How Blood Testing Is Used in DUI Cases

Every state prohibits operating a vehicle while under the influence of drugs or alcohol. And all states have "per se" laws that allow prosecutors to prove a DUI charge by showing the driver had a blood alcohol concentration (BAC) that was above the legal limit (some states also have similar per se drug DUI laws).

The implied consent laws of most states require drivers who are lawfully arrested for a DUI to agree to a blood, breath, or urine test if asked to do so by an officer. Of the three, blood tests inarguably provide the prosecution with the most reliable evidence of BAC or the concentration of drugs in the driver's system. However, obtaining a blood sample might not always be the easiest option for law enforcement due to both practical and legal constraints.

This article explains how blood draws work as well as the benefits and drawbacks of using blood testing in DUI investigations and prosecutions.

How Consumed Alcohol and Drugs End Up in the Driver's Blood

Drugs or alcohol consumed by a driver gets absorbed into the bloodstream in various ways. Alcohol, for instance, enters the bloodstream mostly through the tissue lining of the stomach and small intestine. Drugs that someone ingests by smoking, on the other hand, get absorbed into the bloodstream primarily through the lung tissues.

But whatever the method of ingestion, drugs and alcohol find their way into the bloodstream in a concentration that bears a proportional relationship to the amount the driver actually drank, smoked, or otherwise ingested. In other words, the more the driver ingested, the higher the concentration of the substance in the blood. Though time and a number of other factors certainly play a part also.

Using a Blood Test in a DUI Investigation

A blood draw is a medical procedure. So, police will generally transport DUI suspects to a hospital or clinic to have a blood sample drawn by a licensed phlebotomist or another qualified medical professional.

The sample then goes to a lab for analysis and testing. Test results usually aren't available for a few days or weeks. Once the results are ready, the lab will typically forward them to law enforcement, the prosecution, and the Department of Motor Vehicles (DMV).

Benefits of Blood Testing in DUI Cases

Law enforcement and prosecutors generally prefer blood testing for a number of reasons. Here are some of the most important benefits of DUI blood tests.

Blood Tests are Accurate and Can Detect Alcohol and Drugs

Of the standard DUI testing methods, blood tests are generally the most reliable. Breath tests can't detect the presence of drugs and are less accurate than blood tests for measuring BAC. Urine tests can detect the presence of drugs but provide far less accurate results than blood tests.

In many states, the per se DUI laws specifically refer to blood alcohol or the concentration of a controlled substance in the driver's blood. So, urine and breath test results must be converted to blood concentration. The conversion is just an estimate, which is a source of the inaccuracy with these two types of tests. Blood tests, on the other hand, involve a direct measurement of blood concentration, so conversion error isn't a factor.

Blood Testing Doesn't Require a Willing Participate

Blood tests also have an advantage over breath and urine tests because a blood draw doesn't necessarily require driver cooperation. For example, an unconscious driver can't give a breath or urine sample, but medical personnel can still take the driver's blood. Also, if a suspect is combative or violent, a search warrant signed by a judge allows law enforcement to take blood by force if necessary.

Limitations of Blood Tests for DUI Cases

Although blood tests provide the best form of evidence for DUI prosecutions, they do have some drawbacks.

Blood Testing Is Invasive for the Suspect

Using a needle to extract a blood sample is certainly intrusive and unpleasant. Many people abhor needles and the sight of blood. So, given a choice, most drivers would certainly prefer a breath test or a urine test.

Blood Testing Can be Inconvenient for Law Enforcement

As a medical procedure, a blood draw can generally be performed only by a medical professional (such as a licensed phlebotomist) at a hospital or clinic. For DUIs that occur in rural areas or late at night, getting to a staffed hospital or clinic might be out of the question. In these types of situations, most officers will instead opt for a urine or breath test.

Constitutional Protections Related to Blood Tests

The U.S. Supreme Court has said that drivers generally have a constitutional right to refuse a blood test. So, unless an officer first obtains a warrant from a judge, blood test results are often vulnerable to legal attacks in court. To avoid legal problems, officers sometimes opt for a breath test instead of blood.

State Law Protections for Drivers

Acknowledging their invasive nature, many states limit the use of blood tests as a first resort. In these states, the officer generally must request a breath or urine test before requesting a blood sample. Only if breath or urine testing is unavailable or indicates that additional testing is necessary, can the officer then request a blood test.

Talk to a DUI Attorney

Blood tests, though generally accurate, aren't bulletproof in court. An experienced attorney can sometimes get blood test results thrown out if police didn't follow procedural requirements or violated the driver's constitutional protections. If you've been arrested for driving under the influence, get in touch with a qualified DUI lawyer who can assess your case and advise you on your options.