Tuesday, August 10, 2010

Flawed science behind alcohol breath tests?

We tend to think of flawed forensics happening in high-profile rape and murder cases because that's largely who DNA exonerations have let out when bad forensics falsely convicted them. But the truth is many forensic methods and technologies in use today have not been vetted to a high standard of certainty, including forensics in more common cases like alcohol breath tests used at DWI stops.

Walter Reaves out of Waco points to this recent article by Dr. Michael P. Hlastala from the Journal of Forensic Sciences titled "Paradigm Shift for the Alcohol Breath Test" (pdf) that raises questions about DWI breath test technology. Writes Hlastala:
The breath test is an indirect test, but has been considered to be a good estimate of the BAC because of the assumption that an end-exhaled breath sample accurately reflects the alveolar (or deep lung) air alcohol concentration which is thought to be in equilibrium with the blood in the pulmonary circulation. In spite of the considerable effort that has gone into the studies attempting to validate the breath test, forensic scientists and toxicologists still have only a very basic understanding of the physiological aspects of the alcohol breath test (ABT) and associated limitations.
As Reaves summarizes the research:
The "old paradigm" assumes the amount of alcohol in the breath remains constant as it goes through the lungs. It turns out that is not accurate. In fact, the amount varies - sometimes significantly. The result is that the actual blood alcohol level may be over or under-represented.

The new paradigm recognizes that alcohol is deposited in the airway surfaces during both inspiration and expiration. It also recognizes that the alcohol that comes out in the breath test comes from airway surfaces rather than the alveolar region.
Indeed, found Dr. Hlastala:
All of the alcohol exhaled at the mouth comes from the airway surface via the bronchial circulation. Very little, if any, alcohol originates from the pulmonary circulation surrounding the alveoli. The fact that alcohol comes primarily from the airways is the reason why the BrAC depends on the breathing pattern. This contributes to the very large variation in the ABT readings obtained from actual subjects.
So air breath tests are based on a fundamental misunderstanding of the relation between alcohol in the breath and the bloodstream - a false belief that the exhaled air comes from the deep lungs. The researcher suggests potential policy implications for this updated understanding of breath patterns, concluding that:
Given the variation in the breath alcohol test, it might be appropriate to consider decreasing the importance of threshold levels for penalties. After further experimentation, it might be possible to define the variation due to breathing-related variables and to allow for a magnitude of error in the breath test. Penalties could be graded with a sliding scale that allows for error in the breath test and a continuously graded scale of penalties as the BrAC increases. In any case, this new recognition of the limitations of accuracy of the ABT warrants reconsideration of the breath test protocols used.
In other words, there's a margin of error on the test that's not recognized in the law but which makes using breath tests as the threshold for penalties problematic. Reaves concludes:
In Texas, limits are important for not only determining whether someone is guilty or not, but also for determining whether certain conditions are going to be imposed - such as a interlock device. The validity and accuracy of the breath test results is therefore critical.

This article does not break new ground - problems with breath testing have long been recognized. Those problems must be explored - especially in marginal cases. Where someone's future hinges on a machine, the least we can do is make sure the machine is accurate.

7 comments:

D.A. Confidential said...

How about we use the breathalyser, especially in marginal cases, to establish probable cause for obtaining a search warrant for a blood test?
After all, if this is about making sure our readings are accurate, blood testing doesn't have those same concerns, that I know of.

Gritsforbreakfast said...

DAC writes: "if this is about making sure our readings are accurate ..."

"The time has come," the Walrus said, "To talk of many things: Of shoes--and ships--and sealing-wax-- Of cabbages--and kings-- And why the sea is boiling hot-- And whether pigs have wings."

My point isn't to argue for or against any given defendant but to catalog the array and varieties of flawed forensic science. I find it remarkable that those who created the breath test fundamentally misunderstood what they were measuring, that everybody pretty much accepts that, and yet ABTs are used everywhere.

In the face of that, I can understand why using breath tests as probable cause would be a good backup stance for prosecutors if it's no longer considered good prima facie evidence. But my broader interest is the widespread use of shoddy science or worse and the lack of rigorous testing of most forensic methods and technologies.

Soronel Haetir said...

My understanding is that even the actual blood tests have problems due to inconsistent handling/storage of the vials. If it gets tested right away you will likely have a decent result but there are plenty of cases where that doesn't happen for whatever reason.

Prison Doc said...

It's all a bit of a crapshoot, isn't it?
The MADD folks don't want innocent lives devastated...but inaccurate BAT's and the medieval "roadside sobriety test" devastate their share of innocent lives too. Kind of like throwing a witch in water and if she drowned she was innocent.

Roy said...

Even worse, breathalyzers do not detect alcohol: they detect methyl groups. They are not supposed to be used if someone is chewing gum, using a breath mint, or sipping Sprite -- unless adequate time for the methylated aromatics to evaporate from the mouth, but who ever heard of a cop being willing to wait?

Anonymous said...

For the lawyers that read this blog and post, get the manufacturer's manual for the machine. I have learned that the SOP for the DPS labs do not encompass every step and/or safeguard the manufacturer recommends on gas chromatograph machines. Even for breath machines there are omitted safeguards.

Even if you have to pay for the manuals, it will be worth the expense to show in front of a jury that the lab does not follow every step the manufacturer recommends. When the lab person gets through with their standard spill about how accurate the machine is, most juries will not buy the hog wash from the state's paid lips.

R. Shackleford said...

BAC analyzers are big business. Lots of money made for the state and the manufacturers. If it makes the state money, don't expect the model to change for some time, accuracy or no. For instance, the SCRAM, or SCAM ( as we not so affectionately call it) has an accuracy rating of anywhere from 80% to 87%, UP TO THE FIRST TWO WEEK PERIOD. After that, it's accuracy degrades significantly every single day. Take this in conjunction with the sheer number of daily tests (24 at a minimum), and statistically speaking the SCAM will generate dozens of false positives over the course of a standard 90 day term. In keeping with the usual lack of technical understanding on the part of most judges, and you have lots of folks being locked up for false positives. Add to this the fact that this device picks up any AIRBORNE chemical ending in 'ol', and registers this as a possible drinking episode, and you get a recipe for disaster. I could go on, but I think y'all get the point. Nobody really understands BAC, and to use it as a definitive tool for incarceration is criminal.