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Breath Test DWI

If you are pulled over for a traffic violation such as speeding or failure to use a turn signal and a law enforcement officer notices signs that suggest you may be intoxicated, you may be asked to complete a series of field sobriety tests and submit to a roadside breath test. If the breath test results reveal that your alcohol concentration was 0.08 or above or you fail field sobriety tests, you may be arrested for a Texas DWI.

As a result, you may be taken to jail and asked to perform a second test at the police department or jail. The results of this test can be used to show a driver's blood alcohol content (BAC) in court. The per se legal limit for a DWI for most drivers in Texas is 0.08% or higher. The legal limit for commercial drivers in Texas is 0.04. Minors may be charged with a Class C misdemeanor and receive a ticket and a fine if they are caught driving under the influence of any detectable amount.

Roadside Breath Tests and Chemical Testing After a DWI Arrest

It is important to understand the difference between a roadside breath test and a chemical breath test taken after arrest. A preliminary breath test (PBT) device is carried by police and used to help police gather probable cause evidence to arrest a person on suspicion of drunk driving. These tests are not mandatory in Texas and there are no penalties for refusing a PBT.

However, after an arrest, the police may ask the driver to submit a breath or blood test. This test is required under the Texas implied consent laws. If you refuse a chemical test after an arrest, your license can be suspended for 180 days on a first refusal, and 2 years for a second DWI chemical test refusal.

Admissibility of Roadside Breath Tests

When asked to blow into a breathalyzer, the machine measures the amount of air expelled into the machine. If there is an insufficient sample, the person may be asked to blow into the machine again. When the machine detects that there is enough air in the chamber for it to detect an accurate result, an infrared beam is passed through the sample. The infrared beam detects the amount of ethyl alcohol and the machine calculates the amount of ethyl alcohol in the sample to determine a person's alcohol concentration level.

Portable breathalyzers are less accurate than the breath testing devices that are used after police have detained a subject. This is one of the reasons numerical results of these roadside breath tests are not admissible in court. The requirements for a post-arrest blood or breath test are also more strict. There is generally a waiting period and observation requirement to ensure the test is more accurate.

The fact that a portable breath test was administered and that it revealed the presence of alcohol is generally admissible. However, the quantitative number reading of a PBT is generally not admissible. Only a chemical test taken after a DWI arrest is generally admissible, although those tests can also be flawed.

False Positives for Breath Tests

Ingestion or use of some substances other than alcohol can cause false positives on a breath test. Fumes from paint, cleaning chemicals, and some plastics and adhesives can also cause a machine to register a false positive. The following are some common reasons a machine could register an inaccurate reading, even if you were not intoxicated:

  • Mouthwash and some medications.
  • Acetone – This chemical may be present in diabetics and some individuals following high-protein diets.
  • Improperly maintained machine or failure to use a new mouthpiece for the test.
  • Burping or belching – This can cause a breathalyzer machine to register an alcohol concentration level that is higher than a person's true alcohol concentration level. Correct administration of the test requires the person administering it to restart the waiting period if this occurs.

Challenging Breathalyzer Results

There are several other factors that can affect the accuracy of breathalyzer tests including temperature, breathing technique, certain health conditions, improper calibration of the machine and improperly administered tests. Explaining how these factors can affect a breathalyzer test can raise a reasonable doubt about your guilt, especially if your alcohol concentration level was close to the legal limit.


Temperature is an important factor because the higher your body's temperature is, the more alcohol is diffused from your blood into your lungs. Machines are calibrated to determine an alcohol concentration level based on an assumed body temperature of 34 degrees Celsius. If your body temperature is higher than normal, this can cause the machine to register a higher alcohol concentration than your true alcohol concentration level.

Breathing Technique

Shallow breathing or holding your breath can cause a machine to register a higher alcohol concentration level than it should. Hyperventilation, on the other hand, can cause the machine to register a lower alcohol concentration level.

Health Conditions

If you have a health condition that affects your lung capacity or ability to expel breath such as COPD, this could affect the results of a breathalyzer test. As stated above, if you are diabetic or are on a high-protein diet, this could cause your body to produce acetone which is chemically similar to alcohol. If you have any health conditions or were taking any medications at the time of your arrest which could have affected a breathalyzer test, it is important to inform your attorney.

Improper Machine Calibration

Breathalyzers must be routinely checked and calibrated by an individual with the proper licensing qualifications to calibrate them. The prosecution must provide this information to the defense. If the state fails to introduce information regarding the machine's calibration at trial, this could be grounds for an objection to the introduction of the result of a breathalyzer test into evidence.

Improperly Administered Test

Breath tests must be administered according to a strict protocol. The person who administers the test must have certification to do so, and the certification must be current. The 15-minute waiting period must be observed. Failure to observe the time requirement or follow other protocol may result in a successful challenge to this portion of the state's case.

Consult an Experienced DWI Attorney in Austin, Texas

If you have been charged with a DWI in Texas and have questions about challenging the test results in your case, contact Texas DWI lawyer Jason S. English online or call (512) 454-7548.

Jason S. English has been recognized as one of the top Criminal Defense Lawyers in Austin. 

Jason S. English is an award-winning criminal defense lawyer

If you have recently been arrested in Austin for a DWI, you need the right lawyer to help you and protect your future.  No one calls us because they have been arrested, they call us because of what the arrest can do to their future.  Jason S English Law, PLLC handles misdemeanor and felony DWI cases as a part of its practice - we know how to prepare your case to meet your goals.  Call now at 512-454-7548 to get the help you need for you and your future. 


Office Location

Jason English
505 West 12th Street, Suite 201
Austin, TX 78701