Statewide Iowa DUI lawyer: Does breathalyzer margin of error matter?

The margin of error for a Datamaster DMT is typically +/- .004 g ETOH/210 L breath or five percent, whichever is greater. So what does that mean exactly?

First, we should be clear about what breath test we’re talking about. The Datamaster DMT is the evidentiary, sit-down, desktop breath test machine the results of which are admissible on the issue of intoxication before the jury should the case go to trial.

Here we discuss the difference between the preliminary breath screen, or, PBT, and the evidentiary Datamaster DMT breath test when people refer generically to “breathalyzers.”

With regard to roadside, portable tests called preliminary breath tests, or, PBTs, the short answer is that officers in the field do not apply the margin of error to reduce a preliminary breath test, or, PBT result with the margin of error of that machine. Moreover, the Datamaster DMT test result is evidence in two legal matters: the administrative revocation and the criminal prosecution.

Second, we should be clear that the jury is the ultimate fact finder in any prosecution by which the merits of any case are evaluated. The best evaluative measure of any drunk driving defense is WWJD—what would the jury do if presented with this defense? In a jury trial for Operating While Intoxicated in violation of Iowa Code § 321J.2, the operative jury instruction on the issue of intoxication in the presence of a chemical test of a bodily specimen is as follows:

“The alcohol concentration established by an analysis of a sample of a person’s breath withdrawn within two hours after the person operated a motor vehicle is presumed to be the alcohol concentration at the time he or she operated the motor vehicle.

If you find the defendant was operating a motor vehicle and a sample of his or her breath was withdrawn within two hours after that operation, you may, but are not required to, conclude that the alcohol concentration established by the analysis of the sample was the alcohol concentration at the time he or she operated a motor vehicle.”

So a word about that. The prosecutor does not need to prove the science of the Datamaster DMT machine. In effect, it’s a black box with a breath tube the test subject blows into and the memory test result is a digital fraction. The jury instruction presumes the science of the testing though the Defendant may overcome that presumption by challenging any potential errors during the administration of the test or defects in the machine’s operation. In so doing, they may argue that the machine is a human creation that has a stated margin of error and that reducing the overall result places the Defendant below the presumptive level of intoxication. But the jury is not required to accept that rationale. So margin of error may play a role but not if the jury rejects such a defense.

Pursuant to Iowa DOT regulations, however, if the test result was below the presumptive level of intoxication after applying the margin of error of the machine, the Iowa DOT will not initially revoke a motorist’s privilege to operate a motor vehicle. A revocation may nevertheless still occur if the Defendant is subsequently convicted.

So the margin of error means two different things in two different venues—the criminal prosecution and the Iowa DOT administrative revocation. That is often the confusing part.

If you have been arrested for DUI in Iowa—called Operating While Intoxicated here—contact us for an initial consultation. We'll help you understand it better.