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.

  • Tox Lab Guy

    However, that is an atypical circumstance, and those error ranges are designed to flag the wayward instrument. If you were to look at a large population of replicate results, a very high majority of those would be well within the 0.02 g/dL range. I would be interested in seeing that data, but I doubt it would serve your purpose much.
    In addition, your example improperly refers to a 40% range of error in the second test.  A more accurate depiction would be ± 0.02 g/dL error, and the difference is important. In your example, the replicate readings can be 0.10 g/dL and any one of the following: 0.08 g/dL (20% less), 0.09 g/dL (10% less), 0.10 g/dL (no difference), 0.11 g/dL (10% more), and 0.12 g/dL (20% more). Those numbers are dependent upon the fact that you chose 0.10 g/dL as your benchmark. A higher (or lower) BAC will change those percentages, because the ± 0.02 g/dL requirement is fixed, regardless of the BAC.

  • Lawrence Taylor

    ToxLab Guy:
    I’m not sure what you mean by an “atypical circumstance”.  I agree that a high majority of tests will reflect duplicate test results within the + / – .02% range.  Of course, my point is not that law enforcement is not getting results within this range, but rather that this range is entirely too broad:  a permissible second-test variation of 40% is no indication of acceptable scientific accuracy — nor of legal proof beyond a reasonable doubt.
    As to my “improperly referring” to a 40% range of error, I use this illustrative figure because the second test is designed as an accuracy check on the first.  The question is what the permissible range is for that second test for there to be legal “accuracy”.  If the first test is .10%, the permissible range is .08%-.12%, which represents a .04% range.  This means that the accuracy check on a .10% reading is 20% in either direction, or a total allowable range of 40%
    Of course, the 40% depends upon the numerical value of the first test; the numerical range varies with the vlaue of the initial test.  I could, for example, have chosen .08% as an example, in which case the range of error would be 50%.  I chose .10% as an example because (1) mathematically it is easier to illustrate the point, and (2) in my involvement with thousands of DUI cases over 30 years, it has been my experience that the majority of DUI test results fall within the .08% to .12% range.
    It may well be that we are engaged in semantics.  In any event, I appreciate your well-reasoned comments and continuing contributions to this blog.

  • Tom Barnes

    Bring down the use of breathalyzers period.   They are undependable at best.   All they are is a number generator and they leave out the fact that diabetics, Atkins dieters, people who have an empty stomach, or suffer from hypoglycemia can all blow substantially higher BACs because of ketosis.    We learned all of this after my son was arrested, charged with and convicted of a plea-bargained OWVI in Michigan.   He is 34 years old.   I am writing as his father and I have not had a DUI.  And I am not an attorney; just a parent who watched his son get raked across the coals of an unfair and prejudicial legal system here in Michigan.
    He was stopped for breaking the speed limit on an rural expressway; he was 8 MPH over the limt.   At the time of his traffic stop he was following a strict Atkins diet and had consumed 2 beers over a 3 hour period prior to his drive.   His second beer was finished a half hour before he drove.   But my son works for me and we are in close proximity to eachother often in the work place.   For a months before his DUI I had commented to him on his body and breath smelling of alcohol.    This could be at any time of the day, and I know he wasn't drinking at work.    He said that he wasn't drinking and I believed him as he would be in my presence most the most part, and it would be impossible for him to be drinking while on the job.    I couldn't imagine why this alcohol smell was present in him.   An acquaintance smelled it one day and commented to me as to why my son was drinking on the job!   We work in an office, and we don't keep beer there and he doesn't go into bars, and he doesn't hide beer in his car.    These things I know for a fact.
    Then one day I challenged my son to use my personal breath tester.   I carry a breath tester at all times to make sure that I never drive over the (or even near) .08.      My son had been in my presence all morning, and this day the smell of alcohol on him was present.   It was about 1:00 PM.   He took the test using my alcohol tester and it returned an astounding .12 reading.   I then used the tester and my reading was .00.     We also had noted that my son's hands use to shake whenever he was hungry, especially when he smelled of that daytime alcohol.
    We then started doing some research.   We actually found the article that has started me on my drive to have the use of the breathalyzer stopped all together.   It was the article that informed us that the breathalyzer is not smart enough to be able too tell the difference between ethyl alcohol that in injested while drinking, and acetone or isopropyl alcohol that the body produces when a person is for instance on the Atkins diet and/or doesn't eat any carbohydrates, or who for whatever reasons, just has a very empty stomach.    This is because these conditions produce "ketones" which cause the body to create the isopropyl alcohol. When a person blows isopropyl alcohol into most DUI breath testing instruments, it typically gets mistaken for ethyl alcohol (the type of alcohol that we consume in an alcoholic beverage). As a result, many innocent people are often falsely accused and then wrongfully convicted of DUI.
    My son stopped the Atkins diet, has stopped having "alcohol breath", and we now are preparing to file suit against his attorney for malpractice.   My son was not informed by his supposed "DUI expert" attorney of any of this and his .10 BAC was probably a very inflated number due to the fact of his being on the Atkins diet.   In fact, the attorney told my son to testify untruthfully that my son "felt impaired" (which he didn't) in order for his attorney to appease the court and get a plea bargain down to 4 points on his driving record and a lesser charge of OWVI (operating while visually impaired).   My son can also now blow a .00 in any breathalyzer, which while on the Atkins diet was impossible for him, even totally sober.
    We also learned during this process that the courts are crooked as all hell in this DUI bullcrud.    All the courts want in a conviction and they don't give a damn as to how the conviction is achieved.   Then the money-grab starts!   It's a giant money scam and it's ruining the lives of many members of the 25 – 40 year old generation as well as others.    Even first time offenders have this black blot on thier driving records for life which is very unfair.  Nothing this minor should be on anyone's driving record for life.   For first-time offenders with low-BACs, if in five years there is no repeat, then the charge should come off of thier record.  Some people DO learn by thier mistakes and should be rewarded for doing so.  
    In Michigan, I am hearing of plans for a giant "March on Lansing" in protest to these needlessly harsh punishments for driving with what amounts to sometimes a beer or two.    And as we learned, my son, if on an empty stomach and while in the Atkins diet, could actually blow a .12 while being 100% sober!   I will beat the march if it happens calling for the end of the use of breathalyzers.   Enough is enough.   This DUI thing is completely out of control and a pure money-grab, is needless, takes away the right to a fair trial to the victim of a DUI stop, and is based on the lies and exagerations of fanatical special-interest groups such as MADD.

  • curls

    @Tom Barnes
    Alternatively – don't drink anything before driving & the false levels won't matter.  A beer or two IS IMPAIRED.  Not as much as 10 drinks, but enough to increase changes dramatically of injuring another human being. 
    As a person hospitalized by a person who had a couple drinks at lunch, I'd be happier if they hadn't and I still had my internal organ that was removed, by car, and less blood in my brain to recover from.

  • sam

    A few years ago I was pulled over at a checkpoint in PA
    My breath was a .12- very high
    But my blood came back at  .10- still high ( I understand ) but that is a rather large difference in terms of fines and suspensions. How often do police have a driver blow for an extended amount of time- as soon as they are pulled over (without the 15 min observation period and second test within 10 min )  and use such a large variant producing instrument for probable cause?
    It just seems that courts and counties are trying to balance their budgets on the wallets of tax paying citizens- not to mention that they keep lowering the BAC rates. Is this to make things safer? 10 years ago a .12 was the drunk level-  then .10 – now .08- what next .04?