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.