Alcohol Breath Tests in DUI Cases

How breathalyzer machines are used to measure the amount of alcohol in a driver’s system and when the results are admissible in DUI prosecutions.

Every state has DUI (driving under the influence) laws that prohibit the operation of a vehicle with a blood alcohol concentration (BAC) of .08% or more (.05% or more in Utah). And the easiest way to determine a driver's BAC is with a chemical breath test. Law enforcement uses a variety of machines to test breath alcohol, each with benefits and drawbacks. This article will explain how these machines work and how breath test results are used in DUI investigations and prosecutions.

Types of Breathalyzers and How They Work

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 Devices

EBT accuracy. 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.

Breath test procedures. The testing procedure often starts with a visual inspection of the driver's mouth to ensure no foreign objects or substances are present. If a driver has "mouth-alcohol" or some other chemical (such as chewing tobacco) in his or her mouth, it can affect the test results. To ensure there's no residual alcohol in the driver's mouth, officers are typically supposed to wait for a deprivation period (around 20 minutes) before administering the test. This waiting period substantially reduces the possibility that recently consumed alcohol will still be in the suspect's mouth. However, if the driver burps, belches, vomits, or even coughs during the deprivation periods, alcohol could be regurgitated up into the mouth. So, the officer will normally restart the deprivation period if the driver does any of these things. The officer will also use a fresh mouthpiece, sealed in plastic, to ensure no prior contamination. And each machine type has a number of other steps that must be followed by police to guard against inaccurate results. Because of the extensive and precise requirements for breath tests, defense attorneys meticulously study the testing procedure and results, looking for a way to invalidate the results.

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.

How Are Breathalyzer Results Are Used in DUI Investigations, Court Cases, DMV Proceedings

A driver's breath test results are used in DUI investigations, trials, and administrative license-suspension proceedings.

Establishing probable cause for an 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.

Evidence in DUI trials. Breath test results that come from an EBT machine are often used by the prosecutor in court to prove DUI charges. Once the state proves that the machine was properly maintained and operated, the test results are generally admissible evidence.

DMV license suspension hearings. Typically, breathalyzer results that show a driver to over the legal limit are forwarded by the officer to the Department of Motor Vehicles. In most states, the DMV will administratively suspend the license of any driver who fails an EBT with a BAC of .08% or more.

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