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Challenging the Reliability of Breathalyzer Test Results

In all drunk driving cases, the prosecution must prove that the defendant’s blood alcohol concentration at the time of the offense was at or above a statutory limit. In many states, the limit is .10 percent, but in others it is .08, and there is a national movement afoot to make .08 the limit in all states. In order to prove the requisite level of alcohol in the blood of someone arrested for drunk driving, it is necessary to obtain a suitable sample of the arrestee’s blood, urine, or hair at the time of arrest. The use of a breath test is by far the most popular scientific method for establishing that drunk driving has occurred. Some defendants have been able to challenge the results of such tests in court successfully, thereby preventing a conviction. An attorney experienced in drunk driving defense law is in a strong position to advise a client whether the "Breathalyzer" test results may be subject to challenge in his or her particular case.

Breath Test Results Can Be Challenged on Several Bases

The breath test for drunk driving involves measuring the amount of alcohol in an appropriate sample of exhaled air. While the test is quite simple to perform, the use of breath tests to determine blood alcohol suffers from a major and fundamental weakness in that it is an "indirect method" and thus subject to variability. The temperature, the atmospheric pressure, the chemical composition of the alleged offender’s blood, his or her physical activity, and hyperventilation are just some of the factors that can lead to variable, and thus arguably questionable, results. Some courts have allowed drunk driving defendants to present expert testimony of the scientific inaccuracy of Breathalyzer test results based on these inherent variables.

Test results can also be challenged on the basis of the circumstances of the particular testing at issue, including the skill and experience of the tester and the quality of the particular equipment used. Breath testing machines are also subject to error when the person being tested has been recently exposed to fumes, such as lacquer, paint, gasoline, or dry cleaning fluids. Although some courts have demonstrated a willingness to accept challenges to blood alcohol test results on many grounds, others are more selective and generally appear to accept the test results at face value. An experienced drunk driving attorney knows which challenges are likely to be accepted in local courts and which are not.

In some states and under certain circumstances, breath test results may not be admissible at all, such as when the arresting officer fails to give the required implied consent warning. Such a warning is not always required, however, and is simply written into the drunk driving statute; in those states, consent to testing is implied by everyone with a driver’s license. If the Breathalyzer results are inadmissible for any reason, the prosecutor will have to prove the case based on other evidence, such as the results of field-sobriety tests, if given, or other eyewitness testimony. Without scientific evidence, however, the prosecutor’s case loses much of its clout and the defendant stands a better chance of the charges being dropped.


Many people wrongly assume that if they blow an above the limit reading on a Breathalyzer they are as good as convicted of drunk driving, but that is not always the case. In some cases, the results of Breathalyzer tests can be challenged on general scientific grounds, and in others, the circumstances of the particular test can be brought into question, raising a reasonable doubt about the drunk driving defendant’s guilt. A skilled and knowledgeable drunk driving lawyer can explain what challenges are appropriate in a particular case and possibly even keep the test results out of court entirely.

DISCLAIMER: This site and any information contained herein is intended for informational purposes only and should not be construed as legal advice. Seek competent legal counsel for advice on any legal matter.

Learn More:

Drunk Driving DUI
Drunk Driving/DUI – An Overview
Parole & Probation in Drunk Driving Cases
The Prosecutor’s Role in a Drunk Driving Case
Challenging the Reliability of Breathalyzer Test Results
Frequently Asked Questions about Drunk Driving / DUI
Drunk Driving / DUI Resource Links

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