This installment in our series on the Intoxilyzer 5000 involves the concept of retrograde extrapolation. Retrograde extrapolation is the process of calculating the alcohol concentration of a person at a time earlier (the time of driving) than the time of a breath test. Retrograde extrapolation is not technically a defense against the science or technology of the Intoxilyzer 5000. However, it’s a key component in the defense of any College Station DWI breathtest case in which the Intoxilyzer was used to obtain an analysis of a person’s breath.

At the time of driving the breath alcohol content (BAC) of a person’s breath could be less, the same, or more than the BAC at the time of the breath test. This is because an individual’s BAC is reflected by an alcohol concentration curve beginning at the low point when there is no alcohol in the person’s blood. It then rises as alcohol is absorbed (absorption phase) into the bloodstream until it reaches a peak, that is, the maximum alcohol concentration. Finally, the curve falls as the alcohol is eliminated (elimination phase) from the bloodstream to the low point when there is no alcohol left. This describes an alcohol concentration curve.

In a previous post, we learned the Intoxilyzer 5000 takes two readings from two separate breath samples provided only about two minutes apart. For the purposes of drawing an alcohol concentration curve (ACC) these two samples constitute only a single reading. An accurate ACC would require many more points of data than the Intoxilyzer 5000 provides. Consequently, the “single” reading provided by the machine does not provide enough information to determine whether a person is in the absorption phase or the elimination phase at the time of the breath test.

Consequently, if a person is tested while in the absorption phase their BAC at the time of the test will be higher than their BAC while driving. If tested while in the elimination phase their BAC at the time of the test could be lower than while driving, depending on whether they had reached their peak before or after the stop. Obviously, the greater the length of time between the driving and the test, the greater the potential variation between the two BACs.

This uncertainty exposes yet another weakness with using the Intoxilyzer 5000 to prosecute those accused of driving while intoxicated. The informed College Station DWI defense lawyer will understand retrograde extrapolation, and it’s limitations, to help sow seeds of doubt among jurors who are deciding whether the client’s BAC at the time of driving was higher, lower, or the same as the time of the breath test.