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Court Finds Breath Test Machines May Overstate True Alcohol Level

by ptbarone on July 10, 2009

I have written previously about some DUI defense lawyers being unethical.  Here is an example based on a lack of knowledge.  A client comes into the office with a breath test result of let’s say 0.18.  The attorney asks the client how much they had to drink, and the client answers that they had 3-4 drinks.  The lawyer looks at the 0.18 and concludes from the high level that the client is lying. 

Based on this possibly mistaken belief that the client is lying the unethical lawyer fails to even consider any defense to the breath test results.  This failure is in part due to the lawyer’s lack of knowledge.  The lawyer then convinces the client that they have no defense and promptly takes the client into court and stands next to them as the plead guilty to a crime they may not have committed.

While it is certainly true that people sometimes under-report how much they drank, it is also certainly true or at least certainly possible, that the client is being honest and that the breathalyzer is lying.  The ethical DUI defense lawyer will consider the possibility that the client is right and the test is wrong as he investigates the facts and circumstances of his client’s case.

I have long known that breath test results are not always what they seem to be, and because of this, I know that there are dozens of ways to defend a breath test result in a drunk driving case.  One of them is based on the idea of partition ratio.

Breath test machines across the nation are programmed to “assume” that the driver has a partition ration of 2100/1, meaning that for every one molecule of alcohol in the breath there are 2100 in the blood.  Thus, the amount of alcohol in a person’s breath is multiplied by 2100 to achieve a blood alcohol level.

According to Professor Srikumaran K. Melethil, Ph.D., the determination of BAC by use of a breath test, is by far the most popular scientific test for drunk driving. The breath test involves the measurement of alcohol in an appropriate sample of breath, expired alveolar air. (Alveolar air is that part of the expired air, which is in equilibrium with blood; usually this is taken as the terminal portion of expired air. One likely reason for the high variability observed in partition ratios is the difficulty in obtaining true alveolar, or deep lung air for analysis). This breath alcohol concentration is then multiplied by a factor called the partition ratio to convert the concentration measured in the breath to the corresponding alcohol concentration in the blood. In most jurisdictions, a value of 2100 is used for this ratio by statutory mandate. However, this partition ratio of 2100 can differ from individual to individual or differ in a given individual from time to time. Therefore, while it is quite simple to perform, the use of breath tests to determine BAC suffers from a major and fundamental weakness in that it is an indirect method.

The problem is that some courts, including those in California, said that defense lawyers could not argue partition ratio.  By doing so the courts essentially said obtaining convictions in drunk driving cases is more important than the scientific reliability of the breath test results.  However, in a new case the California Supreme Court has said that partition ratio can be argued.

According to a news report entitled “California High Court: Breath Test Accuracy Can Vary

The scientific problem is that breath-to-blood ratios vary greatly throughout the population and fluctuate individually, influenced by such factors as body temperature, atmospheric pressure, medical conditions and the precision of the measuring device.

That means the same breath-test result for one person’s breath could signal intoxication while for another it could simply mean “just a glass of wine with dinner, officer.”

Compounding the matter are California’s two distinct driving under the influence laws.

The first law, which has been on the books for decades, requires proof that a driver was intoxicated such as slurred speech, bloodshot eyes and the like. Jurors are told they can presume someone is drunk if blood tests show at least a .08 percent level of alcohol.

The second law, passed by the Legislature in 1981 and updated in 1989, simply defined a drunken driver as someone with a blood-alcohol level of .08 percent regardless of appearance or behavior. The state Supreme Court in 1994 extended that definition to include Breathalyzer results, barring drivers charged with the second law from attacking the variability of the breath tests.

That is no longer true, and until California’s law is changed again, defense attorneys will be able to argue partition ratio in their client’s cases.  This despite the fact that prosecutors believe that it will confuse jurors and that the “California ruling will seriously hamper their ability to win convictions in driving under the influence cases.”

I guess the prosecutors believe that the true science of breath testing is simply too confusing and that jurors should wrongfully be told that breath testing is presumed to be scientifically accurate.  After all, if defense attorneys are allowed to confuse jurors by telling them the truth then these prosecutors might fail to convict those wrongfully accused of drunk driving. 

As an Oakland County prosecutor was once heard to say “it’s easy to convict the guilty, the challenge is in convicting the innocent.”  In a California drunk driving conviction this challenge is even greater, and that’s  just as it should be.


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