Disputing the Accuracy of Breath and Blood Tests

The majority of DUI investigations hinge on the results of either a breath or blood test after an arrest. While slurred speech and failed sobriety tests can provide the evidence the court needs to secure a conviction, the amount of alcohol in the driver’s system is the most important proof. Sickness can cause you to slur your speech and physical impairment can result in an unsuccessful walk-and-turn test, yet there is no doubt when your blood alcohol percentage is shown to be over .08%...or is there?

Challenging a Breath Test

Modern breathalyzers are reasonably accurate, but they’re prone to mistakes and errors just like any other similar device. All breath test devices have a built-in margin of error that can be off by as much as .01%. A driver who is just barely over the legal limit has a good argument based on this margin of error, and the prosecution may not be able to prove the charge beyond a reasonable doubt.

A contaminated breath sample can also cause a higher BAC reading than normal. A breathalyzer is designed to measure the concentration of alcohol from the vapor in your lungs, but to get an accurate measurement, no alcohol can be in your mouth. If there is, the results are a combination of your lung and mouth alcohol, which can significantly raise your actual BAC. To avoid this issue, officers are supposed to observe a driver for a 15 to 20 minute period before administering a breath test. If an officer doesn’t adhere to this waiting procedure, you may be able to raise questions about the veracity of the results.

Challenging a Blood Test

Generally speaking, blood tests are more accurate than breath tests, but that doesn’t mean they’re always correct. Below we’ll go through several situations in which a blood test may be challenged:

  • Fermentation: One of the most common sources of errors during a blood test is improper storage. If blood isn’t stored properly it can ferment, which can increase the BAC in the sample.
  • Blood draw without a warrant: Police generally need a warrant to require a blood test. If they obtain one without a warrant, the results could be thrown out.
  • Rising blood alcohol: A “rising-blood-alcohol” defense establishes a person’s BAC was below the legal limit while driving, but rose by the time the testing was conducted since the body takes time to absorb alcohol.

If you have been arrested for DUI, contact our Murrieta criminal defense lawyers at Hanson, Gorian, Bradford & Hanich today. Call (951) 506-6654 or contact us online.


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