Ohio BAC Test Accuracy

Blood-Alcohol Tests

How does the prosecution know that the defendant was eliminating at that rate and not at .005 percent or .3 percent! Quite simply, the prosecution does not know: It merely assumes that the defendant eliminates at the average rate. And, of course, error in such an assumption translates into error in the extrapolation.

This Imaginary "average person" relied on for the accuracy of breathalyzer and other "scientific" alcohol tests is not limited to chemical analysis. When the officer administers the  "horizontal gaze nystagmus'' test as part of the battery of field sobriety tests, he operates on the assumption that the suspect meets the definition of the "average guy."  The officer has been trained to "read" at what angle the suspect's eyes begin jerking. A blood-alcohol reading can theoretically be obtained by subtracting the angle from 50; jerking at 35 degrees, for example, would mean the suspect has a blood-alcohol level of .15 percent. Where does the magic figure of 50 come from? The average person.

An alternative method of administering the nystagmus test is to "flunk" the person if jerking begins before 40 or 45 degrees. Why? Again, because the average person would theoretically have .10 or .05 percent alcohol in his blood at this point.

If you are stopped and asked to perform the test, the police officer has no way of knowing what your "baseline" is.  The important question is not what the average person's reaction is to this test but rather what is the angle at which your eyes would begin jerking when you are sober.  Chances are, you are not physiologically identical to this imaginary "average guy."

Breath-Alcohol Testing Definition
An encyclopedia definition of breath tests that are given to detect alcohol content in the body. 

How Breathalyzers Work
A layman's explanation of the theory and operation of breath alcohol devices used by law enforcement. A thorough treatment by a Ph.D. from the "How Stuff Works" folks.