The Truth About Blood Alcohol Tests in Illinois DUI Cases

Chicago Drivers Shouldn’t Assume a BAC Result Means Conviction

If you’re pulled over and arrested for DUI in Chicago, the police may ask you to submit to a breath or blood test to determine your blood alcohol concentration (BAC). Most people assume these results are ironclad—if the machine says you’re over 0.08%, that’s the end of the story. But the reality is far more complicated.

Illinois law classifies DUI under 625 ILCS 5/11-501. A first-time DUI charge with a BAC of 0.08% or higher is typically a Class A misdemeanor. However, certain circumstances—like causing serious injury, having minors in the vehicle, or having multiple prior offenses—can bump the charge up to a felony. And when that happens, your future is at risk.

From the outset, the reliability of a BAC test may be the most critical element in your case. If the test is faulty, the prosecution’s entire case may be compromised. That’s why we work hard to examine every facet of how and when the test was administered, and whether it meets Illinois’ strict evidentiary standards.

Illinois Law Requires Strict Adherence to Protocol—And Errors Are Common

According to Illinois law, chemical testing must comply with rules set forth by the Illinois Department of State Police. These procedures are detailed in administrative code and must be strictly followed for the test results to be admissible in court.

Blood must be drawn by licensed medical personnel. Breathalyzer machines must be maintained, inspected, and calibrated regularly. Samples must be handled carefully and stored correctly. Any deviation—no matter how minor—can result in contamination or skewed results.

For example, even a hospital blood draw can be challenged if the medical staff were not trained in forensic collection. Blood alcohol fermentation can occur if a sample isn’t stored properly, falsely inflating the BAC. Mouth alcohol from belching, vomiting, or recent mouthwash use can distort breathalyzer results. In each of these cases, the accuracy of the reading becomes questionable.

Just because a test result exists doesn’t mean it’s valid. And just because a machine says “0.10” doesn’t mean you were legally impaired.

When Does a DUI Become a Felony in Illinois Based on BAC Results?

Under 625 ILCS 5/11-501(d), certain aggravating circumstances convert a DUI from a misdemeanor into a felony, which Illinois calls “aggravated DUI.” These include:

• A third or subsequent DUI
• DUI resulting in great bodily harm or death
• DUI while driving a school bus with children
• DUI without a valid driver’s license or insurance
• DUI with a BAC of 0.16% or higher and a child passenger

These charges can range from a Class 4 to a Class X felony depending on the severity of the outcome. Felony convictions in Illinois carry mandatory minimum sentences, license revocation, and permanent criminal records. The higher your BAC, the more likely you are to face these severe penalties.

But even in felony DUI cases, blood alcohol testing must meet strict standards to be admissible. That’s where a knowledgeable criminal defense attorney can make all the difference.

How the Criminal Case Process Works—and Where BAC Evidence Comes In

The DUI arrest process typically begins with a traffic stop—perhaps in a neighborhood like Wicker Park or along Lake Shore Drive. The officer needs only reasonable suspicion to initiate the stop. Swerving, erratic driving, or traffic violations often suffice.

Once pulled over, the officer might ask questions, observe for signs of intoxication, and administer field sobriety tests. If they believe they have probable cause, they’ll arrest you and transport you to the station or a hospital for a chemical test.

The result of that BAC test becomes central to the case. At your arraignment and pretrial hearings, prosecutors will likely present the test as definitive proof of intoxication. However, we know better. The testing process is not flawless, and mistakes happen regularly in Cook County and beyond.

Our team investigates every link in the chain—how the sample was taken, who handled it, how it was analyzed, and whether any rules were broken. If the prosecution’s BAC evidence is compromised, we move to suppress it.

A Case From Chicago: What Happens When Protocol Isn’t Followed

We once defended a man arrested near Navy Pier after rear-ending another car. The police claimed he had slurred speech and bloodshot eyes, and a breath test at the station showed a BAC of 0.12%.

However, our investigation found the breathalyzer machine hadn’t been calibrated for over six months—despite a requirement for monthly inspections. We brought in an independent forensic toxicologist to explain the margin of error. We also introduced surveillance footage showing our client speaking clearly and walking without trouble after the crash.

The judge ruled the breath test inadmissible. The remaining evidence was too weak to support a conviction, and the charges were dismissed.

This is not a rare outcome. Cases like this underscore why a DUI arrest is not the same as a conviction.

Common Types of Evidence in Illinois DUI Cases—and Why They Matter

Besides BAC test results, law enforcement will try to collect a range of evidence during a DUI stop or arrest, including:

• Officer observations (e.g., speech, balance, behavior)
• Field sobriety test results
• Video from squad car or body cameras
• Accident scene reports
• Witness statements
• Statements made by the driver
• Medical records (if blood was taken at a hospital)

Not all of this is reliable. Officers sometimes exaggerate. Field tests are subjective. Video can help or hurt your case depending on the quality and angle. The key is for your lawyer to request all this evidence in discovery and scrutinize it closely.

Why You Should Never Face a DUI Case Without Legal Representation

There are serious and lasting consequences to being convicted of DUI in Illinois—even for a first offense. License suspension, jail time, fines, insurance increases, job loss, and permanent criminal records are just the beginning.

Without an attorney, you’re at the mercy of prosecutors whose job is to secure a conviction. You might be tempted to take a plea deal even if the evidence against you is flawed.

An experienced Chicago DUI lawyer will fight to have the charges dismissed or reduced. We challenge the evidence, hire independent toxicology experts, and protect your rights at every stage of the process. We’ve defended hundreds of clients in Cook County, DuPage County, Will County, and Lake County—and we understand what works.

What Legal Defenses Can Be Used to Challenge BAC Evidence?

There are many ways to fight back against DUI charges, especially when blood or breath test evidence is involved. These may include:

• Improper administration of the breath test
• Malfunctioning or uncalibrated equipment
• Chain of custody issues
• Sample contamination or fermentation
• Medical conditions affecting results (e.g., diabetes, GERD)
• Rising BAC due to post-driving alcohol absorption
• Violation of constitutional rights during the stop or arrest

Each case is different. That’s why we tailor our defense strategy to the facts and the evidence available in your unique situation.

What to Look for When Hiring a DUI Defense Attorney in Illinois

Hiring the right attorney can change the course of your case. Look for someone who:

• Has years of courtroom experience with DUI cases
• Understands forensic science and toxicology
• Knows local judges and prosecutors
• Has a proven record of getting charges reduced or dismissed
• Treats your case with urgency and attention to detail

Ask how often they challenge BAC test results. Ask what types of motions they typically file. Ask if they work with expert witnesses and whether they’ll personally handle your case or hand it off to an associate.

Why The Law Offices of David L. Freidberg Should Be Your First Call

When your future is on the line, you need a fierce advocate who will fight every inch of your case. At The Law Offices of David L. Freidberg, we are relentless in our defense of those accused of DUI. We analyze every piece of evidence, challenge flawed BAC results, and never back down from a fight.

We serve clients throughout Chicago and the surrounding counties—including Cook County, DuPage County, Will County, and Lake County. Whether this is your first DUI or you’re facing a felony charge, we’re ready to step in and protect your rights.

When You Need a Fighter, Call Us

Whether you’re charged in downtown Chicago, Skokie, Maywood, Bridgeview, or Rolling Meadows, we’re ready. We appear regularly in courtrooms throughout Cook, DuPage, Will, and Lake Counties. And we don’t treat aggravated speeding as just another moving violation.

If you were arrested in Chicago, protect your future by contacting The Law Offices of David L. Freidberg. We have decades of experience handling DUI cases in Illinois. Our firm is available 24/7 to provide the legal defense you deserve.

Contact us today at (312) 560-7100 or toll-free at (800) 803-1442 for a free consultation. We’re available 24/7 to serve clients throughout Illinois, Cook County, DuPage County, Will County, and Lake County.