Guidelines for Retrograde Extrapolation of Blood Alcohol

Guidelines for
Retrograde Extrapolation of
Blood Alcohol Concentrations
Brian T. Hodgson and M. Delores Taylor
Royal Canadian Mounted Police
Forensic Laboratory Services Directorate
1200 Vanier Parkway, Ottawa, Canada K1A 0R2
Canadian law proscribes drinking and driving by setting a legal limit of 80 mg
alcohol in 100 mL of blood (80 mg%). As long as the first breath sample (of
two) or a blood sample is taken within 2 hours of the offence, the blood
alcohol concentration (BAC) found is deemed to be the BAC at the time of the
offence unless there is evidence to the contrary. As for samples taken outside
the 2 hours, the law makes no comment and the practice of back extrapolating
BACs has evolved into widespread use in Canadian courts. In recent years,
however, this practice has come under criticism in Canada for being
scientifically unsound(l).
Consequently, we in the RCMP Forensic Laboratory system have reviewed, in
depth, retrograde extrapolation and have established some basic guidelines for
this practice. The preamble of the guidelines states that retrograde
extrapolation is forensically valid, provided an explanation of the presumptions
necessary in making the extrapolation is stated. The presumptions are that
there is no consumption of alcohol between the time of the offence and the
time of sampling; nor is there any consumption approximately 30 minutes
before the time of the offence.
When performing calculations, the following are some factors our Alcohol
specialists consider:
For forensic purposes, the range of BAC elimination rates of 10
to 20 mg %/hour is appropriate.
a.
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It is highly unlikely that a person would have an elimination
rate of less than 10 mg %/hour; and
Alcohol, Drugs and Traffic Safety - T92
Ed. by Utzelmann / Berghaus / Kroj
Verlag TÜV Rheinland GmbH, Köln -1993
b.
A small portion of the population may, under certain
circumstances, eliminate alcohol at rates greater than
20 mg% per hour.
We define the plateau as the period between the maximum BAC
and the start of linear decline, during which the BAC remains
relatively constant.
a.
In social drinking situations a plateau period of between
0 to 60 minutes shall be applied.
b.
For a larger consumption, a plateau period of between
60 to 180 minutes shall be applied.
Social drinking is defined by a rate of alcohol consumption up to
0.6 g alcohol/kg body weight/hour.
In our Ottawa lab, we decided to review the data accumulated over the past 8
years from volunteer drinking subjects (Table 1). In all, the BAC curves from
42 healthy subjects were examined and the data statistically analyzed by
computer to determine how closely the curves fit these guidelines. The 25
male and 17 female subjects ranged in age from the early 20s through the 40s.
The original purpose of these experiments with drinking subjects was to
evaluate breath alcohol analyzers. Hence, the rates of consumption exceeded
social drinking, for the most part, so that the subjects would achieve relatively
high BACs in a short time (about 1 to 1-1/2 hours).
Breath analysis was conducted using either the Borkenstein Breathalyzer Model
900A or the Intoxilyzer Model 4011 AS, both of which are approved for
police use in Canada. Sampling began anywhere from 13 to 87 minutes after
end of drinking (EOD). For the most part, however, sampling began about
one hour after EOD since a post-absorptive state was desired for the original
purpose of the experiments.
In calculating elimination rates, we used only data points from the linear
declining portion of the BAC curve (Table 2). Hence any points on the
plateau portion (if a plateau existed) were not included in these calculations
shown in the table. The BAC range considered lay between about 150 mg %
to 20 mg%. Since the two instruments differ substantially in their principle of
measurement, we separated the data accordingly. Between instruments, the
males and the females do not differ significantly as indicated by the 95 %
confidence interval (C.I.). Between sexes, however, there is a significant
difference.
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Table 1 - METHODOLOGY
42 Healthy Subjects:
25 males and 17 females
Ages:
Ranging from early twenties
to their forties
Consumption Rates:
males ranged from 0.46 - 1.4 g/kg/h
8 of the 25 males were in social range
females ranged from 0.45 - 1.5 g/kg/h
2 of the 17 females were in social range
Breath Analysis:
Breathalyzer Model 900A
Intoxilyzer Model 4011 AS
Sampling began 13-87 min. after EOD.
Table 2 - ELIMINATION RATES
BREATHALYZER
900A
Males: n = 11
Females: n = 6
mean
16.0 mg% per hour
mean
20.5 mg% per hour
S.D.
2.6 mg % per hour
S.D.
2.6 mg% per hour
range
12.8-20.5 mg% / hr
range
19.4-25.2 mg% / hr
95% C.I.
14.2-17.8 mg% / hr
95% C.I.
17.7-23.3 mg% / hr
INTOXILYZER
40011 AS
Males: n = 14
Females: n = 11
mean
17.9 mg % per hour
mean
22.9 mg% per hour
S.D.
3.4 mg% per hour
S.D.
6.9 mg% per hour
range
13.9-26.4 mg% / hr
range
16.2-41.5 mg% / hr
95% C.I.
16.0-19.8 mg% / hr
95% C.I.
18.3-27.5 mg% / hr
The limitation placed on our present interpretation of the data appeared with
examining the BAC curves for plateaus (Table 3). For these experiments a
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plateau was defined as a change of 5 mg% or less in the BAC. We were able
to determine plateaus in 19 out of 42 subjects: a plateau being much more
prevalent in the female subjects than the male. There was quite a wide range
in time of duration in both sexes, however a mode of 20 to 30 minutes did
appear in the women. We could not detect any other predictable pattern
resulting in a plateau, however the design of the original experiments may
preclude any such determinations.
Tahle 3 - PLATEAU (19 out of 42 Subjects)
9 Females (out of 17)
Range: 16-98 min.
Mode: approx. 20-30 min.
10 Males (out of 25)
Range: 19-106 min.
Mode: none
Summary:
In relation to the established guidelines, our data suggest that the lower limit
of 10 mg% per hour is more than generous, especially for women. On the
other hand the data indicate that we should be more flexible with an upper
limit. Indeed the data support the contention of some experts (2) that
employing a range of 10 to 25 mg% per hour is more realistic, again
especially for women. Since the great majority of retrograde calculations are
for charges against drinking drivers, an upper limit is not nearly as crucial as
a lower limit.
The experiments precluded us from making definitive conclusions about the
occurrence of a plateau or its duration. There was no predictable pattern
indicating when a plateau is likely to occur, but the data does support the use
of 60 minutes as a reasonable period in back extrapolation calculations. This
duration could be applied in any drinking situation: social or larger.
References:
1.
The Queen versus WADDELL. Criminal Negligence Causing Death; Impaired
Operation Causing Death; and Driving with a BAC Exceeding 80 mg%.
Court of Queen’s Bench of Alberta, Calgary, Canada, 15 July 1987.
2.
American Medical Association. "Alcohol and the Impaired Driver".
Reprinted 1976, National Safety Council, Chicago, Page 22.
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