Driving with a blood alcohol concentration (BAC) of .08% or more (.05% in Utah) or while under the influence of drugs or alcohol is illegal in every state. But prosecutors need evidence to prove a DUI charge in court, and law enforcement is tasked with gathering that evidence during DUI investigations. One method law enforcement uses to obtain evidence of impaired driving is testing drivers' blood for alcohol and drugs. Blood testing is probably the most direct and accurate way police have to measure the amount of alcohol or other substances in a driver's body. However, blood testing has some drawbacks.
This article explains when police can require drivers to submit to a blood test, how blood draws work, and how prosecutors use blood test results in DUI cases.
State implied consent laws generally require drivers who are lawfully arrested for a DUI offense to take an alcohol or drug test when requested to do so by an officer. For a DUI arrest to be lawful—and for this implied consent requirement to apply—the officer must have a reasonable basis, called "probable cause," to believe the motorist has violated state DUI laws.
Typically, the arresting officer gets to decide which type of test the driver must take. The most common options being blood and breath tests. So, when officers are deciding what test to require, they're generally choosing between these two tests—blood and breath.
Blood tests are generally more accurate than breath tests. But law enforcement sometimes opts for breath testing instead because blood tests are more intrusive and are more likely to run afoul of the driver's constitutional rights. Specifically, the United States Supreme Court has said that blood tests—unlike breath tests—implicate drivers' 4th amendment rights. So drivers generally can't be compelled, coerced, or forced to provide a blood sample unless the officers first obtain a warrant from a court.
Blood tests are typically the most reliable method of measuring the amount of drugs or alcohol in a driver's system. However, in addition to the possible legal issues mentioned above, pragmatic concerns can sometimes dissuade officers from requesting that a driver take a blood test, as opposed to some other type of test.
Blood tests can be intrusive and painful for the suspect, and processing a blood sample requires lab work. To complete a blood test, the arresting officer will generally take the driver to the hospital so that a phlebotomist can properly draw a blood sample. That sample then goes to a laboratory for testing. The blood test results (the concentration of alcohol or drugs in the sample) normally won't be available for several weeks or more. Prosecutors typically won't decide whether to file charges against the driving until they receive the blood test results. So, all this to say, blood testing is somewhat burdensome to everyone involved and necessarily embeds a certain amount of delay in the process.
However, in some situations, blood tests are either the most convenient or only viable method of obtaining a sample from a suspect. For example, blood tests are often used when an intoxicated driver can't or won't comply with testing. Breath and urine tests both require the driver to voluntarily cooperate by providing a sample. Police can obtain a blood sample, on the other hand, regardless of the driver's cooperation. If a driver is unconscious or suffering from serious injuries due to an accident, medical personnel at the hospital will normally take multiple blood draws and send one away for alcohol or drug testing. And, in a case where a driver refuses to provide a blood sample, police might be able to get a warrant authorizing them to take blood from the nonconsenting driver—by force, if necessary.
Blood testing is also a preferred testing method when police suspect the driver of being impaired by drugs. Since breathalyzers can't detect and measure the amount of drugs in a driver's system, a blood test is the best option for reliable results (urine testing isn't as accurate).
Blood test results are crucial evidence in many DUI cases. Prosecutors use blood test results to prove criminal charges, and DMV hearing officers rely on blood test results to determine whether to administratively suspend a driver's license.
Criminal trials. Since all states have per se DUI laws that prohibit driving with a BAC that's higher than the legal limit (usually .08% or more), blood test results can provide prosecutors with nearly conclusive evidence of a defendant's guilt. In other cases, prosecutors might use blood test results correlated with expert testimony to prove actual impairment. In other words, prosecutors can use expert testimony to explain how drugs that showed up in the blood test results might have adversely affected the motorist's ability to drive safely.
DMV administrative hearings. Generally, DUI blood test results are forwarded to the Department of Motor Vehicles. In most states, the DMV will administratively suspend the license of any driver whose blood test results show a BAC of .08% or more.
This article provides some general information about DUI blood testing. However, the laws of every state and the facts of each case are different. So it's always a good idea to talk to a knowledgeable attorney in your area if you've been arrested for driving under the influence.