Approved breath-test instruments
Accuracy and precision considerations
All breath testing equipment, whether
approved for roadside screening or
evidential purposes, is subject to
measurement error or uncertainty. The
main sources of this uncertainty are
analytical variability and biological
considerations. As a result, breath test
devices are no different than any other
quantitative measuring system used by
scientists, engineers, technologists and
technicians.
The uncertainty is deemed acceptable
when it is reproducible and small with
respect to the measured result and
unacceptable when it is large and not
reproducible. The acceptability of an
error level is most often determined by
those who are practitioners in a given
field, after giving consideration to all
factors that may influence an analytical
result.
Precision of measurement
Several elements form the basis for
accepting or rejecting the uncertainty
level produced by a certain method or
instrument. These are things like the
criteria used to select samples, how
instruments are designed, how data is
manipulated and reported and the purpose
of the analysis and the significance of
the intended results.
For a given measurement, there are two
elements of concern. The first is the
precision of a measurement which
reflects the reproducibility
(variability) of the method of analysis.
It is usually determined by the repeated
analysis of a standard substance, such
as a standard alcohol solution, in the
case of breath testing.
Accuracy of measurement
The second consideration is accuracy
which denotes the proximity of a
measurement result to the true value. In
terms of breath testing, 10 breath
readings in succession, each yielding a
result of 100 mg%, would be
considered to be highly precise. They
would also be highly accurate if the
true blood alcohol level (direct
measurement of alcohol in blood) were
also 100 mg%.
On the other hand, 10 breath readings
in succession yielding results of
100 mg% would be considered highly
precise, but highly inaccurate if the
true blood alcohol level at the time of
measurement had been 150 mg% or
50 mg%. It follows therefore that
methods can be highly precise and highly
inaccurate at the same time.
Breathalyzer models 900 and 900A
It is accepted in the forensic
scientific community that the precision
of an approved instrument is
± 10 mg%.For the
Breathalyzer models 900 and 900A, this
precision is reflected in various ways.
Firstly, the BAC scale is graduated in
increments of 10 mg%. Readings to
the nearest 1 mg% must be
interpolated by the operator. Secondly,
a tolerance of ± 10 mg%
is allowed for the blank test, as well
as the standard alcohol test. Values
obtained outside this range require the
operator to start over.
A precision tolerance of
± 10 mg% also applies
to the Intoxilyzer 5000C. This is
affirmed on the manufacturer's computer
print-out card which states,
"calibration checks must be 0.09 g% to
0.110 g%." Finally, the practice of
truncation further supports the
precision considerations outlined above.
Truncation of breath readings
Truncation is the practice of rounding
the breath test values down to the
nearest 10 mg%. Truncation was not
adopted to simply lower a given result
or to provide "some benefit of doubt" to
an accused person. It simply reflects a
well-established scientific principle.
A precision tolerance of
± 10 mg% is deemed to
be necessary for a variety of reasons:
- The instrument used to measure a
breath sample has a series of
tolerances related to how it captures
a breath sample, measures the signal
produced by the alcohol contained in
the human breath, manipulates the data
and reports the results;
- The standard alcohol solution used
for the calibration check is
independently formulated with a
certain tolerance;
- The simulator/equilibrator used is
subject to certain temperature
tolerances and manner of use, unique
to an operator;
- Instrumental service and
preventative maintenance would also
play a role with respect to precision.
No measuring system is absolute in
terms of the values reported because of
the inherent analytical variability.
Even a measurement with a ruler is
subject to precision considerations.
Accuracy depends upon how well a
measuring device agrees with the true
blood alcohol level (determined by
actual analysis of a blood sample). This
aspect depends upon several biological
variables.
Quality of breath sample
In the respiratory system, air moves
back and forth through the same set of
tubes, hence there is continuous mixing
of inspired and expired air. Air passes
from the nose to the trachea to two
tubes (one for each lung) called
bronchi.
The bronchi divide into several
subdivisions, which end in a large
number of tubules. At the end of the
tubules are the alveoli. Each lung
contains several million alveoli.
Surrounding the alveoli is a fine
network of capillary blood vessels.
Deep lung-air required
Exchange of gases, including alcohol,
between the blood and the air in the
lung occurs in the alveoli. As a result,
in order to obtain a good correlation
between blood and breath alcohol levels,
deep lung-air must be analyzed by the
breath tester.
The vital capacity (volume of air that
can be expelled without collapsing the
lung) of a test person may range from 7
L for a trained athlete to less than 1 L
for persons with severely impaired lung
function. If a high minimum volume is
chosen, as the operational parameter for
a breath testing device, there is a
danger that many subjects will not be
able to provide a valid breath sample.
If the minimum volume chosen is too low
then the correlation between breath
alcohol values and blood alcohol levels
will be poor. In this instance, breath
predicted blood alcohol levels will be
lower than the true blood alcohol level.
The more modern instruments have
software algorithms that monitor the
breath sample as it is provided.
Sampling characteristics such as breath
pressure, time, flow rate, exhalation
volumes, rate of change in alcohol
concentration, as well as certain other
features may be monitored to ensure
compliance with those specifications
deemed by a given manufacturer to be
crucial to the suitability of a given
sample. Different manufacturers may use
different ways to comply with the "deep
lung air" requirement which is so
necessary to proper breath testing
procedure.
Variability in breath-test results
The breathing technique employed just
prior to breath testing can also cause
variability in breath test results.
Hyperventilation immediately prior to
sampling has been shown to cause a
decrease in the breath alcohol
concentration of up to 20%. On the other
hand, it has also been reported that a
subject who holds his or her breath for
a short period of time before exhalation
can increase the alcohol content in
exhaled air by up to 15%. As a result,
it is clear that the quality of breath
samples may cause significant
differences between two readings taken
on a single instrument, as well as
readings taken from two different
approved instruments.
Breath temperature
Breath testing for alcohol is based
upon the principle that alcohol is
excreted unchanged in human breath,
because of equilibrium conditions
between lung capillaries and pulmonary
alveoli. At constant temperatures the
amount of alcohol in the breath is
proportional to the amount of alcohol in
the blood, obeying the scientific
principle called Henry's Law.
Obviously, the constant temperature
refers to a body temperature of
37°C. As a result, if the body
temperature fluctuates, even slightly,
the relationship between breath and
blood alcohol content will be altered.
End-expiratory breath temperatures
The temperature of expired air is
always below 37°C because the air
loses its heat as it passes through and
out of the respiratory system. It has
been shown that end-expiratory breath
temperatures can vary 32.4-35.7°C.
Breath testing instruments are based
upon a breath temperature of 34°C. A
lower body temperature would result in
less alcohol in a given volume of breath
and a higher temperature would result in
more alcohol.
It has been shown in a study that a
breath testing instrument will
overestimate the true BAC by about 8.6%
per °C increase in body temperature
(hyperthermia). In a separate study, it
was demonstrated that a breath testing
device will underestimate the true BAC
by about 6.9% per °C decrease in
body temperature (hypothermia).
Time difference between the two tests
In criminal matters, two breath samples
are taken a minimum of 15 minutes apart.
As should be obvious from the previous
comments, alcohol is continually being
absorbed, distributed, metabolized and
excreted. As a result, it is expected
that blood alcohol levels will change
slightly over a 15-30 minute interval.
One can not conclude on the basis of
two readings if a subject's blood
alcohol content was increasing or
decreasing. Several readings in
succession are necessary to make that
determination.
Intoxilyzer 5000C vs. Breathalyzer
Given the implementation of newer more
modern breath testing devices, such as
the Intoxilyzer 5000C, it is not
uncommon to hear evidence in criminal
proceedings that the instruments are
more accurate than the Breathalyzer.
This is not correct.
Comparison testing of these devices has
demonstrated accuracy and precision
parameters that are very similar.
Indeed, the Breathalyzer was used as a
reference instrument when the
Intoxilyzer 5000C was first evaluated as
a candidate for "approved instrument"
status.
Readings can differ by 29 mg%
It is not uncommon to observe a
difference between two truncated
readings of ± 20 mg%.
The Alcohol Test Committee of the
Canadian Society of Forensic Science
(Principle Scientific Advisor to the
Federal Department of Justice with
respect to Criminal Code matters) has
stipulated that this difference is
acceptable on scientific grounds when
prosecuting alcohol-related driving
offenses.
In fact, two actual readings can differ
by as much as 29 mg% to allow for a
truncated difference of
± 20 mg%. The practice
of allowing a difference of
± 20 mg% lends further
support to the inherent variability in
breath samples.
Timing of BAC important
In some drinking scenarios, the BAC at
the time of driving is lower than the
readings obtained later. This
phenomenon, referred to as the "rising
curve," warrants consideration when low
readings are involved.
Since breath tests are conducted up to
two hours after the time of an alleged
offense, a given subject's BAC will
likely have changed in the interim. The
BAC at the time of the alleged offense
could have been higher or lower than the
reading obtained later.
BAC can increase by 20 mg% or more
after the time of an alleged offense. As
a result, caution should be exercised
with respect to the significance of a
BAC taken up to two (2) hours after an
alleged offense, when the readings
obtained are only slightly in excess of
the legal limit. A difference of
20 mg% would represent the
non-absorption of one drink of alcohol
or less for a significant number of
persons within a given population.
Accuracy crucial
It is clear that accuracy and precision
considerations become very important at
BAC levels that exceed the legal limit
only slightly. These concerns are
evident in the standards developed by
the Alcohol Test Committee of the
Canadian Society of Forensic Science and
reinforced by the practice whereby
roadside screening devices are
calibrated to fail at 100 mg% or
higher.
The inherent variability in breath
samples when combined with the passage
of time considerations should result in
extreme caution when relying on BACs in
the vicinity of 80-100 mg% for
criminal purposes. Since a greater
portion of this variability can be
attributed to biological factors rather
than analytical aspects, it is not the
measuring device that is the limiting
factor but rather the subject providing
the sample.
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