The first DUI laws prohibited driving while "under the influence" or "intoxicated" (DWI) by alcohol. So, convictions were based on the driver's actual level of impairment. In other words, prosecutors were required in all cases to prove the driver was actually affected by the alcohol consumed. It's still against the law to drive while impaired by alcohol. But now "per se" DUI laws also make it illegal to drive with a blood alcohol concentration (BAC) .08% or more.
Breath-test devices (also called "breathalyzers") give police a quick and easy way to determine how much a driver has had to drink, and breath-test results are often used in court to prove a DUI charge.
This article discusses the differences between two types of breathalyzers: PAS ("preliminary alcohol screening") and EBT ("evidential breath test") devices.
PAS devices are breathalyzers that police use in the field. (In some states, PAS devices are called "PBTs," for "portable breath test.") Generally, PAS devices are used by police to determine whether the driver has had too much to drink—not necessarily the precise amount of alcohol in the driver's system. In other words, police typically use PAS tests to assess whether there's probable cause for an arrest rather than to gather evidence for trial.
The small size of PAS devices makes them convenient for roadside use. But PAS machines aren't always all that accurate. Accuracy does, however, depend on the specifications of the particular device. Some handheld breathalyzers—especially some of the more expensive models—are fairly precise.
Implied consent laws generally require all drivers lawfully arrested for a DUI to submit to chemical testing (normally, a breath or blood test). However, many states make prearrest PAS tests optional—meaning, there's no legal consequence for a driver who refuses a PAS test.
In some states, PAS results can't be used in court—are "inadmissible," in other words—to prove a DUI charge.
Once a DUI arrest is made, officers typically want to take an accurate measurement of the driver's BAC. The end-goal of a DUI arrest is to get a conviction in court. BAC is crucial for proving a DUI per se charge and can also be helpful in proving an impairment charge.
EBT devices provide police with the easiest way to get a precise BAC measurement. Though blood tests may be slightly more accurate, they require the assistance of medical personnel and are more invasive than breath tests.
EBT machines are typically more accurate but larger than PAS devices. Whereas a PAS device would fit in the palm of your hand, EBT devices are normally big, stationary machines that are kept at the jail or police station.
A driver generally does not have a right to refuse an EBT. (Though some states let drivers choose between blood, breath, or urine testing.) Refusal typical leads to a longer license suspension than would otherwise be the case. And, in most states, prosecutors are allowed to tell the jury at trial that the driver refused DUI testing.