Every state prohibits the operation of a vehicle while under the influence of drugs or alcohol or with a blood alcohol concentration (BAC) of .08% or more (.05% in Utah). Basically, blood alcohol concentration is the measurement of the amount of alcohol in a person's body. When a driver has a BAC that's above the legal limit, he or she can be convicted of a DUI without proof of actual impairment. This article will explain what BAC is and how it is calculated as well as how BAC is used during an investigation and criminal trial.
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 substance.
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).
To obtain a direct blood alcohol concentration measurement, the driver must provide a blood sample. This sample is often taken by a phlebotomist at the hospital and then is sent to a laboratory for testing. The laboratory will test the blood to determine the amount of alcohol (measured in grams) per every 100 milliliters of blood. So BAC isn't a true percentage of alcohol concentration in a person's body. For example, a BAC of .08% is actually.08 grams of alcohol per 100 milliliters of blood.
Blood testing is generally the most accurate way to determine BAC. But for various reasons (for example, some people don't like needles), other methods of alcohol and drug testing are sometimes preferred by law enforcement and drivers. The most commonly used among the testing alternatives is breath testing. Officers can use Intoxilizers or other breathalyzer machines to measure breath alcohol. Many breathalyzers are portable and can be used in the field, eliminating the need to transport a driver to a hospital or police station.
Breath tests are pretty simple. The driver blows into a tube, and the machine is able to gather a sample of the driver's breath and calculate the amount of alcohol present. But the concentration of breath alcohol is not the same as blood alcohol concentration. A BAC measurement of .08% from a breathalyzer actually connotes .08 grams of alcohol per 210 milliliters of breath.
In many states, law enforcement can use urine testing and testing of other bodily fluids in DUI investigations. However, these other methods aren't as accurate or convenient as blood and breath testing, so they're rarely used by police in DUI investigations.
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.
Some breathalyzers that officers use in the field are good at giving an immediate rough estimate of BAC but not particularly accurate. These roadside testing devices are sometimes called preliminary alcohol screening (PAS) machines. Police typically use PAS devices to determine whether there's probable cause for a DUI arrest. But because PAS devices aren't all that accurate, the results aren't always usable in court to prove a DUI charge. Drivers typically aren't required to take a pre-arrest PAS test at the request of an officer.
The more accurate type of breathalyzers are sometimes called "evidentiary breath test" (EBT) devices. The results from EBT machines are typically precise enough to be admissible in court or administrative DMV hearings. Some EBT devices are portable, while others are large stationary machines that are normally kept at police stations.
Blood and urine test results are also generally accurate enough for prosecutors to use in court to prove DUI charges and admissible in DMV license suspension hearings.