Blood and Breath Alcohol Tests in DUI Cases

When police can require DUI suspects to take a blood alcohol test to determine BAC or whether they’ve been using drugs.

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. This article covers the two most common BAC tests: blood and breath testing.

What Does BAC Stand For?

While BAC generally stands for "blood alcohol concentration," it is often used to refer more generally to how much alcohol is in a person's body. The actual measurement can be of blood, breath, urine, or other bodily substances.

How Alcohol Is Absorbed and Dissipated in the Body

When a person consumes alcohol, the body basically breaks it down and metabolizes it. Through this process, alcohol is absorbed in the person's blood. Generally, as a person consumes more alcohol, the concentration of alcohol in the person's blood rises. For purposes of DUI laws, BAC is typically the measurement consisting of the grams of alcohol per 100 milliliters of blood.

But alcohol concentration isn't a static measurement. The BAC in a person's body is constantly changing. After alcohol is absorbed into a person's bloodstream, it begins to dissipate. As time passes, the dissipation of alcohol leads a person's BAC to fall (assuming the person doesn't keep drinking more alcohol).

So, at any given time, a person's BAC is generally in a state of rising (absorption) or falling (dissipation).

How BAC Measurements Are Used in DUI Prosecutions

Law enforcement and prosecutors use BAC test results in various ways in DUI investigations and prosecutions. Generally, BAC test results can supply probable cause for a DUI arrest or evidence for criminal court or DMV license suspension hearings.

DUI Breathalyzer Tests: How They Work and What They're Used For

There are lots of different types of breathalyzer machines. But they generally fall into one of two categories: evidentiary breath test (EBT) machines and preliminary alcohol screening (PAS) devices. These two types of machines basically work the same way, but law enforcement uses them for slightly different purposes.

How Breath Test Machines Measure Alcohol in a Person's System

Breathalyzers measure the amount of alcohol in a person's breath. Police are able to obtain a breath alcohol measurement with these machines by having the driver blow into a small tube. The machine then takes the breath sample and calculates the amount of alcohol present. Generally, the measurement is produced in grams of alcohol per 210 milliliters of breath. A breathalyzer reading of .08 grams of alcohol per 210 milliliters of breath is basically equivalent to a BAC of .08%.

Unlike a blood test, breathalyzer machines don't measure blood alcohol directly. So, in states where the DUI laws specifically refer to "blood alcohol," breath test results must be multiplied by a partition ratio to get blood alcohol. The partition ratio is basically a constant multiplier that, in theory, converts breath to blood alcohol concentration. Generally, the partition ratio works well to make this conversion because there's a proportional relationship between blood and breath alcohol that's fairly consistent from person to person. However, because everyone's body is different, the partition ratio conversion method isn't perfect.

For many years, DUI defense attorneys challenged breath test results in court based on the possible inaccuracies of the partition ration conversion. However, in response, most states amended their DUI laws to make it illegal to drive with a BAC of .08% or with a breath alcohol concentration of .08 grams per 210 milliliters of breath. By defining a DUI offense in this way, states effectively eliminated the partition ration defense.

Evidentiary Breath Test (EBT) Devices

EBT devices are the more accurate of the two types of breathalyzers. However, EBT results can still be off if police don't follow proper procedures or the machine isn't in good working condition. As the Intoxilizer does the same job as laboratories do with blood test samples, the proper use procedures must be followed to ensure reliability. Prior to testing, the machine generally must be approved by the state and be inspected for proper operation. Most states require the device to be tested routinely and certified before being used in the field. Secondly, the officer operating the machine must also be properly trained and certified (often annually) to ensure proper operation.

PAS Breath Test Devices

Officers (and the general public) also have access to handheld breathalyzers called "PAS" or "preliminary breath test" (PBT) machines. PAS machines generally work the same way as EBT machines: When the driver breathes into a PAS, an immediate reading of the approximate BAC level is produced. However, the technology that PAS machines use to measure breath alcohol isn't as precise as that of EBT machines. So PAS results are far less reliable than EBT results and generally aren't admissible to prove BAC in court proceedings.

DUI Blood Tests: The Most Accurate Way to Measure BAC

In terms of accuracy, blood tests are generally superior to breath tests. However, from a law enforcement standpoint, blood tests also come with some drawbacks.

When Police Can Require Drivers to Take a DUI Blood Test

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.

How Blood Testing Works in DUI Cases, Including the Benefits and Drawbacks

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).

How Blood and Breath BAC Test Results Are Used in DUI Cases

BAC test results are crucial evidence in many DUI cases. Prosecutors use BAC test results to prove criminal charges, and DMV hearing officers rely on BAC test results to determine whether to administratively suspend a driver's license. Breath test results, because they're available immediately, can also supply police with probable cause to make a DUI arrest.

Breath Test Results Can Give Police Probable Cause for a DUI Arrest

An officer can make a DUI arrest only if there's probable cause to believe the driver was in violation of the state's DUI laws. In deciding whether to make an arrest, officers can consider lots of different factors. Breath test results can be one of the factors that officers can take into account in making their probable cause assessment. Generally, PAS results are used only during the DUI investigation because the results, while they can help the officers determine if a driver has broken the law, normally aren't accurate enough to be admissible in court.

Breath and Blood BAC Test Results as Evidence in Criminal DUI 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 and breath 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.

Breath and Blood BAC Test Results and 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 or breath test results show a BAC of .08% or more.

Talk to a DUI Attorney

This article provides some general information about DUI blood and breath 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.

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