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Field Sobriety Tests

Why Test?

Alcohol intoxication is legally defined by the blood alcohol concentration (BAC) level. However, taking a blood sample in the field for later analysis in the laboratory is not practical or efficient for detaining drivers suspected of driving while impaired (DWI) or driving under the influence (DUI). Urine tests for alcohol proved to be just as impractical in the field as blood sampling. What was needed was a way to measure something related to BAC without invading a suspect's body.

In the 1940s, breath alcohol testing devices were first developed for use by police. In 1954, Dr. Robert Borkenstein of the Indiana State Police invented the breathalyzer, one type of breath alcohol testing device used by law enforcement agencies today.

Let's take a look at what these tests are based on.

Principle of Testing

Alcohol that a person drinks shows up in the breath because it gets absorbed from the mouth, throat, stomach and intestines into the bloodstream.

Alcohol is not digested upon absorption, nor chemically changed in the bloodstream. As the blood goes through the lungs, some of the alcohol moves across the membranes of the lung's air sacs (alveoli) into the air, because alcohol will evaporate from a solution -- that is, it is volatile. The concentration of the alcohol in the alveolar air is related to the concentration of the alcohol in the blood. As the alcohol in the alveolar air is exhaled, it can be detected by the breath alcohol testing device. Instead of having to draw a driver's blood to test his alcohol level, an officer can test the driver's breath on the spot and instantly know if there is a reason to arrest the driver.

Because the alcohol concentration in the breath is related to that in the blood, you can figure the BAC by measuring alcohol on the breath. The ratio of breath alcohol to blood alcohol is 2,100:1. This means that 2,100 milliliters (ml) of alveolar air will contain the same amount of alcohol as 1 ml of blood.

For many years, the legal standard for drunkenness across the United States was 0.10, but many states have now adopted the 0.08 standard. The federal government has pushed states to lower the legal limit. The American Medical Association says that a person can become impaired when the blood alcohol level hits 0.05. If a person's BAC measures 0.08, it means that there are 0.08 grams of alcohol per 100 ml of blood.

There are several different devices used for measuring BAC.

Since the mid 1970's, the National Highway Traffic Safety Administration (NHTSA), with the cooperation and assistance of the law enforcement community, has conducted research that resulted in the development of a battery of three standardized field sobriety tests (Horizontal Gaze Nystagmus, Walk and Turn and the One Leg Stand) to assist police officers in detecting impaired drivers. These tests, formerly referred to as Improved Sobriety Tests, are now the Standard that the Federal Government wants to be utilized.