False Positives: Medical and Physical Conditions Can Undermine Field Sobriety Tests

Field sobriety tests are routinely presented as objective and standardized. Officers describe them as scientific. Prosecutors rely on them as indicators of impairment to obtain OVI convictions. But the National Highway Traffic Safety Administration (“NHTSA”), which developed these tests, makes clear in its current training materials that the tests are not universally reliable and are not validated for everyone.

This is not a defense invention. It is a limitation built into the system.

NHTSA’s manuals acknowledge that Standardized Field Sobriety Tests are designed to be administered under specific conditions and to specific populations. When those conditions are not met, or when a person has certain physical or medical characteristics, the reliability of the tests diminishes.

Understanding those limitations fundamentally changes how field sobriety evidence should be evaluated.

What NHTSA Actually Acknowledges

In its current SFST training and instructor materials, NHTSA makes several important concessions:

• Standardized Field Sobriety Tests were validated under controlled conditions, not in every roadside environment

• The tests are divided-attention tasks that require balance, coordination, and comprehension

• Certain individuals may have difficulty performing specific tests due to physical or medical factors

• Officers are trained to observe performance, not diagnose the cause of poor performance

NHTSA does not claim that poor performance always equals impairment. Instead, it teaches officers how to administer the tests while recognizing that not all subjects are appropriate candidates for them.

Age, Weight, and Physical Limitations

NHTSA’s materials discuss that balance-based tests, particularly the One-Leg Stand, require lower-body strength, balance, and joint stability. The tests were validated on limited populations and assume a baseline level of physical ability.

Individuals who are older, who have joint, back, leg, or balance issues, or who carry additional body weight may struggle with these tasks for reasons unrelated to alcohol or drugs. The manuals recognize that physical characteristics and limitations can affect performance, even when the person is sober.

The tests measure how well a task is performed, not why.

Injuries and Medical Conditions

NHTSA training materials caution officers to consider whether a subject has injuries or medical conditions that could interfere with performance. Balance, coordination, and divided attention can all be affected by medical issues unrelated to impairment.

When a person has a documented injury, chronic condition, or medical limitation, test results must be interpreted with caution. A stumble, sway, or loss of balance does not automatically indicate intoxication.

Medical context matters.

Fatigue and Environmental Factors

Standardized Field Sobriety Tests assume a baseline level of alertness and physical readiness. Fatigue can impair balance, reaction time, and divided attention, all of which are critical to SFST performance.

NHTSA emphasizes proper administration conditions for a reason. Roadside testing late at night, after long work shifts, or in physically demanding circumstances introduces variables that were not part of the original validation studies.

Comprehension and Communication

SFSTs rely on the subject’s ability to understand and follow detailed verbal instructions. NHTSA training emphasizes that instructions must be given clearly and precisely.

When comprehension is compromised, whether due to language barriers, hearing issues, or processing difficulties, performance may suffer. Difficulty following instructions is not the same as impairment, yet it can be scored as a failure if context is ignored.

What the Tests Do and Do Not Measure

Field sobriety tests do not measure impairment directly. They measure performance on specific tasks. NHTSA does not claim otherwise.

Poor performance can result from medical conditions, physical limitations, fatigue, injuries, or environmental factors. The manuals acknowledge that officers are observing behavior, not determining its cause.

That distinction is critical.

Why This Matters to Jurors

Jurors often assume that standardized tests are fair to everyone. They assume the tests account for normal human variation. NHTSA’s own materials show that this assumption is incorrect.

When medical records, injury history, or physical limitations are present, they provide essential context for interpreting test results. The question is no longer whether a person performed poorly, but whether the test was reliable for that person at all.

The Takeaway

NHTSA’s current SFST materials acknowledge that field sobriety tests have limits. They are not validated for every individual or every circumstance. They do not diagnose impairment, and they do not explain why a person performed the way they did.

Any OVI case that relies heavily on field sobriety tests deserves careful scrutiny. Medical history, physical condition, and testing conditions matter.

Harmony Law evaluates OVI cases by examining the science, the training, and the limitations NHTSA itself recognizes. That analysis often makes the difference between assumption and proof.