San Antonio DWI Lawyer: How to Fight Field Sobriety Test Results in Bexar County

San Antonio DWI Lawyer: How to Fight Field Sobriety Test Results in Bexar County

You were just arrested. The officer told you that you failed the field sobriety tests. Now you’re facing a DWI charge in Bexar County, and you don’t know what to do next.

Here’s what you need to know right now: a failed field sobriety test does not mean you are guilty — and it does not mean you will be convicted.

I’m Genaro R. Cortez, a San Antonio DWI defense attorney with over 20 years of experience. I have helped hundreds of people facing exactly what you’re facing right now. My job is to find every weakness in the State’s case against you — starting with those field sobriety tests.

Can I Fight and Win on a DWI Charge in San Antonio?

Yes. Field sobriety tests are the building blocks of a strong DWI defense. SAPD and the Bexar County Sheriff’s Office must follow strict NHTSA protocols when administering these tests. If an officer skipped a step, gave unclear instructions, or performed the test on a sloped shoulder of I-10 at night, those results can be challenged in court — and potentially thrown out entirely.

Every DWI case is different. But in my experience, many “failed” tests have defensible problems that a skilled attorney can expose before a Bexar County jury.

What the Tests Don’t Tell You: Error Rates

Under ideal, controlled conditions, NHTSA’s own research shows these accuracy rates for detecting a BAC of 0.08% or higher:

The bottom line: Even in a lab, these tests produce false positives up to 1 in 3 times. On a real San Antonio roadside — with flashing lights, wind, uneven pavement, and a frightened driver — those numbers get worse. My job is to show the jury that you were in that margin of error.

The 3 Tests — And How I Attack Each One

1. Challenging the HGN (Eye Test) in Bexar County

Officers call this a”scientific” test, but it’s the test most vulnerable to human error. The Horizontal Gaze Nystagmus (HGN) test tracks involuntary eye jerking.

Here is what I look for in your arrest video:

• Officer Error: Did the officer hold the stimulus (pen or light) 12–15 inches from your nose?
• Did they move it too fast?
• Did they move too slow?
• Environmental Factors: Flashing patrol car lights or passing traffic on a busy San Antonio highway can cause “optokinetic nystagmus,” which looks like intoxication but is actually a natural eye response to moving lights.
• Medical Issues: Certain neurological conditions or even high doses of aspirin can cause eye jerking that an officer might misinterpret as alcohol impairment.

Key Legal Case on Texas HGN Tests

In the landmark case of Emerson v. State, 880 S.W.2d 759, 761 (Tex. Crim. App. 1994) the Texas Court of Criminal Appeals ruled that while the HGN test is scientifically reliable, it is only admissible if the officer follows exact standardized procedures. This means that if a San Antonio officer cuts corners, skips steps, or performs the test in a way that deviates from their NHTSA training, the results can be thrown out of court. Furthermore, Emerson explicitly prohibits officers from using HGN results to estimate a specific blood alcohol concentration (BAC) percentage.

Important Emerson Case Points for San Antonio DWI Defense Cases

1. HGN is “Scientific” Evidence: The court ruled that HGN is not just a “lay opinion” (like a person’s balance); it is scientific evidence. This sets a higher bar for the prosecution. They must prove the officer was specifically qualified and performed the test correctly.
2. No BAC Estimates: This is a huge win for the defense. An officer can say they saw “6 clues,” but under Emerson, they are strictly forbidden from telling the jury, “Because I saw 6 clues, the defendant’s BAC was over a .10.” They can only use the test to indicate qualitative intoxication, not quantitative numbers.
3. Strict Compliance Required: The court held that for the test to be considered “reliable,” the technique must have been applied properly on the occasion in question. As your lawyer, I use this ruling to scrutinize body-cam footage for any slight deviation in how the officer held the stimulus or timed the passes.

2. Common Mistakes in the Walk-and-Turn Test

The Walk-and-Turn is a “divided attention” test. Officers look for eight specific “clues,” but they often ignore the conditions under which you performed the test. Below are real case examples of what we look for when we defend DWI cases:

• Sloped pavement, gravel, or wet pavement from a South Texas rain — all invalid surfaces under NHTSA guidelines
• If you have a back, knee, or hip injury, or are more than 50 pounds overweight, NHTSA says this test may not be a valid indicator of impairment at all
• Being over 65 also affects the test’s validity — your attorney needs to know your physical history

3. Why the One-Leg Stand is Often Unreliable

Standing on one leg for 30 seconds while counting out loud is a feat of balance, not just sobriety.

• The “Nervousness” Factor: Being surrounded by police lights in the middle of the night is terrifying. Anxiety causes tremors and loss of balance that have nothing to do with BAC levels.
• Roadside Distractions: High winds or the “whoosh” of passing semi-trucks can easily knock a sober driver off balance during this 30-second window.

Your Medical History May Be Your Defense

San Antonio and Bexar County police often mistakenly attribute a driver’s poor performance on SFSTs to drinking before driving. However, the following medical conditions can make a sober driver look intoxicated:

• Inner ear disorders (affect balance on OLS and WAT)
• GERD or acid reflux (affects HGN and can skew breathalyzer)
• Diabetic hypoglycemia (mimics intoxication symptoms)
• Fatigue or sleep deprivation
• Anxiety and nervousness during the stop
• Back, knee, or hip injuries (affect WAT and OLS)

When I take your case, I ask you about all of this. Your medical records can be your strongest defense in front of a Bexar County jury.

You Had the Right to Refuse — And That’s Legal in Texas

In Texas, field sobriety tests are voluntary. You had the legal right to decline them. Officers are not required to tell you this. If you refused and were arrested anyway, that’s a different set of facts to work with — and it removes the prosecution’s ability to show a jury footage of you swaying or stumbling.

If you did take the tests, don’t assume the results are airtight. They’re not.

What Happens After a DWI Arrest in Bexar County

If you fail the SFSTs, then the police officer will arrest you and take you to the Bexar County Magistrate’s Office. They will ask you to provide a sample of your breath to test for alcohol. If you refuse, then the police officer will get a warrant to draw your blood and test it for drugs or alcohol in your system.

⚠️ The 15-Day ALR Deadline — Do Not Miss This ⚠️

You have only 15 days from your arrest date to request an Administrative License Revocation (ALR) hearing with the Texas Department of Public Safety. Miss this deadline and your license is automatically suspended — and you permanently give up your right to fight it.Call me today. This deadline cannot be extended.

How I Fight for You

• Motions to Suppress — challenge the admissibility of SFSTs, breath tests, and blood draws
• ALR Hearings — fight your license suspension within the 15-day window
• Trial Defense — take your case in front of a Bexar County jury
• Occupational Driver’s License — get you driving again after a DWI arrest
• Reduced Charges — negotiate plea deals for BAC results above .15
• DWI Deferred Adjudication — help you qualify and keep your record clean
• First-Time Offender Defense — protect your future if this is your first DWI
• MTR / Probation Violation Defense — if your DWI probation is being revoked

Frequently Asked Questions About San Antonio DWI & SFSTs

About Attorney Genaro R. Cortez

I have practiced criminal defense in San Antonio for over 20 years. I know the Bexar County courts, the prosecutors, and the judges. I know how SAPD and BCSO officers are trained to administer field sobriety tests — and I know where they cut corners.

As a solo practitioner, I personally handle every case. When you call me, you get me — not a paralegal, not a case manager. Your defense matters to me personally.

I offer free consultations and serve all of Bexar County, including San Antonio, Converse, Live Oak, Helotes, and surrounding areas. Bilingual service available in English and Spanish.