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Chemical Tests in Illinois DUIs

If you are pulled over by a police officer under the suspicion of driving under the influence in Illinois, you will likely be subject to DUI tests, including chemical testing. If you fail one of these tests, you can be charged with a DUI. Knowing your rights, along with the potential flaws of each test, is imperative when facing such a charge. If you need legal representation following a DUI charge resulting from a failed blood alcohol content test, the team of criminal defense attorneys at Dolci & Weiland is on your side.

Blood Alcohol Content Measurement Tests in Illinois

Three tests are available for measuring blood alcohol content (BAC) in Illinois: breath tests, blood tests, and urine tests. When the driver is suspected of driving under the influence of alcohol, the police generally use a breath or blood test. If the police suspect drug impairment, including prescription drug DUIs or marijuana DUIs, they may use a blood or urine test. What test you will receive after being pulled over under suspicion of driving under the influence depends on a variety of circumstances surrounding your stop.

Breath Tests

Breath tests are the most common form of blood alcohol testing in Illinois. These tests are more reliable than blood or urine tests, are easy to administer, and show results almost instantaneously. Two forms of breath tests exist in Illinois: the preliminary breath test (PBT) and the mandatory breath test.

The PBT is a portable breathalyzer which police officers carry with them in order to prove intoxication at the traffic stop. In Illinois, you are not required to submit to such a test; this test is generally only going to be used against you. However, the mandatory breath test is a stationary machine which provides a more accurate determination of your blood alcohol content.

Blood Tests

If you refuse to submit to a mandatory breath test, you will likely be subject to blood testing. During a blood test, a police officer instructs a qualified individual to draw a sample of your blood, which is then sent off for testing to determine if you were under the influence at the time your blood was drawn. As this test requires more skill and expertise in its administration and has a longer waiting period, blood tests are not used as frequently as breath tests in Illinois.

Urine Tests

Mixed reviews exist in the scientific community regarding whether urine samples reflect an accurate blood alcohol content, and many factors can have an impact on a urine sample's reliability. Urine tests have a high standard of deviation, meaning the results from the test can show a significantly higher or lower BAC than the individual's true blood alcohol content. As such, urine testing is the least common form of testing for blood alcohol concentration.

Challenging Blood Alcohol Tests

When it comes to the accuracy of proving that a person was driving under the influence, prosecutors like to make a jury believe that chemical tests don't lie. While admittedly more accurate than a field sobriety test, these tests do still have flaws. A skilled Illinois DUI defense attorney may be able to challenge the results of your BAC test.


While the breath test is the most common--and most accurate--way of showing that a person was driving while under the influence, it is not a foolproof system. Common challenges made to breathalyzer test results stem from:

  • Inaccurate Administration of the Test: breathalyzer machines require training to be used properly. If the officer who administered your BAC test was not adequately trained or qualified to administer the test, you may have been deemed to be driving under the influence when you were not.
  • Failure to Upkeep the Machine: these machines aren't maintenance-free; rather, they require proper calibration and regular upkeep. If the machine was not checked regularly to ensure its accuracy in proving blood alcohol content, you may have a defense to your charge.
  • Failure to Follow Procedure: when administering a breathalyzer test, officers must follow strict protocol regarding the steps that must be taken and instructions that must be issued. If an officer failed to follow procedure when administering the test, you may be able to challenge the results.

Blood and Urine Tests

When conducting a blood or urine test, the individual tasked with obtaining the sample should take special care in extracting and storing the sample. Failure to obtain the sample in a proper manner can lead to a faulty result. Common ways to challenge a blood or urine sample extracted for purposes of proving you were driving under the influence include:

  • Drawing the Blood: as collecting a blood sample is a medical procedure, it must be performed by someone with proper qualifications and under sterile conditions.
  • Storing the Blood Sample: failure to store the blood sample correctly or store the sample in a timely manner can result in inaccurate results. A skilled criminal defense attorney may be able to prove that your blood sample was improperly stored, resulting in a false positive for intoxication.
  • Margin of Error in Results: no blood test is 100% accurate; as with any analysis, there is a margin of error for the results of this test. If the lab which tested your blood sample failed to report the margin of error that is possible for your blood test, the prosecutor's case against you may be weakened.

No matter the form in which your chemical testing occurred, it's important to obtain legal representation from a seasoned criminal defense attorney who has experience defending against charges of driving under the influence.

Facing a DUI Due to a Failed Chemical Test in DuPage or Chicago? We're Here to Help

If you have been charged with driving under the influence after failing a chemical test, the time to secure legal representation is now. Don't try to defend against your charge alone; your license and ability to drive is on the line. You can entrust your defense to the team of criminal defense attorneys at Dolci & Weiland--our attorneys are dedicated to helping individuals like you protect their rights and defend against the charges they face.

To speak to an attorney about your charge, fill out an online contact form or call the office closest to you. For our DuPage location, call (630) 261-9098, or for our Chicago office, call (312) 238-9007.