Breath Tests in Indiana DUI

DUI Breath Tests

DUI Breath Tests

In Indiana a breath test is the most frequently used method of measuring a driver’s blood alcohol content.  The State of Indiana uses the EC/IR II in measuring the BAC.   In fact, the Indiana State Department of Toxicology (ISDT) has 206 breath test instruments located throughout the state, and four in Hamilton County.  These instruments are inspected, serviced, and certified by ISDT breath test inspectors.

Breath Test Machines

The Indiana Department of Toxicology has rules and regulations regarding the breath test machines.  Indiana Department of Toxicology Indiana Administrative Code 260 states, “A person authorized by the department shall inspect each breath test instrument at the instrument's established location at least once every one hundred eighty (180) days” and, “Any person certified as a breath test operator must be recertified by examination at least every two (2) years from the month of certification or re-certification. It is imperative that your DUI attorney is aware and looks into these legal requirements.


Indiana DUI breath tests rely on several “scientific” assumptions including the “fact” that individuals, (regardless of their health, gender, and weight) discharge alcohol at the same rate from their blood stream into his/her exhaled breath.  Currently numerous scientific studies have shown that alcohol is discharged from the blood stream to exhaled breath at rates unique to each individual, and even differences in this exchange occurring over time with the same individual (sometimes faster, sometimes slower in the same person).

Blood Alcohol Content Results

The BAC of the defendant is a key piece of evidence in every DUI case.  Prosecutors, like Jurors do not often comprehend the unreliability of a DUI breath test.  It is easy for a juror to accept that the test must be scientifically precise or it would not be used.  A DUI lawyer will help the jury understand that the results are not 100% accurate and why reasonable doubt exists in many DUI breath tests. 

BAC Testing Method

Again, Indiana DUI breath tests rely on the “scientific” assumption that the concentration of alcohol in a person's blood is 2100 times greater than it is in the breath.  The machine accepts that 2100 ml of exhaled breath contains the same amount of alcohol as 1 ml of blood (the blood partition ratio).  The machines measure the alcohol in a sample of exhaled breath to estimate an individual’s blood alcohol content based on the 2100 ml ratio being consistent for all individuals.  However, blood and breath are rarely found in consistent ratios in humans. 

You may recall from high school health class that your lungs have small air sacs call alveoli.  The Alveoli is where your blood takes in oxygen and releases carbon dioxide.  But not all people are exactly alike.  And everyone’s body does not work exactly the same.  That goes for how an individual breathes and how his or her alveoli exchange oxygen and carbon dioxide.  In addition to in consistent blood and breath ratio in humans, our body temperature and other factors are also inconsistent between individuals. 

In the mid-1990’s scientists discovered that blood partition ratios can vary greatly between individuals from as low as 1100, to as high as 3000.  Because the “assumption” is that the concentration of alcohol in a person's blood is 2100 times greater than it is in the breath, if an individual’s blood partition ratio is less than 2100 will result in a breath test result that is inaccurately low and therefore will unlikely be found guilty of a DUI.  On the other hand, a breath test result from an individual with a blood partition ratio above 2100 will overestimate that individual’s blood alcohol content.

Why is this important

If we know that the breath test machine result will be used to convict a driver of DUI and that the test result is not 100% accurate the jury needs to be educated as to these issues surrounding the breath test itself.  This is even more important when the test result is close to the legal limit of .08.  Additionally, there are other factors that may lead to a false test result which include:

Unique Transfer of Alcohol to Blood

As we discussed earlier, each individual is unique.  While the “average” person has a blood partition ratio of 2100, the transfer of alcohol from breath to blood is not consistent between individuals.  This may be due to the density of the blood, the permeability of the alveolar membrane, or just how deeply each individual breaths.

Variations in Body Temperature

98.6 degrees is the average body temperature.  However, there is no average individual.  The breath test is dependent upon the exchange of blood and air in the lungs.  The temperature of the blood in the lungs impacts the ratio of alcohol in expelled air.  If an individual has a fever, a breath test will provide an erroneously high result.  Scientists have found that for each 1.0°C change in blood temperature results in an equivalent change of 6.5% in the concentration of alcohol in expelled breath.  So if you have a fever when your BAC is tested you are more likely to be falsely convicted of DUI. 

Increased Heart Rate

It has been shown that exercising prior to a breath test resulted in lower results but up to 25%.

Room Temperature

The temperature of the room where your breath test is conducted can affect your BAC result.  A breath test taken in cooler room will lead to a lower BAC result, while a warm room will lead to a higher BAC result. 

Carmel police officers use breath tests machines that rely on the theory that the blood partition ratio is the same in each individual.  If you have been charged with a DUI after taking a breath test do not plead guilty until you have had an attorney look at all of the evidence.


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