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BAC Test FAQs

Q: What different methods are used to measure blood alcohol concentration (BAC)?

A: There are five different bodily samples that can be screend for a DUI suspect's BAC level: urine, saliva, hair follicles, blood and breath. However, only breath analysis (i.e. breathalyzer) and blood screening are broadly used by law enforcement for gathering evidence. As blood screening is considered more invasive, this method is used less often than breath analysis (mainly after a serious accident or where the suspect has refused a breath test).

Q: How do breathalyzer tests work?

A: There are different machines on the market used for analyzing a DUI suspect's blood alcohol concentration. Some, though rarely used today, employ what is called a "wet chemical" technique to compare one's breath to a sample. The most current generation of breath analysis machines (still commonly referred to as "breathalyzers"), analyze the alcohol content of exhaled vapor through a method called infrared spectroscopic analysis.

The latter method of analysis is based on the scientific principal that captured alcohol vapor absorbs light waves of a particular frequency in the presence of light, depending on the amount of alcohol present. A computer translates this data into the more familiar BAC measurement used to determine the level of alcohol in a person's bloodstream (for example, 0.08 percent BAC).

Q: How accurate are they?

A: While they are not as accurate as blood tests, breathalyzers have been considered acceptably accurate by most courts as tools for gathering evidence. However, some independent studies have determined that breath readings can vary by 15 percent from actual BAC levels (as measured by a blood draw). Some courts have even thrown out breathalyzer results, calling into question the reliability of the machines.

In 1988, a New Jersey court cited the following scientific evidence: (1) high readings for 14 percent of the population due to design flaws; (2) variance in results based on the temperature of the machine itself; (3) different results from the varying body temperatures of test subjects and (4) variances in the presence of hematocrit in the blood also affecting test results. Accuracy also can be hampered by the use of an improperly calibrated machine.

Q: Can you beat a breathalyzer test if you're intoxicated?

A: No. Some popular methods thought to help individuals fool a breathalyzer test, including the ingestion of breath mints, mouthwash, onions and even pennies, have been shown not to work. Mouthwash (which often contains alcohol) may actually have a tendency to raise a person's BAC.

Q: Is it possible to successfully challenge breathalyzer results in court?

A: Absolutely. Breathalyzer machines must be tested routinely to make sure they are properly calibrated. A skilled DUI lawyer will look into the maintenance records of the device used to test his or her client and otherwise determine the validity of the evidence.

Q: How accurate are those small, inexpensive BAC-testing devices sold to consumers?

A: It depends. There is a wide range of products available, some more accurate than others, but it's always best to assume they may be a little off (and they may not be used as as a defense to DUI charges). Saliva alcohol test strips are considered to be among the most accurate consumer BAC tests, since the amount of alcohol in one's bloodstream closely corresponds to the concentration of alcohol in the saliva.

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