How did they come up with that number? This is a question that very few lawyers and even fewer people accused of DUI can answer. Most know that their blood was drawn into a glass tube and that a report was issued by the Tennessee Bureau of Investigations listing a claimed blood alcohol concentration, but know very little about what happens in between. The process of blood testing can and does fill entire books, but this article should provide a good treetops view of what goes on at the TBI lab.

DUI Blood Alcohol Testing in Tennessee

Blood alcohol tests are performed by using a gas chromatograph. It’s box shaped design is to allow for it to be opened from the front just like the small oven that it is. The front panel can be opened and a “column” is installed inside. A column is a very thin flexible tube made of glass, through which the sample travels. The column connects on one end to an “injection port,” where the sample is inserted, and on the other end to a “flame ionization detector.”

The “sample,” in the case of blood alcohol testing, isn’t actually blood, its air. A very small amount of a DUI suspect’s blood is placed in a sealed vial. A needle is inserted into the vial and the air above the blood is removed. That air that is removed is the sample that is tested.

This is possible because alcohol is volatile, meaning that it will move from the liquid to the air and reach a state of equilibrium, so that the amount in the air is proportional to the amount in the blood, at least assuming everything at the lab is done properly. Once the air sample is removed from the vial it is inserted into the machine, goes onto the column and travels all the way through the column until it reaches the detector at the end.

During this travel the different substances in the sample take different amounts of time to reach the detector. That means that for example, alcohol will reach the detector in 1 minute, another substance will reach in 2 minutes, and a third will reach in 3 minutes. That is how the machine decides if there is alcohol present. Only alcohol will take a certain amount of time. Therefore, if something exits the column at that certain amount of time, the machine calls it alcohol. If it takes a different amount of time, the machine says its not alcohol. If alcohol is identified, then the amount that exits the column is measured, and compared against a known standard amount that was tested earlier to “quantify” or determine the content of alcohol in the sample.

That is a very simplified description of how a DUI blood sample is tested for alcohol. It’s a complex process and is subject to error at many stages. If a blood test has come back higher than you think it should have or if it reports alcohol and you hadn’t been drinking, it is important that you contact an attorney who understands the science behind gas chromatography and blood testing. You can reach a DUI lawyer at the Barnes Law Office to answer all your blood alcohol or drug testing questions, and any others you have. Call (865) 999-0294 or email us at

Contact us today for a free consultation!

Your peace of mind, experience, and case result drive everything we do. Taking on a new case is a great responsibility and we treat it that way. We hope you will give us an opportunity to serve you or a loved one facing a criminal prosecution or DUI.

  • This field is for validation purposes and should be left unchanged.