One of the most important things to understand about DUI arrests is that chemical tests used to determine your blood alcohol concentration are not foolproof. Although chemical tests may give a quantitative number to an individual's blood alcohol concentration (BAC) level, that number does not directly correlate with an individual's level of intoxication or impairment.

What Is Blood Alcohol Concentration?

BAC is the amount of alcohol present in a person's body at a given time. Massachusetts law defines BAC as the "percentage, by weight, of alcohol in a person's blood, as measured by a test of the person's breath or blood." G.L. c. 90, §24. A person’s BAC is measured by the number of grams of alcohol per 100 milliliters of blood; or the number of grams of alcohol per 210 liters of breath. 501 CMR 2.02. Alcohol is quickly absorbed into the bloodstream, and is mostly broken down by the liver, which is a slow process. For most adults, a blood alcohol content of more than 0.40% percent is lethal. These tests are not as precise as you may think—although they are able to measure the presence of chemicals in the body, these tests may not accurately differentiate between types of alcohol.

How is Alcohol Absorbed?

Alcohol is generally ingested by way of the mouth. It travels to the stomach where it is held until the brain identifies the stomach's contents to determine if digestion is needed. Alcohol does not require digestion and can be released immediately. If consumed with food, the body will hold the alcohol in the stomach with the food until the food has been properly digested for nutritional absorption. Once released from the stomach into the small intestines, the alcohol is readily absorbed into the bloodstream and carried throughout the body.

Alcohol will be deposited in various organs in proportion with the organ’s water content. The brain has a very high water content. Hence, there will be a lot of alcohol deposited into the brain. Additionally, alcohol, as a “slightly soluble gas,” will be deposited into the lungs, where it can be exchanged between the blood and the breath.

How is My BAC Determined?

The blood to breath ratio is used as a conversion factor to translate the amount of ethanol (alcohol) in a breath sample into a quantitative blood alcohol concentration. The blood to breath ratio, based on Henry’s Law, is a ratio of the amount of alcohol in the blood and the amount of alcohol in the breath. Henry’s Law predicts how gasses will dissolve in the alveoli and bloodstream during gas exchange. Essentially, the amount of molecules of a volatile substance (i.e. alcohol) in a liquid (i.e. blood) will be represented by a proportional amount of the same molecules in the air immediately “above” the liquid (i.e. in the lung). In any given volume of liquid there will be a much greater concentration of molecules than in the same volume of the air above.

The standard conversion ratio of 2100:1 is programmed into the breath testing device and used to calculate BAC, which is then used as the evidence to arrest and charge an individual with OUI. This ratio, however, has not been found to be reliable by the scientific community. In fact, studies have shown wide variations in conversions, ranging from 1500:1 to 3000:1. Important factors that create variations include age, gender, genetics, and level of intoxication. Variations also exist between breath testing devices. These variations result in a significant under or overestimations of BAC that could have serious ramifications for the individual being tested. There is no agreement in the scientific community as to a suitable, reliable conversion ratio to determine BAC.

Moreover, the breath test result may itself be influenced by various factors. Simple differences in body temperature (such as having a fever) or breathing patterns (such as holding one’s breath or brief hyperventilation) can produce inaccurate BAC readings.

How quickly your body processes and eliminates alcohol can also influence a BAC result. This is known as the elimination rate. Age, gender, and drinking experience all influence elimination rate. Older people, women, and more experienced drinkers have higher elimination rates than younger people, men, and less experienced drinkers. Your elimination rate becomes important in calculating BAC when a blood or breath test is done some time after an arrest is made. Since the Commonwealth must prove you were under the influence at the time of the offense, elimination rates may be used to adjust a BAC result based on the amount of time passed. Variations in elimination rates are not taken into consideration, and the calculated BAC may not be accurate.

Retrograde Extrapolation and BAC

Retrograde extrapolation is a mathematical calculation used to estimate a person's blood alcohol level at a particular point in time by working backward from the time the blood alcohol level was tested and factoring in rates of absorption and excretion. Commonwealth v. Colturi, 448 Mass. 809 (2007). The length of time necessary for alcohol to be absorbed into a person's bloodstream depends on a variety of factors, including the presence and type of food in the stomach; the person's gender, weight, age, mental state, and drinking pattern; the amount and type of beverage consumed; and the time period of alcohol consumption. When the alcohol reaches the brain and nervous system, the characteristic signs of intoxication begin to appear. At some point after drinking has stopped, the person's blood alcohol concentration will reach a peak. After the peak, the concentration will begin to fall as alcohol is eliminated from the person's body.

Depending on where your body is in the process of absorption or excretion, your BAC may be rising to its peak or falling towards elimination. If a motorist is stopped on suspicion of OUI and subsequently arrested, it is possible that they have a higher BAC level when they submit to a breath test later at a police station than the BAC they had at the time they operated a motor vehicle. This could occur if the motorist’s BAC level was still rising towards the peak when they were operating. But it is important to remember that the law punishes for one’s impairment at the time of operation, not at the time of testing. Therefore, presenting an expert to testify about retrograde extrapolation and its relation to your BAC level may be critical for an OUI defense.

In Massachusetts, the Supreme Judicial Court has permitted the prosecution to admit evidence of a BAC level that was obtained within three hours of an arrest without having to present an expert to testify about retrograde extrapolation. Commonwealth v. Colturi, 448 Mass. 809 (2007). However, if the BAC level is obtained after that three hour window, the prosecution must present an expert in order to admit evidence of a BAC level. Depending on the prosecution’s ability to admit the BAC test at trial, it may be in a defendant’s best interest to present an expert to explain how a BAC level may have been lower at the time of operation.

What are Interfering Substances?

Interfering substances are substances other than ethanal that may interfere with the breath sample. The Draeger Alcotest 9510 machine utilized in Massachusetts uses two types of technology to obtain an alcohol measurement - Infrared Spectroscopy (IR) and electric chemical fuel cell oxidation (EC). The IR reads the alcohol measurement while the EC is used to verify the IR reading and detect interfering substances. There are specific limits on how closely the results of the two tests must be in order for the results to be considered valid. Some chemicals, other than ethanol, could be measured and reported without triggering the EC’s interferent detection system. Some examples of potential interferents include 1-Butanol (n-butanol, butyl alcohol), which is a natural product of fermentation present in foods and beverages, and Butyl Acetate, found in fruits and synthetic fruit flavorings.

Does BAC Determine My Level of Intoxication?

At best, a chemical BAC test may only offer speculation as to an individual's level of intoxication. BAC tests may not account for many of the situational and biological factors in a situation. Differences in body height, weight, metabolism, and other factors play an essential role in how an individual processes alcohol, but this is not accounted for in the results of a BAC test.


When it comes to a DUI charge, these factors may play a vital role in your defense. The Massachusetts DUI lawyers at The Law Offices of Joseph D. Bernard P.C. understand how important it is to take these factors into account when preparing a defense for the firm's clients.

Contact The Law Offices of Joseph D. Bernard P.C. for a free case consultation!