DWI Blood Testing

Blood testing is a huge part of DWI Evidence. That is just a reality. DWI blood testing in Fort Worth employs a process known as gas chromatography. This process separates the different parts of your blood to identify the relative quantities of each compound. That number that the jury sees is arguably the most deadly evidence against you. Your lawyer must raise serious doubt about that number. One effective way to do that is if you have a good video of you performing the field sobriety tests.

How can the alcohol in DWI blood testing be identified from all the other stuff in your blood? This article is about the analytical testing of blood. For more on how the blood is drawn, click here. For more on breath tests and how those work, click here and here.

The Blood Testing Process

The blood testing procedure actually tests the air above the blood and not the blood itself! The area about the blood is called called the headspace. According to Henry’s Law, when a volatile chemical is dissolved in a liquid inside of a closed air space, there quickly develops a certain ratio between the concentration in the liquid and in the air. Therefore, you can measure the chemical in the liquid by measuring the air. Alcohol is a volatile chemical, and when it is in a liquid, your blood, by testing the gas or the air above the liquid in the tube, you can accurately measure the alcohol in the blood.

The Blood Testing Machine

The gas chromatograph machine is large and processes a batch of many different samples at one time. Here is what it looks like:

The gas chromatograph processes many vials of blood at the same time

When the process begins, a DWI blood testing sample is heated rapidly until it vaporizes. The vapors are transported through a column by which the components are separated and identified based on their boiling points. After time, the heat applied to the compounds will separate them. They are represented individually on a graphic printout called the chromatograph. At the end of it all, your DWI blood testing is reflected in a graphic printout, detailing the concentration of ethanol and other compounds in your blood.

It looks like something like this:

A printout will show the results of a blood alcohol test

Batch Testing

DWI blood testing analyzes vials of blood in batches. A batch consists of many vials all of which look identical. The only thing which determines your blood from someone else’s is a number, often written on a label, sometimes penned in sharpie on the vial. This can be a troubling thought. What if they get it mixed up? The accuracy of your DWI blood test can rely on the analyst or lab technician putting many identical looking vials in the same order. It is important for defense counsel to challenge all aspects of the blood test.

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