Criminal Defense Lawyer | DWI, Drug, Theft & Assault Charges

By Dallas and Collin County Criminal Defense Lawyer Jeremy Rosenthal

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There are many strategies to specifically attack either a breath or blood result, but today I’m going to compare blood and breath samples very generally.

Generally speaking — the most vulnerable aspects of either test is due to the degree that a human can either intentionally or negligently effect the outcome.

Jurors tend to have a bit of a natural skepticism against the science and technique regarding the breath test, but jurors unfortunately don’t scrutinize blood tests quite the same way.  On the other hand, the process for administering the breath test is ‘idiot proof’ whereas the procedure for taking, shipping and testing the blood is filled with human contact.

The Breath Test

The breath test is based on extracting the alveolar breath from one’s deep lungs.  That breath sample has to (1) come from the deep lung in the first place;  then (2) travel through the lungs, esophagus, and mouth, through a tube on the intoxylizer machine, and into a test chamber roughly the 1/3 the size of a coke can.  Pollutants which contain hydroxyl molecules at any place can corrupt the sample.  But, compared to the blood test, the breath test is scored right on the spot.  The operator of the breath test machine needs very little training and experience to administer the test — and they should not be able to affect the test.  If the operator makes a mistake, chances are that the machine will invalidate the result or that it will be revealed on the video of the test being taken.

Blood Draws

Challenging the chain of custody and the testing of blood can be very frustrating.  This is because the person drawing the blood, the people processing and sorting the blood samples at the lab, and the lab technicians simply don’t remember YOUR specific blood test.  Places where blood is drawn and the labs that test them are mills where they process 20, 30 or 100 different blood samples any given day.  But don’t worry — they’ll be sure to testify at trial that they never make mistakes when they draw the blood, put it on the carousel to be tested, or process it in the mail room.  The manufacturer of the blood vials puts in powdery chemicals into the vial to preserve the blood specimen.  Again, challenging the amount or quality of the chemicals can be like howling at the moon in front of a jury.

A recent opinion from the U.S. Supreme Court, Bullcoming v. New Mexico, at the very least allows defendants the opportunity to cross examine the personnel that test the blood.  In addition, it contains a far more in depth discussion of blood draws and is worth the read if you are interested.  Prior to Bullcoming, prosecutor’s were able to simply proffer a sheet of paper with the blood result which is impossible to cross-examine and a spokesperson to talk about the underlying science.  Jokingly, it is not much different that calling the receptionist at the lab who just tells the jury that in her experience “everyone is guilty.”

A Must if You’re Challenging Either Breath or Blood

Challenging the blood or the breath test, though, can only be done when the jury is told over and over that it is impossible to show where some scientific test went wrong — only that the result can’t possibly be right.  Jurors tend to have the expectation that someone can go back into a lab or into a breath test machine, recreate the exact circumstances, and prove exactly where a test went wrong.  But a good DWI trial lawyer needs to debunk that expectation and demonstrate to the jury that you can tell, for example, that a clock is wrong not by examining it by the finest timekeeper in switzerland — but because the clock says it’s night time and the sun is out.

*Jeremy Rosenthal is Board Certified in Criminal Law by the Texas Board of Legal Specialization and licensed by the Supreme Court of Texas. Nothing in this article is intended to be legal advice. For legal advice on any topic, you should consult an attorney directly.  Contacting the attorney through this blog does not create an attorney-client privilege and communications in response to this article are not subject to the attorney-client privilege.