How Reliable Are Blood Chemical Tests in Illinois DUI Cases?

 Posted on August 06, 2026 in DUI/DWI

Blood alcohol tests are often treated as strong evidence in Illinois DUI cases, but they aren't as foolproof as you’d think. Errors in testing, handling, and storage can all affect the accuracy of the results. If you're facing DUI charges based on a blood test in 2026, our Chicago DUI defense lawyer can help you understand whether the results in your case can actually be trusted.

How Does Blood Alcohol Testing Work in Illinois DUI Cases?

When someone is arrested for DUI in Illinois, police may request a blood test to measure blood alcohol concentration (BAC). Under 625 ILCS 5/11-501.2, Illinois law sets rules for how chemical tests, including blood tests, must be administered and how the results can be used in court.

A blood sample is typically drawn at a hospital or by a trained professional, then sent to a lab for analysis. The lab measures the amount of alcohol in the blood and reports this as a percentage. In Illinois, a BAC of 0.08 percent or higher is considered legally intoxicated for most drivers.

Why Do Police Use Blood Tests Instead of Breath Tests?

Blood tests are sometimes used instead of breath tests because they can also detect drugs, not just alcohol. Police may request a blood test if they suspect a driver is impaired by something other than alcohol, or if a breath test isn't available or refused.

What Factors Can Affect the Accuracy of a Blood Alcohol Test?

Blood tests are often seen as more reliable than breath tests. However, this doesn't mean they're immune to error. Several factors can affect the accuracy of the results.

Common issues that can compromise a blood test include:

• Improper storage of the blood sample before testing
• Contamination during the drawing or handling of the sample
• Errors in the chain of custody, meaning gaps in who handled the sample and when
• Fermentation of the blood sample, which can artificially raise alcohol levels over time
• Equipment that wasn't properly calibrated or maintained

Any one of these problems can throw off the final result, sometimes significantly.

Can a Delay in Testing Affect the Results?

The longer it takes to draw and test blood after a driver was stopped, the more room there is for error. Alcohol levels change over time as the body processes it, so a delay between the traffic stop and the actual blood draw can create a gap between the driver's BAC at the time of driving and the BAC shown on the test.

What Is Chain of Custody and Why Does It Matter?

Chain of custody refers to the documented path a blood sample takes from the moment it's drawn until it's tested in a lab. Every person who handles the sample along the way is supposed to be recorded, along with the date and time.

If there are gaps in this documentation, or if the sample was left unsupervised at any point, this can raise serious questions about whether the sample that was tested is even the same one that was drawn from the driver. A defense attorney can request these records and look closely for any breaks in the chain.

Can a DUI Blood Test Result Be Challenged in Court?

A blood alcohol test isn't automatically accepted as accurate just because it came from a lab. Defense attorneys can challenge these results in several ways, including questioning whether the blood draw followed proper medical procedures, whether the lab's equipment was properly calibrated, and whether the chain of custody was fully documented.

An attorney can also request the lab's testing records and calibration logs to check for irregularities. If the lab has a history of errors or if the specific technician who handled the sample made mistakes in the past, this information can be used to challenge the reliability of the results in your case.

Do Medical Conditions Affect Blood Alcohol Test Results?

Certain medical conditions can affect BAC readings in ways that don't reflect actual impairment. For example, people with diabetes can sometimes produce naturally elevated readings due to how their bodies process sugar. Certain medications can also interact with testing methods in ways that produce inaccurate results. An attorney familiar with these issues can determine whether a medical condition may have played a role in your case.

Call Our Cook County, IL DUI Defense Attorney for a Free Consultation

Attorney James DiQuattro brings an accomplished, aggressive, and dedicated approach to every DUI case, going the extra mile to fight for his clients. He represents individuals facing state and federal criminal investigations, giving him broad experience in challenging evidence and protecting clients' rights at every stage of a case.

Contact The Law Offices of James F. DiQuattro at 312-627-9482 to talk to our Chicago DUI defense lawyer today.