DUI Blood Alcohol Testing

How law enforcement uses blood tests to measure blood alcohol concentration (BAC) in drunk driving investigations and how the defense can challenge the results in court.

By , Attorney · University of San Francisco School of Law

The implied consent laws of all states require drivers who are lawfully arrested for driving under the influence (DUI) to agree to take a chemical test. Chemical test results often provide crucial evidence for prosecutors to prove a DUI charge in court. In most cases, the test will be a breath or blood test. Here are how blood tests work and some of the ways defense attorneys use to discredit the reliability of blood test results.

Blood Testing Procedures

Once a suspect is arrested for a DUI, police generally want to conduct chemical testing to determine the amount of alcohol or drugs in the suspect's system. A blood test is a medical procedure, so police normally need to transport the suspect to a medical facility or other location where a qualified person (usually a phlebotomist) can perform the blood draw. The DUI laws of many states specify how and by whom DUI blood testing is to be done.

Blood test results aren't instantaneous. After the blood sample is obtained, it must be sent off to a lab for analysis. In many cases, the results won't be available for several weeks or more. With breath tests, police get the results immediately, but blood tests are generally more accurate and—unlike with breath tests—will show the presence of drugs and alcohol in the suspect's system.

If the blood test results come back and show the suspect had a blood alcohol concentration (BAC) of .08% or more (.05% or more in Utah) or an amount of drugs sufficient to be intoxicating, chances are the prosecution will file DUI charges.

Challenging Blood Test Results in Court

Defense tactics for challenging blood tests in court generally fall into one of two categories: challenging test result reliability and arguing the blood sample was obtained by an illegal search and seizure.

Blood Test Reliability

Blood tests are generally more reliable than breath tests, but they aren't bulletproof.

Blood samples must be properly handled and preserved. Otherwise, the blood sample can be contaminated and will no longer give an accurate measure of the suspect's BAC level. For example, a laboratory's lack of care can theoretically lead to a blood sample fermenting in the vial. Fermentation causes alcohol content to increase in the blood sample. If fermentation does occur, there's no way to know how much alcohol is from what the suspect drank and how much alcohol is due to fermentation.

Another shortfall of blood testing comes from the delay time between when the suspect was driving and the blood test is conducted. A person's blood alcohol is always changing and it takes time for alcohol to absorb into and dissipate from the bloodstream. If a defendant drank shortly before getting behind the wheel, the alcohol could still be in the process of absorbing into the defendant's bloodstream at the time of the arrest. As the alcohol continues to be absorbed, the defendant's BAC will continue to climb. As a result, the defendant's BAC will have been lower at the time of driving than it will be an hour or so later when the blood draw is conducted. Defense attorneys call this the "rising-blood-alcohol" defense. Basically, the argument is that, although the BAC was above the legal limit at the time of the blood draw, it was below the limit at the time the defendant was actually driving. (Some states, like Colorado and Nevada, have defined driving under the influence in a way that eliminates the possibility of this defense.)

Illegal Search and Seizure

The Fourth Amendment of the U.S. Constitution protects against unreasonable searches and seizures. Generally, a search and seizure is considered "unreasonable"—and violates the constitution—if conducted without a warrant.

The Supreme Court has said that blood draw is within the search-and-seizure protections of the Fourth Amendment and that police normally need a warrant or the suspect's consent to lawfully draw blood. However, in certain circumstances—like the suspect being unconscious—police are excused from having to get a warrant before ordering a blood draw.

So, in many cases where police require the suspect to submit to a blood test without first getting a warrant, the defense tries to get the results of the blood test thrown out as the product of an illegal search and seizure. If defense efforts are successful, there the blood test results can't be used by the prosecution in court and there's a good chance the charges will be dismissed.

Getting Help From a DUI Attorney

DUI law is complicated, especially when it comes to chemical testing. If you've been arrested for a DUI, it's best to get in contact with a local DUI attorney. An experienced DUI attorney can evaluate your case and let you know if you have any viable defenses to fight the charges.

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