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How accurate are breathalyzer tests?

Breathalyzer testing is the most frequently used method of determining whether or not a person is above the legal blood alcohol limit to operate a motor vehicle. The accuracy of these tests is often a subject of debate, but advances in technology have made results more reliable.

Understand the legal limit
While any BAC level over .08 percent is considered over the legal limit, understand that a BAC determined by a breathalyzer test is only one determinant of your ability to drive, and you can be arrested for DUI if you display other symptoms of intoxication or fail a field sobriety test.

Any lack of accuracy in the machine must be assumed in both a positive and negative direction, so it's as likely that your test will show a lower than actual BAC as it may a higher than actual ratio.

Determining error
Breathalyzer tests make a number of assumptions about the person being tested. The ratio of blood alcohol content to breath alcohol content is assumed to be 2100:1, though the actual ratio for a person can range from 1700:1 to 2400:1 depending on factors like height and weight, sex and metabolism.

Strenuous physical activity can reduce BAC by up to 25 percent, leading to inaccurate readings in some cases.

Margin of error
Most courts recognize a 0.1 percent error when it comes to breathalyzer tests, and many law enforcement agencies require multiple tests to fall within a certain range for a breathalyzer test to be considered as evidence of DUI.

While the determinant range suggests accuracy in testing, some DUI attorneys argue that the process is flawed. In California, an arresting officer can test as many times as necessary until the breathalyzer comes up with results within a .02 percent range on two consecutive tests. Skeptics say that while tests of .04, .09, .12, .03, .08 and .10 would be considered accurate and the driver would be over the legal limit (because the final two tests fall within a range of .02 and are equal to or above .08), the overall range of the tests (from .03 to .12) suggests that the machine may not be functioning properly.

Advances in technology
Although many defendants and DUI defense lawyers have considered breathalyzer test results suspect for years, advances in breathalyzer technology are making the devices more accurate. The most frequently used testers, need to be calibrated often and measure ethyl alcohol by identifying molecules in the larger methyl group, sometimes leading to inaccurate results.

Fuel cell testers, which are gaining in popularity, measure a smaller group of compounds with a more accurate sensor.