During administration of the field sobriety tests, the officer is looking at more than your performance; in fact, your appearance and behavior can later be used as evidence against you. More specifically, the officer will make note of the odor of alcohol, your speech and how your eyes look to make the case that you were intoxicated. However, these observations are entirely subjective and could be refuted in court.
When pulled over for DUI, the officer will pay close attention to your breath to determine if you smell like alcohol. Unfortunately, this is not an accurate indicator of impairment. One study tested 20 police officers to see if they could correctly identify the type and amount of alcohol on the test subjects' breath, but the results showed no correlation between what the officer smelled and the subject's blood alcohol content (BAC) level.
As people get intoxicated, their speech may become slurred. While a number of studies have proven that speech patterns change significantly as BAC levels become higher, they also show that a percentage of the test subjects exhibited no changes at all. Research also shows that speech is not a good indication of impairment at lower levels.
There are a number of conditions that could cause slurred speech, including stress or neurological disorders. If the officer never heard the driver speak before being pulled over, how could he or she state that the driver's speech pattern has changed due to alcohol consumption?
Officers may cite bloodshot or watery eyes as a sign of intoxication; however, there are a number of common conditions—including fatigue, crying or allergies—that can result in red or glassy eyes as well. An attorney could argue that bloodshot eyes cannot be used to prove an elevated BAC level.