What Does BAC Mean
blood alcohol concentration
“BAC” is short for “blood alcohol concentration”—a measurement of how much alcohol is in a person’s blood.
Generally, BAC rises as a person drinks. So there’s a correlation between BAC and impairment. That’s why DUI laws use BAC as a marker for whether a person is under the influence. (To get a rough idea of how BAC relates to the number of drinks consumed, see our BAC chart and BAC calculator.)
How Police Measure BAC
Blood alcohol tests and breath alcohol tests are the two most common methods police use for measuring how much a person has had to drink.
For drivers and police alike, blood testing can be troublesome. Many drivers simply don’t like having blood drawn—it can be painful and some even have a needle phobia. Other drivers have medical conditions, such as hemophilia, that make blood testing a bad option.
Police often disfavor blood tests because they involve burdensome procedures. The officer must take the suspect to a place—usually a medical facility or police station—where a phlebotomist or other medical personnel extracts the blood sample. And blood results aren’t quick: It can take a month or more for a lab to analyze the sample and report the results.
Breath tests, on the other hand, don’t suffer the same flaws. Police don’t need medical personnel to use a breathalyzer, and the results are instantaneous. Also, most DUI suspects much prefer blowing into a breathalyzer tube to having a phlebotomist jab them with a needle.
The Relationship Between Breath and Blood Alcohol
Because of the advantages, most officers favor using breathalyzers to testing a suspect’s blood. However, breath-test devices don’t measure blood alcohol—they measure breath alcohol concentration.
For many years, “per se” DUI laws prohibited driving with a BAC of .08% or more but failed to mention breath alcohol concentration. To be used in court, breath test results had to be multiplied by a “partition ratio.” Because actual partition ratios vary person-to-person and for the same person over time, defense attorneys often challenged the accuracy of this breath-to-blood conversion.
Most states—including Texas and Florida—put an end to the recurring legal battles over partition ratios by changing their per se DUI statutes. In these states, it’s now illegal to drive not only with a blood alcohol concentration at or above .08%, but also with a breath alcohol concentration of .08% or more.
(Blood alcohol is usually measured in grams of alcohol per 100 milliliters of blood, whereas breath alcohol is typically in grams of alcohol per 210 liters of breath.)
So, blood and breath alcohol concentration are basically equivalent for purposes of determining whether a driver is guilty of a per se DUI. Indeed, many DUI attorneys use “BAC” as a generic term both.
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