DWI Chemical Testing 101

January 1, 2022 | By Shane Phelps Law
DWI Chemical Testing 101

Driving while intoxicated is a serious crime, and Texas law enforcement officers constantly look for drivers who break the law. So, how does testing work, and what happens during a DWI?


Blood alcohol concentration is the measure of alcohol in a person's bloodstream at the time of testing. The legal limit in Texas is 0.08% which means anyone with a BAC at or above the limit is breaking the law and will face the consequences. Depending on a person's BAC, they may face criminal DWI charges.



Testing BAC is done in two ways: breath testing or blood sampling. The most common type of blood alcohol testing is done with a breathalyzer which measures a person's BAC by testing their breath. Breathalyzers do not provide a precise measurement of intoxication, but they can give a close estimation of intoxication levels.

In the same way that you may smell alcohol on a person, a breathalyzer test detects alcohol concentration. The device has a small straw-like spout that the driver breaths through. The device has a limit of 0.08% and will notify the officer if the BAC is above that percentage.

Breathalyzers are portable and efficient, making them a go-to for officers on duty. However, these testing devices do not provide the most dependable measurement and may be less effective depending on the person. Everyone processes food and alcohol differently, and depending on age, weight, gender, and time the alcohol may be digested at a different rate.

Blood Testing

To remove variables around BAC, law enforcement may use blood testing to measure alcohol content more accurately. In most cases, when the police arrest someone for DWI, they often take additional steps with a blood test to get a precise BAC measurement.

As mentioned in the previous section, how the body processes chemicals are different from person to person. That said, the way alcohol is absorbed into the blood may change depending on a number of physical factors, which means that drinks are not a one-to-one ratio. In other words, one drink does not equal a percentage point.

Blood tests give a clearer picture of BAC, but they still do not account for how individuals process alcohol. In some cases, a person's BAC may increase between the initial breathalyzer test and the blood test at the police station. The inverse can also be true.


Breath and blood tests can measure BAC, but everyone has different body chemistry and composition, which changes how they digest alcohol and release it into the blood. Because of these variables, a breathalyzer or blood test is not the end of the road for your DWI case.

DWI Basics

Almost every DWI case relies on BAC to determine whether the law has been broken. If a driver hits a pedestrian, they could be tested for intoxicated driving. In other cases, BAC may be used to increase the charges in a DWI to an aggravated charge or felony offense.

However, while being accused of driving while intoxicated may seem like the end of the road, the prosecution has to rely on variable evidence that may not hold up under scrutiny. For example, even though Texas has implied consent laws, there are still unlawful searches and seizures that could take place under the guise of investigating a DWI.

Sobriety tests are evidence collection methods, but like most tools, they are not infallible. If you are accused of a DWI, don't lose hope – your case isn't over because of a sobriety test.

Protect Your Freedom

Driving is a privilege that can be lost due to a DWI. However, your case isn't lost because a breathalyzer showed that your BAC was over the legal limit – you still have a chance.

Shane Phelps Law. understands how devastating criminal charges can be, so we provide clients with comprehensive legal counsel custom fit to their unique case. Our team leaves no stone unturned, and we are dedicated to protecting your freedom.

Contact the Law Office of Shane Phelps, P.C. today for more information.