Breathalyzers: 40% Margin of Error is Acceptable Accuracy?

Posted by Lawrence Taylor on June 20th, 2006

With more than a little federal coercion, all states have now passed laws making it a criminal offense to drive with .08% alcohol in your blood. And most people suspected of violating that law are given breath tests to determine their blood-alcohol concentration (BAC).

The breathalyzer will take a small sample of the suspect’s breath and estimate how much alcohol is in it — and, then, estimate how much may be in the blood. And what that machine says is pretty much the end of it. There will be no second tests. There will be no cross-examination of the machine.

Are these machines so reliable and accurate that we have permitted them to become judge and jury and to determine guilt beyond a reasonable doubt?

Ignoring the many flaws of the machines for the moment (see “How Breathalyzers Work — and Why They Don’t”), scientists universally recognize an inherent error in breath analysis, generally of plus or minus .01%. That means that if everything is working perfectly (an unlikely scenario), a .13% breathalyzer test result can be anywhere from .12% to .14%. This has been acknowledged by courts across the country (see, for example, People v. Campos, 138 Cal.Rptr. 366 (California); Haynes v. Department of Public Safety, 865 P.2d 753 (Alaska); State v. Boehmer, 613 P.2d 916 (Hawaii), recognizing an even larger .0165% inherent error).

What does that tell us about the accuracy of these breathalyzers? Well, let’s take a common test result of .10%. Taking inherent error into consideration — and assuming the machine was working perfectly, the officer administers the test correctly, and the suspect’s physiology is normal and perfectly average — the true BAC could be anywhere from .09% to .11%. In other words, the true BAC can be 10% in either direction — or, put another way, anywhere within a 20% margin of error.

These machines have a 20% margin of error?

That’s right. A person accused of driving with over .08% BAC can be convicted by a machine whith a built-in 20% margin of error. Would you be comfortable with an airline pilot who worked with a 20% range of error? With a bank teller who gave you $90 cash for a $100 check? How about the sole evidence in a criminal case where guilt must be proven beyond a reasonable doubt?

But it gets worse. Most states have laws which establish standards for breath alcohol analysis. These set forth minimum levels of accuracy in a given test, usually determined by the requirement that two separate tests produce results within a given range. California’s requirements, for example, are fairly typical: to be admissible in court there must be two test results that are within .02% of one another.

What does that mean? Well, let’s again assume that you breathe into the machine and it produces a .10% reading. You are now required to breathe into the machine a second time. What test result would be necessary for the evidence to be considered acceptably accurate and legally admissible? .08? .09? 10? .11? .12? Actually, any of these readings would satisfy the .02% requirement: anything from .08% to .12% would render the test results “accurate”.

In other words, a 40% range of error in that second test is deemed sufficiently accurate to sustain the prosecution’s burden of proof “beyond a reasonable doubt”.

Close enough for government work…in a DUI case.