DUI Defenses: Excluding and Disputing the Accuracy of Breath and Blood Tests

How you might defend against a DUI charge by challenging breath- or blood-test results.

DUI charges come in two types: charges based on actual impairment and DUI “per se” charges. To prove an impairment DUI, the prosecution typically relies on the arresting officer’s observations. For example, the officer might testify the driver had slurred speech and failed field sobriety tests. (Read about defending against an impairment DUI charge.) With a per se DUI charge, on the other hand, the focus is generally on the amount of alcohol in the driver’s system. Drivers with a blood alcohol concentration (BAC) of .08% or more are deemed per se under the influence.

Proving a per se DUI charge involves producing chemical-test results showing the driver was above the legal limit. Implied consent laws generally require all drivers lawfully arrested for driving under the influence to submit to testing. So, in many cases, the driver will have taken a blood or breath test.

Here are some of the more common ways defense attorneys attack DUI-test results.

Challenging Breath-Test Results

Modern breathalyzers are fairly accurate. However, they aren’t perfect, and not following proper procedures when using a breathalyzer can lead to significant error.

All breath-test devices have an inherent margin of error. With many devices, the results can be off by as much as .01%. So, a driver who’s just barely above the legal limit may have a good argument—based on margin of error—that the prosecution can’t prove the charge beyond a reasonable doubt. In other words, there’s no way to know whether the excessive BAC was a true result or attributable to the inaccuracy of the machine.

A tainted breath sample can also cause an artificially high BAC reading. Breathalyzers are designed to measure the concentration of alcohol from the vapor in a person’s lungs. However, to get an accurate measurement, no alcohol (or substances registering as alcohol) can be in the person’s mouth. Otherwise, the breath-test results will reflect a combination of the person’s lung and mouth alcohol, which can be significantly higher than the person’s actual BAC. To avoid this problem, officers are supposed to observe the driver for a 15-to-20-minute waiting period prior to administering a breath test. In cases where the officer doesn’t adhere to the waiting-period procedure, the defendant might be able to raise questions about the accuracy of breath-test results.

Challenging Blood-Test Results

Generally, blood tests are more accurate than breath tests. But a skilled defense attorney might still be able to cast doubt on blood-test results or find a way to get the results excluded from evidence.

Fermentation. One of the more common sources of blood-test errors is improper storage. Blood that isn’t stored properly after collection can ferment. Fermentation can increase the alcohol concentration in the sample and lead to a misleadingly high BAC measurement. With a good understanding of blood-testing procedures, an attorney can hone in on any errors that may have led to inflated BAC results.

Warrantless blood draws. Implied consent laws impose administrative consequences (license suspension) for refusing to submit to DUI testing. However, the Supreme Court has said that because blood tests are so invasive they implicate a criminal defendant’s constitutional rights. Therefore, police generally need a warrant to require a blood test. If police obtain your blood without a warrant, you might have a legal argument that the results should be thrown out.

Rising blood alcohol. Even with a BAC that’s above the legal limit and no good way to attack the accuracy or legality of the testing, a defendant may still have a line of defense. With the “rising-blood-alcohol” defense, the defendant tries to establish that his or her BAC was below the legal limit while driving but rose above the legal limit by the time the testing was conducted.

This defense is based on the how the human body metabolizes alcohol. Basically, it takes time for alcohol to absorb into your system and affect your BAC. And there’s always going to be some delay between when a motorist was driving and BAC testing takes place. So, if you get pulled over while alcohol is still absorbing into your body, chances are your BAC will be higher when it’s measured than it was when you were actually behind the wheel.

Like most other BAC-related defenses, the rising-blood-alcohol defense tends to work best when your BAC is only marginally above the legal limit. And the defense typically requires the testimony of an expert with knowledge of alcohol metabolization.

Get in Contact With a DUI Lawyer

Mounting an effective defense against DUI charges requires specialized knowledge and experience. If you’ve been arrested for driving under the influence, get in touch with a qualified DUI attorney. A good DUI lawyer can look at the facts of your case and determine whether you have any viable defenses to the charges.

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