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Measuring the accuracy of roadside breath tests

In Florida and throughout the country, law enforcement officers often use roadside breath tests to measure drivers’ blood alcohol content levels. Drivers who have a BAC level of 0.08 or higher may be charged with a DUI. 

Yet, studies show that breath test device readings are not always accurate or reliable. They often give inflated results, which could cause a wrongful DUI charge or conviction. 

Checking the facts

BAC levels obtained through an actual blood test is an accurate measure of how much ethanol alcohol is in a person’s system. Breath test devices, obtain readings by measuring the amount of ethanol alcohol present in a person’s exhaled breath sample, and then converting this amount to a BAC level. When comparing blood test results to breath test readings, however, the differences can vary by more than 15%, according to the State University of New York at Potsdam. That means at least one in four people tested with a breath test device will show inflated BAC results. 

Identifying the factors

What is it that causes the difference between blood test results and breath test readings? Not only do these machines measure the amount of ethanol alcohol in a breath sample, it picks up other substances that have similar molecular structures. There are factors that can alter the results, which include the following: 

  • Residual food, drink, vomit or blood in the mouth 
  • Dirt, cigarette smoke or pollution in the air 
  • Gasoline and cleaning fumes 
  • Relative humidity and temperature of the air 
  • Electrical interference from officer radios and cellphones 

When breath test devices are not used properly or are not calibrated regularly, they may also give inaccurate results. 

Personal physiological differences in hematocrit levels can also influence breath test results.