You’re pulled over on the side of the road, heart racing, palms sweating. Would you rather a police officer ask you to take a sobriety test or a blood alcohol test? These tests might seem like they serve the same purpose — proving you’re impaired — but they’re actually drastically different in how they work, how accurate they are, and most importantly, how they can be challenged by an experienced DWI attorney.
So, what are the biggest differences between field sobriety tests vs. BAC tests in Texas?
| Category | Field sobriety tests (FSTs) | BAC tests (breathalyzer/blood) |
| Types | HGN (eye test), walk-and-turn, one-leg stand | Breathalyzer (breath test) or blood draw |
| What they measure | Physical coordination, balance, and ability to follow instructions | Blood alcohol concentration level (numerical result) |
| When they’re administered | Roadside, before arrest | After arrest (usually) or at roadside for preliminary breath tests |
| Legal requirement | Completely voluntary — you can refuse | Subject to implied consent laws (but you can still refuse with consequences) |
| Accuracy | Highly subjective and notoriously inaccurate | More objective, but still prone to errors (especially breathalyzers) |
| How results are used | Officer’s subjective observations used as probable cause for arrest | Numerical BAC reading used as primary evidence of intoxication |
| Consequences of refusal | No legal penalties, but likely still arrested | License suspension, potential evidence of “consciousness of guilt” |
| How easy to challenge | Very challengeable — dependent on officer’s training, bias, and conditions | Breathalyzer vs field sobriety test: both challengeable, but BAC tests require attacking machine calibration, administration protocol, and medical factors |
Here’s what you need to understand: field sobriety tests are designed to make you fail, while BAC tests are designed to produce a number that prosecutors can wave around in court. The game is rigged against you, but neither one is foolproof, and both can be torn apart by an attorney who knows what they’re doing.
Mark Thiessen is an ACS-CHAL Lawyer Scientist and triple Board-Certified DWI lawyer in Houston who understands the science behind these tests better than most prosecutors and can expose every crack in their case. Call Thiessen Law Firm at (713) 864-9000 or contact us online to start fighting back.
Types of sobriety tests in Texas
Field sobriety tests (FSTs)
Field sobriety tests in Texas are physical coordination exercises that police officers use to assess whether you’re impaired. These aren’t scientific measurements — they’re subjective observations based on how well you can balance, walk, and follow directions while standing on the side of the road under high stress.There are three standardized field sobriety test types that the National Highway Traffic Safety Administration (NHTSA) has approved for use:
- The horizontal gaze nystagmus (HGN) test: An officer shines a light in your eyes and watches how your pupils react as you follow an object. They’re looking for involuntary eye movements that supposedly indicate intoxication. (They always say they see it even thought it’s millimeters of jerking at night. But how do you prove that they didn’t? Cops know this and lie.
- The walk-and-turn test: You’re asked to walk heel-to-toe along a straight line, turn around, and walk back while keeping your arms at your sides. One stumble, one arm raise, or starting too early can be marked as a “clue” of impairment. Only take 2 clues to fail. The clues are: steps out of instruction phase; starts too soon; steps off line; wrong number of steps; raises arms for balance; misses heel to toe by more than half an inch; stops while walking; or improper turn.
- The one-leg stand test: You stand on one leg for about 30 seconds without swaying, using your arms for balance, or putting your foot down. Even people in good shape struggle with this one.
The problem? These tests are designed to be difficult, and officers are trained to look for “clues” of impairment that could apply to anyone — drunk or sober.
BAC tests: breathalyzer and blood tests
BAC tests measure the actual amount of alcohol in your system and produce a numerical result. Unlike field sobriety tests, these tests are supposed to be scientific and objective — though as you’ll see, they’re far from perfect.There are two main types of Texas sobriety tests that measure BAC:
- Breathalyzer tests: You blow into a machine that analyzes your breath to estimate your blood alcohol concentration. These can be administered roadside (preliminary breath tests or PBTs) or at the station (evidentiary breath tests).
- Blood tests: A sample of your blood is drawn and sent to a lab for analysis. Blood tests are generally considered more accurate than breathalyzers, but they’re also more invasive and come with their own set of problems.
While BAC tests give prosecutors a number they can point to in court (.08 being the legal drinking limit in Texas), that number doesn’t always tell the whole story. Breathalyzers can be thrown off by everything from your diet to the machine’s maintenance schedule, and blood tests can be contaminated or mishandled.
What do these tests actually measure?
What field sobriety tests measure (and why that’s problematic)
Field sobriety tests don’t measure alcohol in your system — they measure your ability to perform arbitrary physical tasks under pressure. Officers are trained to follow specific field sobriety test instructions and look for “clues” of impairment, but here’s the problem: these clues are incredibly subjective.
Did you sway because you’re drunk, or because you’re standing on an uneven shoulder with cars flying by at 70 mph? Did you raise your arms because you’re impaired, or because you’re a normal human being trying not to fall over? The officer decides. And their decision is based on their interpretation of your performance, which means bias, poor training, and environmental factors all play a massive role.
Even worse, many of the “clues” officers look for can be caused by:
- Nervousness or anxiety
- Medical conditions (inner ear problems, injuries, arthritis)
- Age or weight
- Footwear or clothing
- Road conditions and weather
- Flashing police lights
Are field sobriety tests accurate? Absolutely not. But police use them anyway because they’re an easy way to justify an arrest.
What BAC tests measure (and their limitations)
BAC tests measure the concentration of alcohol in your blood, expressed as a percentage. A reading of .08 or higher means you’re legally intoxicated in Texas, and prosecutors will use that number as the centerpiece of their case against you.
But that number isn’t as reliable as they want you to think.
Breathalyzers measure the alcohol in your breath and use a formula to estimate your blood alcohol content. The problem? Your breath alcohol doesn’t always match your actual blood alcohol. Factors like mouth alcohol, acid reflux, diabetes, and even your body temperature can throw off the results.
Blood tests directly measure the alcohol in your blood, which makes them more accurate than breathalyzers — in theory. But blood samples can be contaminated during collection, improperly stored, or mishandled in the lab. Chain of custody issues are shockingly common, and a good attorney can tear these test results apart.
Bottom line: BAC tests are better than the subjective nonsense of field sobriety tests, but they’re not infallible. The science can be challenged, the machines can fail, and the people administering these tests make mistakes.
When are you required to take these tests?
Field sobriety tests: completely voluntary
Here’s something most people don’t know: you are not legally required to take a field sobriety test in Texas. Not the eye test, not the walk-and-turn, not the one-leg stand. None of it.
These tests are completely voluntary, and you have every right to refuse them. There’s no law that says you have to perform circus tricks on the side of the road to prove you’re sober.
Why don’t police tell you this? Because they know that most people will comply out of fear or confusion, and every test you take gives them more ammunition to use against you in court.
Will refusing help you avoid arrest? Probably not. If an officer thinks you’re impaired, they’re likely going to arrest you anyway. But refusing field sobriety tests means there’s less subjective “evidence” they can point to later.
Stay calm, be polite, and remember: you don’t have to do anything other than provide your license, registration, and proof of insurance.
BAC tests: implied consent laws in Texas
Unlike field sobriety tests, BAC tests fall under Texas’s implied consent law, which means that by driving on Texas roads, you’ve automatically agreed to submit to chemical testing if an officer has reasonable suspicion that you’re impaired. But here’s the thing: you can still refuse.
If you refuse a breathalyzer or blood test, you’ll face administrative penalties, including:
- Automatic driver’s license suspension (180 days for a first refusal, 2 years for subsequent refusals)
- An Administrative License Revocation (ALR) hearing where you can fight to keep your license
- The refusal itself can be used as evidence of “consciousness of guilt” in court
Can you refuse a field sobriety test but take a breathalyzer? Absolutely. In fact, this is often the smarter move. Refusing the subjective roadside tests while agreeing to a breathalyzer (which is easier to challenge in court) can give your attorney more room to fight your case.
One more thing: Texas has “no refusal” weekends and holidays where judges are on standby to issue warrants for blood draws. If you refuse during one of these periods, police can get a warrant and take your blood by force.
Continue reading: Why “do not blow” is bad advice
Accuracy: Can you trust these tests?
Field sobriety test accuracy issues
We’ll just tell you: field sobriety test accuracy is a joke. These tests are riddled with problems, starting with the fact that they’re completely subjective. There’s no machine spitting out a number, no lab analyzing a sample — just an officer watching you try to balance on one leg and deciding whether you “pass” or “fail.”
Here’s what makes these tests so unreliable:
- They’re hard even when you’re sober. The one-leg stand? Difficult for anyone over 40, overweight, or with any kind of physical limitation. The walk-and-turn? Try doing that perfectly while stressed, tired, and standing on the side of a highway.
- Environmental factors destroy accuracy. Uneven pavement, poor lighting, bad weather, tight clothing, and high heels can all affect your performance. But officers rarely account for these factors when they’re marking you down for “clues.”
- Officers aren’t perfect. They’re supposed to follow strict protocols, but many don’t. They skip steps, give unclear instructions, or let their bias color their observations. And once they’ve decided you’re drunk, they’re looking for reasons to confirm that belief.
- Medical conditions mimic impairment. Inner ear problems, neurological conditions, injuries, and even anxiety can cause the exact same “clues” that officers associate with intoxication.
Why do cops do field sobriety tests instead of breathalyzers? Because field sobriety tests are easy to administer, require no equipment, and give officers the “probable cause” they need to arrest you and demand a BAC test. It’s a low bar, and that’s exactly the point.
BAC test accuracy: Better, but still flawed
BAC tests are more objective than field sobriety tests, but don’t mistake “more objective” for “accurate.”
Breathalyzers have a margin of error, and it’s bigger than you think. These machines assume that the ratio of alcohol in your breath matches the alcohol in your blood, but that ratio varies from person to person. Add in factors like:
- Mouth alcohol from burping, acid reflux, or recent drinking
- Improper calibration or maintenance of the machine
- Operator error during administration
- Medical conditions like diabetes or hypoglycemia
…and you’ve got a test that can easily produce a false high reading.
Blood tests are more reliable than breathalyzers, but they come with their own problems. Blood samples can be:
- Contaminated during collection
- Improperly stored or transported
- Mislabeled or mixed up in the lab
- Analyzed by technicians who make mistakes
Even when BAC tests are done perfectly, they don’t account for things like rising BAC (where your BAC was lower while you were driving but higher by the time you were tested) or individual tolerance levels. The takeaway? BAC tests are harder to challenge than field sobriety tests, but they’re far from bulletproof.
How test results are used against you in court
Field sobriety tests as probable cause
Field sobriety tests exist for one reason: to give police officers the probable cause they need to arrest you. Once you “fail” a field sobriety test — which, let’s be honest, is almost guaranteed — the officer can justify hauling you in for a breathalyzer or blood test. Your performance on these roadside tests becomes part of the narrative they’ll use to build a case against you.
Here’s how it plays out in court: The officer testifies about how you swayed, stumbled, raised your arms, or couldn’t follow instructions. They’ll describe how your eyes showed “nystagmus” during the HGN test. They’ll point to dash cam footage showing you struggling to walk a straight line.
And here’s the kicker: even if you think you passed, the officer’s report will say otherwise. They’re trained to document every tiny deviation as a “clue” of impairment, and those clues add up fast.
The good news? A skilled DWI attorney can tear these observations apart by challenging the officer’s training, pointing out environmental factors, and showing the jury just how difficult these tests are even for sober people.
BAC tests as “proof” of intoxication
Prosecutors love BAC tests because they can hold up a number in court and say, “See? The defendant was over the legal limit.” In Texas, a BAC of .08 or higher is considered “per se intoxication”, which means you can be convicted of DWI based on that number alone — even if you weren’t actually impaired while driving.
But that number isn’t the smoking gun prosecutors want it to be. Your attorney can challenge BAC results by attacking:
- The machine’s calibration and maintenance records
- The officer’s training and certification to administer the test
- The time gap between when you were driving and when the test was administered (rising BAC)
- Medical conditions or substances that could have affected the results
Breathalyzer results are especially vulnerable to challenges because these machines are notoriously finicky. One small error in administration or maintenance can invalidate the entire test. Blood tests are harder to challenge, but not impossible. Chain of custody issues, contamination, and lab errors happen more often than you’d think. Bottom line: a BAC test result is just the beginning of the fight, not the end.
What happens if you refuse?
Refusing field sobriety tests
If you refuse field sobriety tests, here’s what will happen: absolutely nothing — legally speaking.
There are no legal penalties for refusing to do the walk-and-turn, the one-leg stand, or the HGN test. You won’t lose your license. You won’t face fines. You won’t be charged with an additional offense.
Will you still get arrested? Probably. If the officer already suspects you’re impaired (which is why they pulled you over in the first place), they’re likely going to arrest you whether you take the tests or not.
But here’s why refusing can help your case:
Without field sobriety test results, the officer has less “evidence” to point to in court. No video of you stumbling. No report of you swaying or raising your arms. Just their word that they smelled alcohol or noticed slurred speech — which is much easier for your attorney to challenge.
The key is to refuse politely and calmly. Don’t get aggressive, don’t argue, and remember that you’re probably being recorded. Simply say, “I respectfully decline to perform any field sobriety tests.”
Refusing BAC tests
Refusing a breathalyzer or blood test is a different story. Under Texas’s implied consent law, refusing a BAC test triggers an automatic driver’s license suspension:
- First refusal: 180-day suspension
- Second or subsequent refusal: 2-year suspension
You’ll also have to deal with an Administrative License Revocation (ALR) hearing, where you can fight to keep your license or request an occupational license so you can still drive to work, school, or medical appointments. On top of that, prosecutors can use your refusal against you in court. They’ll argue that you refused because you knew you were guilty — “consciousness of guilt,” they call it.
So should you refuse? It depends.
If you’ve been drinking heavily and you know you’re way over the legal limit, refusing might be the better move. A suspension is temporary, but a high BAC reading is permanent evidence that can destroy your case.
If you’ve only had a drink or two, taking the test might be smarter — especially if you’re close to or under the legal limit.
This is why having an attorney who understands the science and strategy behind DWI cases is critical. Mark Thiessen has the expertise to evaluate your situation and build a defense regardless of whether you took the test or refused it.
Which test is easier to challenge in court?
Challenging field sobriety test results
Field sobriety tests are a defense attorney’s dream because they’re so easy to pick apart.
Here’s what a good DWI lawyer will attack:
- Officer training and certification: Was the officer properly trained to administer these tests? Can they prove they followed NHTSA protocols? If not, the results are unreliable.
- Test administration errors: Did the officer give you clear instructions? Did they demonstrate the test correctly? Did they conduct the test on a flat, well-lit surface? Any deviation from protocol can invalidate the results.
- Environmental conditions: Was it raining? Windy? Dark? Were you standing on an uneven shoulder or gravel? All of these factors affect your ability to perform the test and can be used to challenge the officer’s conclusions.
- Medical and physical conditions: Do you have a bad knee? Inner ear problems? Are you overweight or over 50? These factors make it nearly impossible to pass certain tests, and a jury will understand that.
- Clothing and footwear: Were you wearing heels? Tight jeans? Work boots? All of these can affect your balance and coordination.
The beauty of challenging field sobriety tests is that they rely entirely on the officer’s subjective interpretation. Once your attorney shows the jury how flawed and biased that interpretation can be, the whole case starts to crumble.
Challenging BAC test results
BAC tests are harder to challenge, but they’re far from unbeatable — especially if you have an attorney who understands the science.
For breathalyzer results, your lawyer can attack:
- Machine calibration and maintenance: When was the machine last calibrated? Are there maintenance records? If the machine wasn’t properly maintained, the results are unreliable.
- Operator error: Was the officer certified to use the machine? Did they follow proper procedures, including the 15-minute observation period to ensure you didn’t burp or regurgitate?
- Mouth alcohol: Did you have alcohol in your mouth from recent drinking, acid reflux, or GERD? Mouth alcohol can cause falsely high readings.
- Medical conditions: Do you have diabetes, hypoglycemia, or a high-protein diet? These can produce acetone in your breath, which breathalyzers sometimes mistake for alcohol.
For blood test results, your lawyer can attack:
- Chain of custody: Was the sample properly labeled and tracked from collection to the lab? Any break in the chain can raise doubts about whether the results are actually yours.
- Contamination: Was the collection site properly cleaned? Was the sample stored at the correct temperature? Blood samples can ferment if stored improperly, which increases the BAC reading.
- Lab errors: Technicians make mistakes. Your attorney can subpoena lab records and look for inconsistencies or procedural violations.
- Rising BAC: Your BAC might have been below the legal limit while you were driving but rose by the time the test was administered. This is a legitimate defense that can get charges reduced or dismissed.
Mark Thiessen is an ACS-CHAL Lawyer Scientist, which means he has the scientific training to dismantle the state’s BAC evidence piece by piece. He knows how these machines work, what can go wrong, and how to expose those weaknesses in court.
Fight flawed DWI tests with Thiessen Law Firm
Whether you’re weighing field sobriety tests vs. BAC tests or trying to understand which one destroyed your case, the truth is both are flawed. Field sobriety tests are subjective nonsense. Breathalyzers spit out inaccurate readings. Blood tests get contaminated and mishandled. And all of it can be challenged and torn apart in court by an attorney who knows what they’re doing.
Mark Thiessen is an ACS-CHAL Lawyer Scientist and triple Board-Certified DWI lawyer in Houston who doesn’t just know the law — he knows the science. While prosecutors wave around BAC numbers and dash cam footage, Mark’s dissecting machine calibration records, challenging officer training, and exposing every procedural error they hoped you wouldn’t notice. He’s secured 140+ Not Guilty verdicts and thousands of dismissals for clients who were told their cases were unwinnable.
A charge is not a conviction. As long as that gavel hasn’t dropped, you have options. Call Thiessen Law Firm at (713) 864-9000 or contact us online to start building your defense today.
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