According to science, breath alcohol tests in DUI cases can be as much as 230 percent higher than corresponding blood tests. Because blood transports consumed beverage alcohol from the stomach to the brain where it can reach sufficient levels to cause impairment, a person’s blood alcohol level is what really matters. Therefore, in the context of a DUI case, breath alcohol only relevant to the extent that it accurately reflects blood alcohol content. This is true because breath alcohol does not have the capacity to cause intoxication.
To understand just how significant this fact is, consider a hypothetical case where a driver’s breath test comes back at .18. This would likely result in the driver being charged with an enhanced DUI, or what Michigan calls “super drunk driving,” a charge applicable to drivers with a BAC of .17 or above. While this breath test evidence might look bad for the driver, it is well within the realm of scientific possibility that this same driver has corresponding or simultaneous blood alcohol level of .063, or well below the legal limit of .08. Understanding why this is so, and why breath testing can be so pernicious, requires a basic understanding of alcohol metabolism.
Pharmacokinetics and the Absorption, Distribution and Elimination of Alcohol
Understanding why there can be such a vast difference between breath and blood testing requires an understanding of “pharmacokinetics,” which is the scientific discipline involving the absorption, distribution, and elimination of drugs including alcohol. According to A.W. Jones, one of the world’s foremost scientists studying ethanol, pharmacokinetics addresses “what the body does to the drug” as opposed to pharmacodynamics, which addresses “what the drug does to the body.”
The path that alcohol takes within the body begins in the stomach then continues into the small intestine. Absorption begins almost immediately, and as the alcohol is absorbed it moves in the arteries first into the liver, then into the lungs, then back to the liver, all the while being distributed within the body. In other words, generally speaking, the alcohol is absorbed into the bloodstream, distributed throughout the body, and then eliminated by the liver. Let us look now at each of these steps along the way.
Beverage alcohol, more properly called ethanol (EtOH), is most commonly consumed orally. What follows assumes oral consumption and this is important because different rules apply when alcohol is injected or administered anally. (Yes, alcohol enemas are a thing).
The absorption of alcohol begins almost as soon as it comes into contact with the oral mucosa. However, alcohol absorption into the blood stream does not begin in earnest until the alcohol passes from the stomach into the small intestine. By approximate numbers, 20% of the alcohol can be absorbed through the stomach lining, whereas 80% is typically absorbed through the small intestine. The number one factor governing when alcohol passes from the stomach into the small intestine is the contents of the stomach. If the stomach is full of calorie rich foods, such as fats and proteins, this food, and therefore any alcohol, will remain in the stomach far longer than low calorie foods that “burn” quickly, such as potato chips.
Full absorption, meaning the point at which an equilibrium between absorption and elimination takes place, usually takes place about 45-60 minutes after the last drink is consumed. However, some studies have shown far longer and shorter periods are required for full absorption. Full absorption, also known as equilibrium, occurs in as little as 15 minutes but may take as long as 138 minutes. Some studies have shown that certain circumstances and medical conditions can delay full absorption even longer than 138 minutes. In a drunk driving case it’s important to know when full absorption occurs.
Absorption of alcohol takes place by the process of simple or passive diffusion through the various tissues in the body. Alcohol passes rather easily through all tissues in the human body, including the blood/brain barrier, and in part this is owing to the fact that alcohol is fully miscible with water and is hydrophilic.
The Distribution of Alcohol
Because ethanol is hydrophilic, meaning “water friendly,” it is distributed within the human body according to the amount of water present in each tissue. This means that tissues with lots of water, like the brain, will “pull” a lot of alcohol out of the bloodstream, whereas tissues with little water, such as bone tissue, will pull out very little alcohol. Either way, the distribution of alcohol within the body is relatively rapid.
Accordingly, the alcohol concentration in a body tissue is generally proportional to the water content of that tissue. Because alcohol is distributed by passive diffusion, if a tissue has 8% more water content than blood, this tissue will have 8% more alcohol then does the blood. It is on this basis that scientists can estimate blood alcohol concentrations in a particular tissue when a person’s blood alcohol level is known. The opposite is also true. Knowing the alcohol concentration of a particular tissue allows a scientist to estimate the amount of alcohol in a person’s blood. In postmortem tests, eye fluid is thought to be the most reliable way to determine a person’s pre-death BAC.
The distribution of alcohol also depends somewhat on total body water, which in turn depends on a person’s gender, age and body mass or amount of body fat (obesity). Women tend to have less water and more fat in their bodies when compared with men. One way to measure water content is by using of the “total body compartment” method. Studies have shown that the total body compartment for non-obese men comprises about 55–60% of body weight and for non-obese women is about 50–55% of body weight.
The Elimination of Alcohol
Alcohol is eliminated in the human body through its interaction with an enzyme called “alcohol dehydrogenase” or ADH. While the greatest concentrations of ADH occur in the liver, ADH is present throughout the body, including in the mouth and stomach. The elimination of alcohol during the post absorptive elimination phase takes place through what is called “first order kinetics” which means that it is not dose dependent. No matter how much alcohol is in the bloodstream it can only be eliminated based on the amount of ADH present to break it down. This is one of the main reasons blood alcohol concentrations can be deducted, including the derived BAC from a given number of drinks, and how long a given number of drinks will take to be eliminated.
To perform alcohol calculations there must first be agreement relative to the terms used to define the volume consumed. Scientists usually use “units” to define alcohol volume such that a unit of alcohol, which is defined as one 5% 12 oz beer, one 1.5 oz of 80 proof liquor or one 5 oz glass of 12% wine has the potential to raise a person’s BAC by between .02 and .025, depending on the scientific literature consulted. The easy way to calculate a BAC therefore is to simply divide the units of alcohol by a factor of 2. This means 4 units are necessary to reach the legal limit of .08. But this simple math does not take into account the factor of alcohol elimination.
Zero‐order kinetics also means that BAC decreases at a constant rate per unit of time. Alcohol is generally eliminated at a rate of .015 per hour for men, and .018 per hour for women. Women tend to eliminate quicker than men because they have larger livers as a percentage of total body mass. The literature suggests that the range of elimination starts as low as .09 per hour to as high as about .039 per hour a nearly four-fold difference. Most of this difference is attributable to the drinker’s prior exposure to alcohol, meaning a naïve drinker, who has never been exposed to alcohol, or has not had anything to drink for a long period of time will eliminate at the lower range whereas alcoholics with liver damage and/or who are exposed to large amounts of alcohol will eliminate at the higher range. However, there are many factors that can impact a person’s elimination rate. Jones has reported that for DUI suspects, the average elimination rate was .019 per hour, and the range was .009-.029.
Breath Alcohol Testing vs. Blood Alcohol Testing
As beverage alcohol is consumed, and alcohol is collected in the stomach and small intestine, a person’s BAC will increase until the amount of alcohol in the blood is greater than the amount of alcohol in the digestive track. There will be a brief period of equilibrium, and thereafter, a person’s BAC will begin to decrease.
Because of the way alcohol moves through the body, during the absorptive phase arterial blood will be higher than venous blood. This means that a breath test will generally be higher than a simultaneous blood test. During the elimination phase, the opposite will be true, the breath alcohol test will be lower than a corresponding blood alcohol test. The only time it would be expected that both tests would be the same is during the brief period of equilibrium. Here is how Jones describes it:
During the absorption phase of the BAC time profile the concentration of alcohol in the arterial blood is higher than in the venous blood and the magnitude of the arteriovenous difference depends in part on the rate of absorption of alcohol from the gut: rapid absorption exaggerates the difference; slow absorption minimizes it. During the postabsorptive state the alcohol concentration gradients between blood in artery and vein are reversed and venous BAC now exceeds arterial and capillary BAC.
The reason this is all so significant in DUI prosecutions is that breath alcohol testing follows arterial blood whereas blood alcohol testing follows venous blood. Jones indicates that the venous breath-blood ratio is a moving target. One study has shown that arterial breath alcohol tests can overestimate venous blood alcohol concentrations by more than 100% for a significant amount of time after drinking stops.
According to this study, the maximum breath alcohol testing error found for four individual subjects was +230%, +190%, +60%, and +30%.
The scientist who wrote this study indicted that these unacceptably large deviations demonstrate that quantitative evidential breath alcohol tests are far less accurate for the absorptive state than they are during the post-absorptive state.
The differences between arterial and venous blood-alcohol profiles are time dependent and relate to the amount of time it takes were shifted in time owing to the time it takes to reach full absorption, which in turn is dependent on how long it takes for alcohol to reach equilibrium between arterial blood and tissue water. The concentrations of alcohol in arterial and venous blood are the same only when the amount of alcohol has fully dispersed in total body water.
Stowell, Toxicology/Alcohol, in Encyclopedia of Forensic Sciences (Second Edition), 2013.
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G Simpson, Accuracy and precision of breath alcohol measurements for subjects in the absorptive state, Clin Chem 1987 Nov;33(11):2130-1
Jones AW, Lindberg L, Olsson SG. Magnitude and time-course of arterio-venous differences in blood-alcohol concentration in healthy men. Clin Pharmacokinet. 2004;43(15):1157-66. doi: 10.2165/00003088-200443150-00006. PMID: 15568892.