Motorists charged with drunk driving should understand that a Breathalyzer -- a blood-alcohol concentration (BAC) test -- is but one of several types of evidence that may be used in court. Passing the test does not guarantee acquittal, nor does failing such a test guarantee conviction. The following article discusses the role BAC tests, particularly the ubiquitous Breathalyzer, play in a DUI case.

FindLaw's Sobriety Tests section contains additional articles and resources to help you better understand the law.

Evidence in a DUI Case

Research by the National Highway Traffic Administration has identified 20 symptoms that may indicate a person is driving impaired. These driving patterns give police probable cause to stop a vehicle and further investigate for DUI. Sworn testimony from an arresting officer and video footage of the vehicle prior to the stop may not be compelling evidence by itself, but may serve as the basis for a case.

Statements from the driver are also admissible evidence. When asked how many drinks you've consumed (unless you've had nothing), it is wise to politely decline to answer questions until you've spoken to an attorney. That is your right. Admitting to having any number of drinks will be included in the police report, and will be used against you in court. Also, if you are excessively hostile or jovial, your demeanor will weigh against you in court.

BAC Testing and Implied Consent

When asked to submit to a BAC test, you are required to comply under implied consent laws. If you refuse to submit to any chemical test -- breath test, blood test, or urinalysis -- this refusal typically will be admitted in court as "consciousness of guilt" and result in an automatic suspension of driving privileges in most states. Failing the test with a BAC of .08 or higher will result in immediate arrest. Additionally, state laws usually allow a driver to be charged with DUI for a BAC between .05 and .07 (if other evidence shows impairment).

How Breathalyzers Work and Sometimes Fail

The Breathalyzer (the brand name of an early breath-testing device) measures alcohol vapor in the driver's breath, and is the most commonly used chemical test for DUI suspects. The equipment has several drawbacks, however, so the results are not generally enough to convict without other evidence supporting intoxication.

A Breathalyzer presumes the entire volume of air being measured is from the driver's lungs. It is therefore susceptible to falsely elevated measurements from any alcohol trapped in the mouth or esophagus. Alcohol may be retained in dental work, such as bridges or caps, and may remain in the esophagus after vomiting or due to any sort of gastric reflux disorder. Mouthwash, breath mints and other products may also contain alcohol and cause an erroneous BAC reading.

It is important to note that alcohol is not instantaneously absorbed into the blood, so BAC results can sometimes be refuted with a "rising BAC" defense. A test that is delayed up to an hour after the initial stop may indicate intoxication, even though the alcohol had not been sufficiently absorbed at the time of driving to cause impairment. This argument works both ways, depending on the delay between the driver's last drink and the administration of the test (since an impaired driver can sober up while waiting to take the test).

Regardless, the driver must be proven intoxicated while actually driving.

Defenses to Field Sobriety Tests

Police may administer a variety of field sobriety tests (FST), often recorded on video. These tests are typically administered after an officer has decided to make the arrest for the sole purpose of building a more solid case. This supporting evidence may be used to counter claims that a breath test was inaccurate due to improperly calibrated equipment or other anomalies. Field sobriety tests are intended to highlight clear physiological or mental impairment, but they can be misleading if the driver has other disabilities that may interfere with passing the tests.

Police officers are not medical professionals and therefore not qualified to expertly judge certain physical cues. For example, the horizontal gaze nystagmus test (requiring a driver to visually follow a pencil, finger or penlight) may be inadmissible in court because an officer cannot reasonably determine nystagmus with his or her limited expertise. This is the sort of evidence that may lead to conviction despite a passing BAC test result.

Also, you may have the right to request an independent blood test, the most reliable measurement of BAC. Results may be used to defend against Breathalyzer results that might otherwise lead to conviction.

Get Legal Help if You've Been Charged with a DUI After Passing a Breathalyzer Test

Breathalyzer tests, field sobriety tests, being over the legal limit -- the laws surrounding DUIs can be confusing. Understanding the consequences of a DUI conviction, whether a first-time offense or a third, can mean the difference between a long jail sentence and a possible plea bargain. For this reason, it's important to speak with a local DUI attorney to learn how DUI laws apply to your case and to have an advocate on your side.