In Canada, blood alcohol concentration (BAC) is often measured more than two hours after a drink driving offence. Thus the practice of back extrapolation BACs has become widely used in Canadian courts, although it has recently been criticised as unscientific. This paper presents an in-depth review of retrograde extrapolation by the RCMP Forensic Laboratory in Ottawa, and of the resulting basic guidelines for this practice. The guidelines state that retrograde extrapolation is valid, as long as the necessary assumptions are explained. The assumptions are that: (1) no alcohol is consumed between the time of the offence and the time of sampling; and (2) no alcohol is assumed approximately 30min before the time of the offence. The Laboratory decided to review all data accumulated over the past eight years from 42 volunteer drinking subjects. Breath analyses were conducted, using either the Borkenstein Breathalyser Model 900A or the Intoxilyser Model 4011 AS, both of which are approved for police use in Canada. The data support the use of 60min as a reasonable period in back extrapolation calculations; this duration can be applied to any drinking situation.
Abstract