Reliability of BAC Tests

If you are currently being charged with driving while intoxicated (DWI), the prosecution has evidence you were either over the blood alcohol concentration (BAC) at the time of the stop, or that there is evidence you were too impaired to drive. This evidence typically includes the results of a blood, breath, and/or field sobriety tests (FSTs).

Police and prosecutors move through the court system as if BAC tests are entirely valid and reliable. The truth is that these tests are susceptible to human error and others issues, which lead to false positives and inaccurate results.

Roadside BAC Tests
After you’ve been pulled over on suspicion of DWI, the officer needs additional information before they can arrest you. An officer only needs reasonable suspicion to stop your vehicle. Reasonable suspicion must be based on fact, but it is a low threshold. To arrest you, an officer needs probable cause, which is a higher standard.

One way an officer may attempt to gain more information is by asking you to complete FSTs. The National Highway Traffic Safety Administration (NHTSA) has standardized three field sobriety tests:

  • The horizontal gaze nystagmus (HGN) test
  • The walk-and-turn test
  • The one-leg stand test

When you hear that these FSTs have been standardized by a government agency, you may think they are highly reliable. However, there is little evidence to back up this assumption. An American Automobile Association (AAA) DUI Justice Link study found that standardized FSTs only accurately determined when people were over the legal BAC limit (.08 percent for drivers 21-years-old and older, .04 percent for commercial drivers) between 79 and 88 percent of the time. The margins of error are also significant. Seemingly small mistakes or differences in how officers administer the tests can drastically alter the outcome.

Another method officers can use to obtain evidence of your intoxication is through a roadside breath test, commonly referred to as a breathalyzer. These tests are performed with pocket-sized devices that sample your breath. The device quickly translates the amount of alcohol in your breath to an estimated BAC result. Similar to standardized FSTs, you may think breathalyzers are more accurate and reliable than they truly are.

Roadside breath tests can return false positives due to:

  • Improper or lack of recent calibration
  • Improper maintenance of the device
  • Outside influences, such as medication, certain foods, breath sprays, and mouthwashes
  • Medical conditions such as diabetes or gastroesophageal reflux disease
  • Lack of monitoring before administration of the test

Unfortunately, many police officers rely on roadside breath tests in determining whether or not to arrest someone for a DWI, even though a breathalyzer may show someone is over the legal limit when they are not.

BAC Tests at the Police Station
Whether or not you are charged with a DWI may not be based on the results of a roadside test. Instead, the police will likely conduct a more formal breath or blood test at the police station. The test that’s widely considered the most accurate is a blood test. This utilizes gas chromatography. However, blood tests are hardly infallible.

Human error is a common problem with police station BAC tests. While taking a blood or breath sample to determine, police officers may do the following:

  • Improperly label and store your blood sample.
  • Switch blood samples.
  • Improperly maintain and calibrate machines.
  • Carry over Ethanol (alcohol) from a previous blood test.

If you were required or agreed to submit to a blood test after a DWI arrest and believe it wrongly determined your BAC to be above .08 percent, then contact an experienced attorney who specializes in DWI. A lawyer will investigate the police department’s procedures and the method through which your test was performed to look for possible errors.

Ned Barnett is a Houston DWI defense attorney with nearly 30 years of experience. He is board-certified as a criminal attorney by the Texas Board of Legal Specialization. He is a founding member of both the National College of DUI Defense, and the American Association of Premier DUI Attorneys. Visit The Law Offices of Ned Barnett to learn more about how he can help.

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One Response to “Reliability of BAC Tests”

  1. Joe says:

    Researchers say the Alcotest 9510 a police breathalyzer, produces incorrect breath test results:

    Jason Lantz, a Washington-based defense lawyer, enlisted a software engineer and a security researcher to examine its source code. The two experts wrote in a preliminary report that they found flaws capable of producing incorrect breath test results. The defense hailed the results as a breakthrough, believing the findings could cast doubt on countless drunk-driving prosecutions.

    The two distributed their early findings to attendees at a conference for defense lawyers, which Draeger said was in violation of a court-signed protective order the experts had agreed to, and the company threatened to sue.

    Their research was left unfinished, and a final report was never completed.

    Draeger said in a statement the company was protecting its source code and intellectual property, not muzzling research.

    “Pursuant to a protective order, Draeger provided the source code to both of the defense experts in Snohomish County,” said Marion Varec, a spokesperson for Draeger. “That source code is highly proprietary and it was important to Draeger that the protective order limit its use to the purposes of the litigation at issue.” Draeger says it believes that one of the experts entrusted to examine the source code was using it in violation of the protective order, so Draeger sent the expert a cease and desist letter. Draeger says it “worked with the expert to resolve the issue.”

    Of the law firms we spoke to that were at the conference and received the report, none knew of Draeger’s threat to launch legal action. A person with a copy of the report allowed ZDNet to read it.

    The breathalyzer has become a staple in law enforcement, with more than a million Americans arrested each year for driving under the influence of alcohol — an offense known as a DUI. Drunk driving has its own economy: A multi-billion dollar business for lawyers, state governments, and the breathalyzer manufacturers — all of which have a commercial stake at play.