the offence as a result of the drinking evidence.

If defence is not challenging the accuracy of the results, they only need to call evidence on the last criteria, and have the expert give evidence why it would not be inconsistent with the breath sample readings. (This is typically called the bolus drinking scenario.)

If the prosecutor is unable to rely on the presumption of identity (usually because the first reading was taken outside of the two hours), they can still “read-back” the readings by calling their own expert evidence.

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Author: AllOntario Team

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