Best DUI Defense Nine: Inaccurate Breath / Blood Testing

In Michigan, in order to prove their UBAL (unlawful bodily alcohol level) theory of OWI, the prosecutor must prove beyond a reasonable doubt that your breath or blood test was in fact accurate, precise and reliable.  There are literally dozens of reasons this may not have been true in your case.  Here is a brief explanation: 

  1. 1. Inaccurate Chemical Test Instruments

The machine used by the police to test a person’s breath throughout the state of Michigan is called the DataMaster.  Like any machine used to take measurements, it is simply not possible for it to be 100% accurate all the time.  The DataMaster is subject to inherent error and scientific limitations. Like the rising blood alcohol defense, a through and detailed explanation of breath testing is beyond the scope of this writing.  However, here is a brief outline:

Breath testing is based on infrared testing of a breath sample expelled into a sample chamber. Once the sample acceptance parameters are met (see below) the sample chamber closes, and an infrared light is passed through the chamber and its intensity measured as it exits the other end. After it passes out of the sample chamber, the light is “pulsated” meaning changed from DC to AC by way of a chopper wheel. There are also two filters and a focusing lens. Then there is a detector that measures the amount of alcohol in the chamber by determining how much is “absorbed” by the alcohol, that is, how much the IR signal is decreased.

Relative to the “inner workings” of the DataMaster, the machine tracks the breath using a device called a thermister. The Datamaster uses this device to track the length and strength of the blow according to the sample acceptance parameters, and will refuse the sample if it does not conform to these requirements. There is nothing to suggest that this “internal check” was working at the time of the test in this case. Relative to the sample acceptance criterion, there are four of them, and these include; that the driver must have maintained the minimum flow rate (1), then, while meeting this minimum flow rate, must have provided a MINIMUM of 1.5 L (2)  the rate must have fallen below the minimum (3) while not triggering the slope detector (4), meaning the slope leveled off (as represented by blue line).  The leveling off again suggests deep lung air has been expelled into the machine.  Once all four are met, the sample chamber is closed, and a final reading is taken.

Armed with a sophisticated understanding of the science behind breath testing it is possible to devise dozens of viable breath test defenses.

When blood is drawn as part of a drunk driving investigation in Michigan the blood sample will be sent to the State Police Laboratory for testing.  At the lab the State Police use whole blood GC/FID[1].  It is generally well accepted that GC/FID is a very sound testing protocol with the potential to have a high level of scientific integrity.  But again, like breath testing, blood testing is hardly unassailable.  This topic is also discussed in detail in Defending Drinking Drivers.  It is also discussed in detail in Mr. Barone’s Michigan Bar Journal Article: Understanding Quantitative Blood Testing in Drunk Driving Cases.

While the DataMaster and the GC/FID are the two most common methods of testing in a Michigan OWI case, it is also possible to have a drunk driving case based on a serum blood test or even an electrochemical breath test (as with the PBT).  Both serum blood testing and preliminary (electrochemical) breath testing are completely insufficient for forensic purposes.  If you face a drunk driving case based only on a serum blood test be sure to file a motion to seek that the serum test be suppressed.  A sample motion is included in the materials included with Defending Drinking Drivers.

Keep in mind also that while it is usually necessary to retain an expert to help educate the jury regarding these potential inaccuracies of the breath or blood test instruments, retaining an expert is no substitute for obtaining your own understanding of the science behind these tests.


[1]GC/FID stands for “gas chromatography/flame ionization detector

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 Best DUI Defense Nine: Inaccurate Breath / Blood Testing

Patrick T. Barone

Principal at Barone Defense Firm
Patrick T. Barone has successfully handled 1000s of Michigan DUI cases. He has an “AV” (highest) rating from Martindale-Hubbell, and since 2009 has been included in the highly selective US News & World Report’s America’s Best Lawyers while The Barone Defense Firm appears in their companion America’s Best Law Firms. He has been rated “Seriously Outstanding” by SuperLawyers, rated “Outstanding/10.0” by AVVO and has recently been appointed to the advisory board for the Michigan edition of Leading Lawyers Magazine. He is also the author of two books on DUI defense including the well-respected two volume treatise Defending Drinking Drivers, and Michigan DUI Law: A Citizen’s Guide. He had also authored chapters in Defending DUI Vehicular Homicide Cases, 2012/2014 eds. (Aspatore Books). Mr. Barone formerly served as the executive editor of The DWI Journal: Law & Science (Whitaker Newsletters, Inc.). Mr. Barone is also an adjunct professor at the Thomas M. Cooley Law School where he teaches Drunk Driving Law and Practice, on the faculty of the Michigan Trial Practice College, and is a graduate of the Gerry Spence Trial Lawyer’s College.
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