Wednesday, November 9, 2011

Blood or breath?

Is a blood test more accurate than a breath test? Is either one reliable when it comes to proving whether a motorist had an alcohol concentration of .08 or higher?

The answer in Austin may very well be no and no.

An Austin defense attorney reported that his client had a blood sample taken on a No Refusal weekend. That sample was tested by the Austin Police Department's crime lab. The result came back showing his client had an alcohol concentration of .10. The defense attorney had the blood retested by the Southwestern Institute of Forensic Science in Dallas. Their test showed an alcohol concentration of .08.

What happened?

While it is true that as alcohol metabolizes in the body the alcohol concentration decreases, when blood is stored in a sealed tube there is no way for the alcohol in the blood to metabolize. In fact, it is more likely that the alcohol concentration would increase due to contamination of the blood.

First a little primer on blood testing. When a blood sample is drawn in a DWI case, the person drawing the blood typically makes two draws in two separate tubes. One tube is the "state's" tube and the other tube is there in case the defense wants to retest it. The technician in the lab will prepare two samples from the tubes for testing in a gas chromatograph. This gives us four test results. Typically the lowest result is the alcohol concentration used at trial.

In Austin, however, they average the four results to come up with an alcohol concentration. That raises questions of which blood sample is the actual sample to be used at trial. It also raises questions about the validity of the method by which APD determines alcohol concentration.

APD officials weren't concerned about the 20% discrepancy in the test results. What's a couple hundredths of a gram anyone, right?

APD claims that the differing results could have been caused by evaporation of the alcohol in the test tube -- except that the tubes are supposedly sealed and refrigerated to prevent degradation of the samples. And what about that pesky Henry's Law that states whenever you mixed water and alcohol in a closed system, the concentration of alcohol in the water and the concentration of alcohol in the headspace is the same? A sealed test tube is a closed system.

Just a little something to think about.

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