How Did They Come Up With My BAC?  How Does DUI Blood Alcohol Testing Work? (My BAC is too high!)

These are questions 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.”

DUI Blood Alcohol Testing Machine - Gas Chromatograph
This is a Gas Chromatograph made by Agilent. Its the machine used by the Tennessee Bureau of Investigation to test blood samples for alcohol content.

Your Blood is Never Actually Directly Tested!

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.

DUI Blood Alcohol Testing - Headspace Vials.
These are “headspace vials.” They are where blood is deposited before having the air above the blood injected into the gas chromatograph.

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.

The Most Important Point About Gas Chromatography is That the Machine Itself Can’t Tell Us Anything

The machine has to be “taught” what its looking for and where.  The process of teaching the machine is filled with potentials for human and machine error.  Gas chromatography, when done correctly, is a legitimate and well respected testing mechanism, but like any other machine, its not magic and it can be wrong.

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 Firm to answer all your blood alcohol or drug testing questions, and any others you have. Call (865) 805-5703 or email us at