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Also known as "mouth alcohol bias"
Ethanol remaining in the mouth following a recent drink (last 15 minutes) or as the result of vomitting, GERD, burping, mouthwash may exaggerate breath test readings dramatically. Evidentiary breath instruments are usually designed so as to alert the qualified technician that mouth alcohol or stomach alcohol has been detected rather than deep lung air alcohol. This is accomplished through a slope detection algorithm in the software which looks for a peak in the readings coming in near the beginning of the breath sample with a rapid drop off followed by a combined effect of deep lung air alcohol and the mouth. The instrument shuts down and prints a warning often stated as "Invalid Sample".
Unfortunately, although the software usually does what it is supposed to do (i.e. watch for the peak and report), mouth alcohol bias can still occur due to the combined analysis of a small amount of mouth alcohol combined with a fairly low true blood alcohol concentration. It is very possible for an individual with a true BAC of 30 mg/100mLs to blow a 120 mg/100mLs reading and for a person with a true BAC of 115 mg/100mLs to blow over 300 mg/100mLs. A person whose true BAC is just under the legal limit (eg. 78 mg/100mLs) could blow just over the legal limit (eg. 87 mg/100mLs).
In some jurisdictions there is a requirement for two tests. If those tests are well spread out in time (eg. 15 minutes apart) and the two tests are in good agreement with each other (eg. within 20 mg/100mls of each other truncated) then mouth alcohol bias is less likely.
Another best practice to reduce the possibility of mouth alcohol bias is a deprivation period (observation period) of 15 to 20 minutes before the breath tests wherein the qualified technician keeps the subject under continuous observation for that period to ensure that he or she does not vommit, consume alcohol, gum or breath strips, belch or burp.
Non-evidentiary breath instruments used as screening devices generally do not have mouth alcohol slope detectors.
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Alcohol Test Committee Recomended Standards 2003 Can. Soc. Forens. Sci. J. Vol. 36. No 3 (2003) pp. 101–127 at pages 114-115:
I OPERATIONAL PROCEDURES
A. Approved Instruments
l. The subject shall not have consumed or placed alcohol (or any other substance that may interfere with the test) in the mouth for at least fifteen minutes prior to the collection of a breath sample.
5. Two samples of deep lung breath collected at least fifteen minutes apart shall be tested.
a. Readings of breath tests shall be truncated before being reported.
b. If the reported results of two tests differ by more than 20 mg/100 mL, a third sample should be collected and tested.
c. If more than two samples of breath are necessary for a “proper analysis” as specified in the Criminal Code, a certificate of a Qualified Technician should not be tendered into evidence; the Qualified Technician should present viva voce testimony.
B. Approved Screening Devices
7. A test on a subject shall not be conducted until at least fifteen minutes after the time the subject states alcohol has last been consumed.
8. The Screening Device shall be operated according to the manufacturer’s recommendations.
See also: observation period
See also: Fifteen Minutes Apart
See also: Two Tests
See also: mouth alcohol
See also: mouth alcohol delay
Links to legislation in International Referral Database:
Ontario : 3 Day Suspension Over 50 mg/100mL
Breath Instruments Links
Breath and Blood Departments and Standards Links
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