When a Houston police officer administers field sobriety tests on the side of the road, the officer is not a physician, a neurologist, or an unbiased observer. What the officer sees — unsteady balance, eye movement, difficulty walking heel-to-toe — may have nothing to do with alcohol. Dozens of medical conditions, medications, and physical factors produce the exact same signs that officers are trained to call “clues of intoxication.”
This article explains exactly how the three standardized field sobriety tests work, what scientific accuracy they actually claim, and the twelve medical conditions that regularly produce false positives — signs that lead officers to arrest sober or only marginally impaired drivers every day in Harris County.
The Three NHTSA-Standardized Field Sobriety Tests
The National Highway Traffic Safety Administration (NHTSA) has validated three field sobriety tests for use in DWI investigations. “Validated” here means they were studied in the 1970s and 1980s under controlled laboratory conditions — not on the shoulder of I-10 at 2 a.m. in the rain.
1. Horizontal Gaze Nystagmus (HGN)
The officer holds a stimulus (pen, finger, or light) in front of your eyes and moves it slowly from side to side. The officer is looking for nystagmus — involuntary jerking of the eyeball. Alcohol affects the cerebellum and causes nystagmus to appear at smaller angles of gaze than it would otherwise. Officers look for six clues total (three per eye): lack of smooth pursuit, nystagmus at maximum deviation, and nystagmus onset before 45 degrees.
NHTSA’s own research claims 88% accuracy for HGN when all six clues are present. What NHTSA’s studies don’t disclose prominently: nystagmus is caused by dozens of conditions completely unrelated to alcohol.
2. Walk-and-Turn
You take nine heel-to-toe steps along an imaginary line, turn on one foot, and return. The officer looks for eight possible clues: inability to maintain the starting position, starting too early, stopping, stepping off the line, raising arms for balance, an improper turn, incorrect number of steps, and failure to touch heel to toe.
NHTSA claims 79% accuracy for walk-and-turn. Two or more clues indicates impairment — but any physical condition affecting gait, balance, or leg function produces clues with no alcohol involved.
3. One-Leg Stand
You raise one foot six inches off the ground and count (“one thousand one, one thousand two…”) for 30 seconds. Officers look for four clues: swaying, using arms for balance, hopping, or putting the foot down.
NHTSA claims 83% accuracy. NHTSA itself notes the test is less reliable for anyone over age 65 or more than 50 pounds overweight — but officers rarely ask about either factor before administering the test.
You Can Refuse Field Sobriety Tests in Texas — With No Automatic Penalty
Before getting into the medical conditions, this is critical: field sobriety tests are voluntary in Texas. Refusing a field sobriety test carries no automatic legal penalty — unlike refusing a breath or blood test after arrest, which triggers an automatic license suspension. You have the constitutional right to decline, and doing so cannot be used against you as evidence of guilt at trial.
If you have any of the conditions below, you should know this right before you are ever pulled over.
12 Medical Conditions That Produce Field Sobriety Test False Positives
Conditions Affecting the HGN Test
1. Prescription Anticonvulsant Medications
Anticonvulsant drugs — including phenytoin (Dilantin), carbamazepine (Tegretol), valproic acid (Depakote), phenobarbital, and lamotrigine (Lamictal) — cause nystagmus as a known side effect at therapeutic doses. The NHTSA training manual itself lists “certain anticonvulsants” as a cause of nystagmus. Millions of Americans take these medications for epilepsy, bipolar disorder, and chronic pain — none of whom are intoxicated when the drug creates HGN clues.
2. Pathological Nystagmus from Neurological Conditions
Multiple sclerosis, brain lesions (from tumors, strokes, or trauma), cerebellar ataxia, and certain inner ear disorders cause pathological nystagmus — nystagmus that is present regardless of alcohol consumption. An officer looking at six HGN clues cannot distinguish pathological nystagmus from alcohol-induced nystagmus without medical training. Officers have none.
3. Benign Positional Vertigo (BPPV) and Inner Ear Disorders
The inner ear’s vestibular system controls eye movement stability. BPPV, labyrinthitis, Meniere’s disease, and vestibular neuritis all disrupt this system, causing nystagmus, vertigo, and balance problems. These conditions are extremely common — BPPV alone affects approximately 2% of the adult population — and their symptoms are indistinguishable from alcohol impairment on an HGN test administered by a non-physician.
4. GERD and Acid Reflux: The Mouth Alcohol Problem
Gastroesophageal reflux disease causes stomach contents — including stomach vapors that contain alcohol absorbed from any food or drink — to travel up the esophagus into the mouth and throat. When alcohol vapor is present in the mouth during a breath test, it significantly skews the reading upward. But GERD also affects the HGN test: officers are supposed to check for nystagmus only after confirming no mouth alcohol is present. Belching, hiccupping, or reflux during the observation period can introduce mouth alcohol that creates artificial HGN clues.
Conditions Affecting the Walk-and-Turn Test
5. Inner Ear Disorders: Vertigo and Balance Impairment
The same inner ear conditions that affect HGN also destroy performance on the walk-and-turn test. Someone experiencing a BPPV episode, acute labyrinthitis, or a Meniere’s attack cannot walk a straight line regardless of sobriety. The spinning, vertigo, and directional pull these conditions cause are subjectively overwhelming — and objectively indistinguishable from DWI impairment on a roadside test.
6. Parkinson’s Disease and Movement Disorders
Parkinson’s disease affects gait, balance, coordination, and the ability to follow multi-step instructions — all of which are scored in the walk-and-turn test. The characteristic Parkinsonian gait (shuffling, small steps, impaired turn) generates multiple clues on the walk-and-turn. Related conditions including essential tremor, progressive supranuclear palsy, and drug-induced parkinsonism produce similar effects and are frequently misread as intoxication by officers untrained in neurology.
7. Peripheral Neuropathy
Peripheral neuropathy — nerve damage affecting sensation and muscle control in the feet, legs, and hands — is one of the most common long-term complications of diabetes, affecting approximately half of all diabetic patients. Neuropathy causes numbness, tingling, weakness, and proprioceptive impairment (difficulty knowing where your feet are in space without looking). Walking heel-to-toe on an imaginary line with numbness in your feet is physically difficult even when completely sober — and the walk-and-turn test scores exactly the performance deficits neuropathy causes.
8. Musculoskeletal Injuries and Surgical History
Knee replacements, ACL repairs, hip replacements, ankle fractures, plantar fasciitis, and chronic back conditions all affect gait, balance, and the ability to walk heel-to-toe. NHTSA’s own research notes that the walk-and-turn may be less reliable for individuals with leg, knee, or back problems — but this advisory appears in the research, not in what officers tell suspects before conducting the test. Many clients with obvious surgical scars on their knees were never asked whether they had any physical limitations before the test began.
Conditions Affecting the One-Leg Stand
9. Obesity: The Factor NHTSA Acknowledges But Officers Ignore
NHTSA’s own research acknowledges that the one-leg stand is less reliable for people who are 50 or more pounds overweight. The biomechanics are straightforward: a significantly overweight person’s center of gravity is higher and less stable, their ankles bear far more weight than in research subjects, and the physical demand of holding one leg elevated is substantially greater. Officers routinely administer the one-leg stand to visibly overweight individuals, note swaying and arm-raising as clues, and cite this as evidence of intoxication — despite NHTSA’s explicit acknowledgment of reduced reliability for this population.
10. Cerebellar Ataxia and Balance Disorders
The cerebellum coordinates voluntary movement and balance. Cerebellar ataxia from any cause — hereditary ataxia, stroke, multiple sclerosis, cerebellar degeneration — produces exactly the same observable signs as alcohol on the one-leg stand: swaying, inability to maintain position, foot placement to prevent falling, and use of arms for balance. These are not clues of intoxication. They are symptoms of neurological disease.
Conditions Affecting All Three Tests
11. Fatigue and Sleep Deprivation
Sleep deprivation produces cognitive and physical impairment that is functionally indistinguishable from mild alcohol intoxication in research studies. A person who has been awake for 24 hours shows attention, reaction time, and balance deficits comparable to someone with a .08 BAC. Shift workers, long-haul drivers, new parents, and medical professionals pulled over late at night after a double shift are particularly vulnerable to this effect. Fatigued drivers also often display red, glassy eyes, slurred speech, and slow reaction times — the same observations officers note as signs of intoxication.
12. Hypoglycemia and Diabetic Ketoacidosis
Hypoglycemia (low blood sugar) causes shakiness, sweating, confusion, slurred speech, impaired coordination, and behavioral changes that can appear identical to DWI impairment. Officers confronting a hypoglycemic person on the roadside often cannot distinguish the presentation from alcohol intoxication. More dangerously, diabetic ketoacidosis produces acetone as a metabolic byproduct — and acetone is detected by some breathalyzer technologies as if it were alcohol, producing false positive readings above .08 even from a completely alcohol-free person.
Environmental Factors Officers Are Required to Control — And Often Don’t
Beyond medical conditions, the standardized field sobriety tests require specific environmental conditions for valid administration. Officers are trained on these requirements but frequently skip the assessment:
- Surface: The walk-and-turn requires a “hard, dry, level, non-slippery surface with room for 9 heel-to-toe steps.” Cracked asphalt, gravel shoulders, grass, slopes, and wet surfaces violate this requirement.
- Footwear: High heels, thick-soled boots, flip-flops, and dress shoes affect walk-and-turn and one-leg stand performance. Officers are supposed to offer to remove inappropriate footwear — and almost never do.
- Lighting: HGN requires adequate lighting to observe the eyes. Flashing patrol car lights create involuntary nystagmus by themselves — the nystagmus induced by strobe-like stimulation is called “optokinetic nystagmus” and is indistinguishable from alcohol-induced nystagmus during an HGN test.
- Temperature and wind: Cold, wind, and rain affect performance on outdoor balance tests in ways officers are not trained to account for.
Officer Training and Administration Errors
Officers who administer field sobriety tests are certified through an NHTSA-standardized training program. The training ranges from as little as a few hours (basic certification) to a multi-day Standardized Field Sobriety Test (SFST) course. Herman subpoenas every officer’s training records and looks for:
- HGN administered too fast (the stimulus must move at a rate that allows accurate nystagmus detection)
- HGN administered without checking for resting nystagmus first
- Walk-and-turn initiated before the instruction stage was complete
- One-leg stand not counting properly or stopping before 30 seconds
- Failure to ask about medical conditions, injuries, or medications before testing
- Administering tests on an inappropriate surface without documenting the limitation
Each of these errors is documented in the officer’s dashcam or bodycam footage — which Herman reviews in detail before any pre-trial motion or cross-examination.
What Juries Actually Think About Field Sobriety Tests
In Herman’s experience defending DWI cases in Harris County, jurors are not blank slates on field sobriety tests. Many have taken the tests themselves, have seen them administered on reality TV, and have their own opinions about their fairness. When the defense explains that the walk-and-turn was conducted on a crumbling shoulder in the rain, or that the client was 67 years old with a known inner ear condition, jurors respond to that information.
The goal isn’t to convince a jury that the tests are universally useless — it’s to create reasonable doubt about what your performance on that night under those conditions actually proved.
If you were arrested for DWI in Houston and field sobriety tests were a key part of the state’s case, call Herman Martinez at (713) 242-1779 for a free case evaluation. Herman will review the dashcam footage, the officer’s training records, and your medical history to identify every challenge available to your case.
