The DUI Field Sobriety Test Flaw: How Tharp’s Equation Challenges NHTSA’s Claims

When you’re pulled over on suspicion of DUI in Georgia, law enforcement officers often rely on the Horizontal Gaze Nystagmus (HGN) test as part of the standardized field sobriety tests (SFSTs) endorsed by the National Highway Traffic Safety Administration (NHTSA). The HGN test checks for involuntary eye jerking nystagmus as you follow a stimulus, like a pen, with your eyes. According to NHTSA, if nystagmus begins before your eyes reach a 45-degree angle, it’s the last two “clues” of six total clues that your blood alcohol concentration (BAC) is 0.10 grams or higher while four out of six clues is supposed to mean 0.08 grams or higher. 0.08 grams or higher is the legal limit currently in Georgia. Sounds like straightforward science, right? Not quite. It is “sciency” like a lab coat on a meter maid but it is not scientific. Dig into the science or lack thereof behind this claim, and you’ll find a contradiction that could make all the difference in your DUI case.

NHTSA’s Position: A Hard Line At 45 Degrees

NHTSA’s training manual for SFSTs states that an onset of nystagmus prior to 45 degrees for each eye are the last two sequential clues of six possible clues in the HGN test. If an officer observes four or more clues across both eyes, NHTSA claims this indicates a BAC of 0.80 grams or greater with 77% accuracy in a laboratory study assuming the test is performed correctly. For Georgia drivers, this can be a critical piece of evidence leading to an arrest, as officers use it to establish probable cause. The implication is clear: if your eyes start jerking before that 45-degree mark (the last two clues of six), NHTSA says you’re likely over the legal limit of 0.08% in Georgia by 0.02 grams or you are 0.10 grams or higher.

But here’s where it gets shaky. NHTSA’s assertion hinges on a simplified interpretation of a formula known as Tharp’s equation, which doesn’t fully align with their black-and-white cutoff.

Tharp’s Equation: A Spectrum, Not A Threshold

Developed by Dr. Vance Tharp as part of early HGN research, Tharp’s equation attempts to estimate BAC based on the angle at which nystagmus begins. The formula is simple: BAC (grams) ≈ 50 – (angle of onset). For example:

• Onset at 45 degrees: 50 – 45 = 5, or a BAC of 0.05%.
• Onset at 40 degrees: 50 – 40 = 10, or a BAC of 0.10%.
• Onset at 42 degrees: 50 – 42 = 8, or a BAC of 0.08%.

This equation suggests a sliding scale nystagmus onset at different angles corresponds to different BAC levels. If onset occurs at 42 degrees, the predicted BAC is 0.08%, Georgia’s current legal limit, not the 0.10% NHTSA claims as a minimum for pre-45-degree onset. Right away, you see the problem: Tharp’s equation doesn’t support NHTSA’s rigid stance that onset before 45 degrees always means a BAC of 0.10% or higher.

The Contradiction: Science Vs. Standardization

NHTSA’s position treats the 45-degree mark as a definitive threshold that you are over 0.10 grams, implying that any early onset reliably signals significant impairment. Yet Tharp’s equation the very research NHTSA once leaned on shows that nystagmus onset at, say, 43 or 44 degrees could indicate a BAC well below 0.10 grams, or even below 0.08 grams. Worse still, studies have found that nystagmus can occur before 45 degrees in completely sober individuals due to fatigue, medical conditions, or even natural variation in eye movement. There are in fact over 40 different factors that can cause nystagmus other than alcohol. This directly undermines NHTSA’s claim of a hard-and-fast rule.

Even NHTSA admits the HGN test isn’t foolproof. Their own data pegs its accuracy at 77% when perfectly administered in a laboratory setting a far cry from the certainty officers often project on Georgia roadsides. Factor in poor lighting, improper technique, or an officer’s failure to screen for non-alcohol-related causes of nystagmus, and that reliability drops further.

Medical School Text Books Echo This Sentiment

In Duane’s Clinical Ophthalmology, Volume 2, Chapter 11, p. 20, by William Tasman, MD and Edward Jaeger MD (1994 Edition), states in plain terms,

“Unfortunately, the fact that alcohol can produce horizontal gaze-evoked nystagmus has led to a “roadside sobriety” test conducted by law enforcement officers. Nystagmus as an indicator of alcohol intoxication is fraught with extraordinary pitfalls: many normal individuals have physiologic end-point nystagmus; small doses of tranquilizers that wouldn’t interfere with driving ability can produce nystagmus; nystagmus may be congenital or consequent to structural neurologic disease; and often a sophisticated neuro-ophthalmologist or oculographer is required to determine whether nystagmus is pathologic. It seems unreasonable that such judgments should be the domain of cursorily trained law [enforcement] officers, no matter how intelligent, perceptive, and well[-]meaning they might be.”

What This Means For Your Georgia DUI Case

If you’ve been charged with DUI in Georgia based on an HGN test, this contradiction between Tharp’s equation and NHTSA’s guidelines could be a game-changer. An experienced DUI lawyer like me, George Creal can challenge the validity of the HGN results in court. Here’s how:

1. Questioning the Science: Tharp’s equation shows that early nystagmus doesn’t guarantee a BAC over 0.08 grams, let alone 0.10 grams. We can argue that the test overstated your impairment or is inadmissible as junk science.
2. Highlighting Variability: If your onset was close to 45 degrees, the BAC prediction could fall below the legal limit, casting doubt on the officer’s conclusions.
3. Exposing Flawed Administration: Officers must follow strict NHTSA protocols. Any deviation like failing to check for medical conditions weakens the test’s credibility.

In Georgia, where DUI convictions carry steep penalties like jail time, license suspension, and hefty fines, poking holes in the prosecution’s evidence is critical. The HGN test might sound scientific, but its flaws are real and exploitable.

Fight Back With Knowledge

Don’t let a roadside test dictate your future. The gap between Tharp’s equation and NHTSA’s oversimplified standard is just one example of why DUI cases deserve scrutiny. As a Georgia DUI lawyer with years of experience, I’ve seen how these tests can mislead juries and judges when left unchallenged. If you’re facing a DUI charge, call my office today. Let’s dig into the details of your stop and build a defense that holds up because the science isn’t as settled as the state wants you to think.

George Creal is a seasoned DUI attorney based in Georgia, dedicated to defending clients against unjust charges. Contact us at 770-961-5511 for a consultation.