How a Breathalyzer or Blood Test Work

Posted: July 30, 2015 Category: Legal Update

It’s common knowledge that when you are pulled over for DWI, you will be tested, either through a breath test or a blood draw, to determine your overall blood alcohol content (BAC). But how exactly do those tests work? We’ll break down the basics behind both testing methods.


When a person drinks, it shows up in their breath as it gets absorbed from the mouth, throat, stomach, and intestines into the bloodstream. As blood goes through your lungs, some of the alcohol moves across the membranes of the lung’s air sacs, or alveoli. After leaving the alveoli, the alcohol goes into the air and evaporates.

The concentration of alcohol in the air exhaled from your lungs and alveoli is related to the concentration of the alcohol in the blood and detectable by any type of breath testing device.

There are three major types of chemical breath tests used among law enforcement officers, each based on different principles:

  • Breathalyzer: Shows results through a chemical reaction involving the alcohol in the breath to produce a color change in the test device.
  • Intoxilyzer: Detects alcohol by infrared (IR) spectroscopy to display results.
  • Alcosensor III or IV: Detects a chemical reaction of alcohol in the breath in a small fuel cell in the device.

Due to the differences between these types of breath tests, the results may be challenged in court, based on accuracy and degree of training required to accurately interpret test in the field.

Blood Test

If an officer feels that a chemical breath test was inaccurate in its measurements in the field, he may request a blood test to get a more accurate BAC measurement. Because of Texas’ implied consent law, you are required to consent to the draw; if not, you could face additional fines and punishments.

Blood tests are considered the most accurate readings of blood alcohol levels. Their accuracy can be affected by the time between drinking and when the blood is taken, so the test must be conducted as quickly as possible.

There are a few cases where BAC levels could be misleading. These cases may involve those who have diabetes or high blood ketones, or anyone who has taken cough medicines or herbal supplements. BAC levels could also be misread depending on whether whole blood is tested or just the blood serum. Serum readings could produce BAC levels up to 25% higher, so be aware of how your test is conducted.

If you feel that your breath test or blood draw was conducted inaccurately, call the Law Office of Carlo Key today. He will fight any inaccuracies in your test to get you a fair verdict.

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