Here’s what most people don’t realize: blood alcohol concentration (BAC) tests aren’t infallible. In fact, their results are incorrect all the time, and a “positive” result doesn’t always mean you were actually intoxicated. Medical conditions, testing errors, and even what you ate for dinner can all conspire to produce a falsely elevated reading that could cost you your license, your job, and your freedom.
So, what can cause a false high BAC test? More things than you might think:
- Diabetes and ketosis (including low-carb or keto diets)
- GERD and acid reflux
- Certain medications
- Contaminated blood samples
- Breathalyzer calibration issues
- Mouth alcohol
- Fermentation in blood vials
- Radio frequency interference
- Rising blood alcohol
The bottom line? A high BAC reading doesn’t automatically equal guilt, and challenging these results requires an attorney who understands the science behind the numbers. Mark Thiessen of Thiessen Law Firm is an ACS-CHAL Lawyer-Scientist* with over 100 Not Guilty verdicts and thousands of dismissals. He knows exactly how to attack faulty BAC evidence and dismantle the prosecution’s case.
If you’re facing DWI charges based on a BAC test you know is wrong, don’t wait. Call Mark Thiessen today at (713) 864-9000 to start building your defense.
*Continue reading: Why should I hire a lawyer scientist?
Diabetes and ketosis (including low-carb and keto diets)
If you have diabetes or you’re pre-diabetic, your body might be producing ketones — specifically acetone and isopropyl alcohol — as it breaks down fat for energy instead of glucose. Here’s the problem: breathalyzers can’t tell the difference between ethyl alcohol (the kind you drink) and these ketones. The result? A falsely elevated BAC reading that has nothing to do with how many drinks you had.
Following a ketogenic or low-carb diet? Your body is burning fat instead of carbohydrates for energy, which means it’s producing acetone — essentially the same issue that diabetics face. Even if you haven’t had a drop to drink, your breathalyzer reading could come back elevated simply because of what you ate (or didn’t eat) that day. This isn’t some far-fetched defense theory — it’s backed by science, and it’s exactly the kind of challenge that separates a DWI specialist from a general criminal defense attorney.
A skilled Houston DWI lawyer knows how to challenge these results by examining your medical history and proving the machine detected ketones, not alcohol.
GERD and acid reflux
Gastroesophageal reflux disease (GERD) and acid reflux can wreak havoc on breathalyzer results. When you have these conditions, stomach contents — including any alcohol you consumed — can be pushed back up into your mouth and throat. This creates what’s called “mouth alcohol,” which causes breath testing devices to measure alcohol vapor from your stomach rather than deep lung air.
The breathalyzer doesn’t know the difference, so it registers an artificially high reading. Even if you only had one drink hours ago, GERD can make it look like you’re well over the Texas legal limit BAC of 0.08%.
Over-the-counter and prescription medications
Plenty of over-the-counter and prescription medications can cause false positives on both breath and blood tests. Cough syrups, liquid cold medications, and certain inhalers often contain alcohol. Other medications like diuretics, antihistamines, or even some asthma medications can affect how your body metabolizes alcohol or produce compounds that mimic alcohol on chemical tests.
In some cases, your DWI blood test results might show substances that were prescribed by your doctor but get misinterpreted as impairment. This is why understanding your rights during a traffic stop matters — the less information you volunteer about what medications you take, the less ammunition you give prosecutors to twist against you later.
Contaminated blood samples
Blood samples are only as reliable as the people handling them. If the technician drawing your blood didn’t properly clean the site with a non-alcohol-based disinfectant, that alone can contaminate the sample and inflate your BAC.
Chain of custody issues are another huge problem — if your blood sample wasn’t properly labeled, stored, or tracked from the moment it was drawn to the moment it was tested, there’s no guarantee the BAC lab test results are even yours. Add in the possibility of cross-contamination at the lab itself, and you’ve got plenty of room for reasonable doubt. A good attorney will demand every scrap of documentation about how your sample was handled and stored.
Breathalyzer calibration issues
Breathalyzers are machines, and like all machines, they break down, malfunction, and give inaccurate readings when they’re not properly maintained. Texas law requires these devices to be calibrated regularly, but here’s the dirty secret: it doesn’t always happen the way it’s supposed to.
If the device wasn’t calibrated within the required timeframe, or if calibration records are missing or falsified, the results are garbage. The Intoxilyzer 9000, the most common breathalyzer used in Texas, is notoriously finicky and prone to errors. Challenging the calibration and maintenance records of the specific device used in your case is a cornerstone of successful DWI defense, and it’s one of the first things an experienced attorney will investigate.
Mouth alcohol
GERD isn’t the only cause of mouth alcohol. As we mentioned, breathalyzers are designed to measure deep lung air, but they can’t distinguish between alcohol vapor from your lungs and alcohol residue in your mouth. This residue can come from mouthwash, breath spray, breath mints, or even burping shortly before the test.
Officers are supposed to observe you for 15 minutes before administering a breathalyzer to make sure you don’t burp, vomit, or put anything in your mouth — but they don’t always follow protocol.
Fermentation in blood vials
Blood samples aren’t meant to last forever, and when they’re stored improperly, bad things happen. If your blood sample wasn’t refrigerated correctly, or if it was stored for too long before testing, the blood can actually ferment inside the vial. Yeast and bacteria naturally present in blood can convert sugar into alcohol, artificially inflating your BAC reading weeks or even months after the sample was drawn.
This is especially problematic in cases where there are delays in testing. The chemical test for DWI requires strict adherence to storage protocols, and any deviation creates an opening to challenge the results. A knowledgeable attorney will scrutinize the storage conditions and timing of your blood test to expose these flaws.
Radio frequency interference
Believe it or not, electronic devices can interfere with breathalyzer readings. Cell phones, police radios, and other radio frequency (RF) devices can cause the breathalyzer to register a falsely high BAC. Officers are supposed to keep RF devices away from the breathalyzer during testing, but in the chaos of a traffic stop, this doesn’t always happen. If the officer had their radio on their belt or was standing near squad car equipment while administering your test, that’s a legitimate challenge to the accuracy of the results.
Rising blood alcohol
Your BAC continues to rise for 30 to 90 minutes after your last drink as your body absorbs the alcohol. This is called the “absorption phase,” and it creates a huge problem for DWI prosecutions.
Let’s say you had your last drink at 9:00 PM, got pulled over at 9:30 PM, and weren’t tested until 10:00 PM. Your BAC at 9:30 PM when you were actually driving might have been under the legal limit, but by the time they tested you, it had risen above 0.08%. Texas law requires that you be intoxicated while operating the vehicle — not an hour later at the police station.
This is called a “rising BAC defense,” and it’s a powerful tool in the right hands. An experienced attorney will calculate backwards to determine what your BAC actually was at the time of driving, not at the time of testing.
BAC testing — FAQs
Can a BAC blood test be wrong?
Absolutely. BAC blood tests can be wrong for dozens of reasons, including contaminated samples, improper storage leading to fermentation, chain of custody errors, and lab mistakes. These tests are only as reliable as the people and procedures handling them, and when protocols aren’t followed perfectly, the results are worthless.
What medical conditions cause high BAC?
Diabetes, GERD, and acid reflux are the most common medical conditions that cause falsely elevated BAC readings. Auto-brewery syndrome — a rare condition where your gut ferments carbohydrates into alcohol — can also produce genuine alcohol in your system without you consuming a single drink.
What can cause a false positive for alcohol on a blood test?
False positives on blood tests typically come from contamination issues, like using alcohol-based disinfectants when drawing blood, improper storage causing fermentation, or cross-contamination at the lab. Certain medications and mouthwashes can also show up as alcohol in blood tests, especially if they contain ethanol or compounds that metabolize similarly. If the sample wasn’t handled, stored, or tested correctly at any point in the chain of custody, the results can be completely unreliable.
Can you refuse a breathalyzer? A sobriety test?
Can you refuse a field sobriety test? Yes, but the police will still use other methods of probable cause development to arrest you for DWI. If you refuse a sobriety test you will likely face an immediate breath or blood test.
What happens when you refuse a breathalyzer? You can also refuse a breath test, but doing so has immediate consequences. By driving at all in Texas, you automatically consent to BAC tests (due to something called “implied consent”), but it is also unconstitutional to force you to take a BAC test without a warrant. So what happens?
First, they suspend your license automatically, for anywhere from 180 days to 2 years. Next, they’ll request a warrant for your blood, and they’ll probably get it. What should you do? It’s complicated, but you can read our take on it here: Refusing a Breathalyzer in Texas: Why “Do Not Blow” is Bad Advice.
Charged with a DWI in Houston? Worried about your BAC test?
If you’ve been charged with a DWI and are wondering what can cause a false high BAC blood test, you’re asking exactly the right question. BAC results aren’t always accurate, and challenging them requires an attorney who understands the science, not just the law.
Mark Thiessen of Thiessen Law Firm is Board Certified in DWI Defense by the National College for DUI Defense. In fact, Mark is one of only three lawyers in all of Texas to be Board Certified in Criminal Law by the Texas Board of Legal Specialization and DWI Law by the National College for DUI Defense. He has secured thousands of dismissals for his clients and won over 100 Not Guilty verdicts.
Mark is also one of around 25 lawyers in the state to have been designated by the ACS-CHAL as a qualified Lawyer-Scientist. In other words, he is uniquely qualified to identify, investigate, and defend you against faulty BAC test results.
Ready to test your test results? Contact us today at (713) 864-9000 or use our online form to request a free consultation.
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