DUI Defense – Challenge the Blood Alcohol Reading

Last Modified: August 10, 2023

DUI Defense – Challenge the Blood Alcohol Reading

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Challenging Breath Tests in a DUI Prosecution

In a driving under the influence case, the prosecution relies heavily on the results of the chemical tests performed on the defendant after he or she has been arrested for suspicion of driving under the influence. One such test is known as the “breath test”. There are actually two different tests that may be administered in the course of a DUI investigation. One is known as the preliminary alcohol screening (PAS) test and this is typically performed at the scene of the arrest. The officer is supposed to use this test in order to help determine whether there is sufficient probable cause to arrest the person for driving under the influence. The other breath test is typically given at the police station or jail once someone has been arrested for suspicion of DUI. This test is known as the evidentiary breath test.

These tests involve using machines that are designed to analyze the content of alcohol in a person’s breath and then convert that into a percentage of alcohol in the person’s bloodstream. Many people believe that this number is always accurate but that is not always the case. In fact, there are several problems with the way these tests are administered and the way the machines arrive at their results that may be challenged by the defense in a DUI prosecution.

Breath Blood Partition Ratio

The devices used in breath tests do not actually measure the content of alcohol in someone’s blood. They analyze a sample of the subject’s breath in order to determine its alcohol content. The machine is then programmed to perform calculations that supposedly convert the amount of alcohol that the device measured in the breath into the amount of alcohol in the person’s bloodstream. It is important to note that in law in California states that it is illegal to drive with a 0.08% or higher Blood alcohol concentration – not breath alcohol concentration. It is possible to use this to the defense’s advantage in a DUI case.

The breath test devices are programmed to recognize that for every one part of alcohol in a person’s breath there are 2100 parts of alcohol in that person’s blood. This ratio (sometimes referred to as “partition ratio”) is not the same for everyone. It simply represents an average ratio. In fact, people display a very large range of partition ratios. They range from 900:1 to 3400:1. The machine does not compute the partition ratio of the individual subject. It simply uses an average based on a large sample of the population and then uses that average to compute a result.

If an individual’s actual ratio of parts of alcohol in his or her blood to his or her breath is less than the 2100:1 average rate used by the device, the device will actually artificially inflate the blood alcohol level for that test.

Impact of temperature on Breath Alcohol – Henry’s Law

There is another assumption made by the testing device that may result in inaccurate results. The temperature of the sample tested by the device will affect the results that the machine outputs. A scientific principle known as “Henry’s Law” states that in a closed system, like the human body, the amount of a volatile substance contained in a gas in contact with liquid is directly related to the volume of the subject dissolved in the liquid and temperature. In other words, if the heat in the system is increased the results will also increase.

Again, the machines are programmed to use an average temperature of 34 degrees Celsius. However, one size does not fit all. The person’s breath may have been at a different temperature which could lead to unreliable results. In fact, a study published in the Journal of Forensic Science reported a breath alcohol increase of 8.6% for every one degree Celsius increase in core body temperature. This will result in a falsely high reading. Many machines do not record the temperature of the individual tested, they just use an average value.

It is important to remember that temperature is a variable that can not only affect the reliability of the results when the test was administered to the defendant, but it also can impact the effective calibration of the breath machine. This is because the testing and calibration of the machine involves the use of a simulator solution that contains a certain percentage of alcohol. If the temperature of the calibration solution is not exactly correct, the machine will put out unreliable results.

A skilled defense attorney can use this to cast reasonable doubt on the accuracy of the breath test results.

Mouth Alcohol and Slope Detector

The term “Mouth Alcohol” refers to any alcohol that remains in someone’s mouth due to recent ingestion, alcohol being trapped in cavities, or any other way that the alcohol remains in someone’s mouth after drinking it. If this mouth alcohol is blown into the breath test device, the machine will include this mouth alcohol in its measurement leading to an artificially high BAC reading.

The police are supposed to take steps to account for this possibility but they do not always do so. These steps include:

  • Observing the subject for at least 15 minutes prior to administering the test. This is to help ensure that the subject does not ingest any alcohol and to make sure that the subject has not belched, vomited, or regurgitated any alcohol from his or her stomach into his or her mouth. A skilled defense attorney may be able to attack the observation period reported by the officer or in some cases, show that the officers did not even observe the subject for the appropriate amount of time.
  • Agreement of tests – Officers are supposed to collect and test at least two samples of the subject’s breath. These tests should be conducted a few minutes apart. If the two tests disagree by more than .02% they should be run again because that would suggest the presence of mouth alcohol in the system. Officers do not always follow this procedure and the defense can use this to its advantage.
  • Slope Detector – Some breath test devices contain a feature referred to as a “slope detector”. It is supposed to help guard against the test results being affected by mouth alcohol. The slope detector is supposed to check for a rapid fall-off in the breath alcohol readings as that may be a sign of the presence of mouth alcohol in the system. These slope detectors do not always work reliably and may or not have been working properly at the time the test was administered. The defense can present evidence of the unreliability of the slope detector which can cast reasonable doubt on the prosecution’s evidence.

The main thing to take away from this discussion is that the breath test results are far from infallible in a DUI case. There are numerous ways to challenge the reliability of the results including problems with the technology and problems with the methodology employed by the arresting agency. Even medical conditions such as diabetes, hypoglycemia, GERD, and certain diets can affect the results and reliability of breath tests. If you are arrested for DUI it is critically important that you retain the services of an attorney who can analyze your specific case and use the uncertainty of these test results to help beat your case.

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