The breath test for drunk driving involves measuring the amount of alcohol in an appropriate sample of expired air. While it is quite simple test to perform, the use of breath tests to determine blood alcohol suffers from a major and fundamental weaknesses 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 variables, 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 appear to generally 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. In Washington, we have successfully challenged the admissibility of breath test results because of incomplete or misleading implied consent warnings. 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 a breath test result, however, the prosecutor's case loses much of its clout and the defendant stands a better chance of an acquittal.