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Alcohol
Cannabis
Stimulants
Prescribed Drugs
The
Inexperienced Driver
Publications
Alcohol is a relatively easy drug to measure because it is:
- Taken in large doses compared to other drugs;
- A simple compound for which several accurate methods of measurement are known;
- Distributed throughout the entire body water; and
- Excreted unchanged in breath and urine, which allows indirect estimation of blood concentration.
BLOOD SAMPLING
Blood from a peripheral vein is usually used to estimate the blood alcohol concentration. Alcohol in blood is remarkably stable unless contaminated with fermenting organisms . Delays in performing the analysis make no significant difference to the measured blood alcohol concentration. Post-mortem blood provides a reliable measure of the ante-mortem blood alcohol concentration provided the sample is collected early, away from the stomach, and the body not putrefying.
BREATH ANALYSIS
The recognition that exhaled air contained a small but measurable amount of alcohol lead to the development of specialised instruments to estimate the concentration of alcohol. The lower respiratory tract contains about 300 million tiny air cells (alveoli) covered with a rich network of fine blood vessels (capillaries). The walls of these air cells are 0.001 millimetre thick. Oxygen diffuses across these thin membranes and carbon dioxide is removed. At this microscopic level, the concentration of alcohol in the air is directly dependent on the concentration in the adjacent blood stream and the temperature (Henry's Law). Human bodies maintain a remarkably constant temperature so that if the concentration of alcohol in the deep lung air can be measured the blood concentration can be estimated.
The challenge is to get an uncontaminated sample of deep lung air. Each of the devices currently in use has some mechanism to satisfy the operator that a proper sample has been obtained. Properly performed breath analysis is an accurate and reliable measure of breath alcohol concentration.
Converting breath alcohol concentration to an equivalent blood alcohol concentration relies on the assumption that there is a constant relationship between the two. The most common value of this ratio for law enforcement purposes is 2100:1. Using this figure assumes that the normal population share a common blood/breath ratio, all having the same body temperature and lung anatomy. In reality the real ratio for an individual may vary slightly from the population norms. Other jurisdictions use a ratio of 2300 which gives higher blood alcohol concentrations for given breath measurements and increases the agreement between blood and breath. With a blood/breath ratio of 2100, breath analysis tends to slightly under-represent the blood alcohol concentration. I
Mouth Alcohol
Alcohol present in the mouth is a possible source of falsely elevated breath analysis readings. Possible sources of alcohol in the mouth are:
- recent ingestion of an alcoholic drink;
- regurgitation/vomiting of stomach contents with alcohol;
- eructation of stomach gas;
- alcohol based dental or mouth rinses;
- alcohol containing breath fresheners.
Realistically such effects are small and short-lived. A 20-minute wait between last ingestion of alcoholic beverage and breath analysis is sufficient to completely eliminate such effects. A blood sample may be desirable to establish the validity of a breath test, especially when the subject is dissatisfied with the reading obtained, but in many jurisdictions the result will be higher than a breath test taken at the same time.
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