Tuesday, July 01, 2008

Debating DWI breath test accuracy from 'The Defense Perspective'

According to Houston attorney Mark Bennett, there are only two times when you should refuse a breathalyzer test at a traffic stop: When you're guilty and when you're innocent. The reason innocent people should refuse, he says, is the machine's high error rate (up to 25%) and the risk of a false positive.

It was poor accuracy in breathalyzer tests that first turned this blog's attention toward the reality that accuracy appears optional in many forensic science endeavors, with error rates of 10% or more routinely accepted in a variety of forensic fields.

Indeed, in some states breathalyzer results must be accepted in court "regardless of whether the particular breathalyzer was broken, defective, given incorrectly or otherwise inaccurate."

Stephen Gustitis at The Defense Perspective has been posting an excellent blog series on the subject of scientific flaws with the Intoxilyzer 5000, the primary forensic tool used in most jurisdictions for DWI breath tests. Gustitis has offered up these detailed critiques so far:
And more's coming, he promises. Bottom line, says Gustitis, the scientific assumptions behind breathalyzer tests assume results are measured in a closed system, which the human lungs are not. The tool works fine in the lab, in other words, but probably isn't accurate enough for field work involving humans. E.g., the device assumes a constant breath temperature, and slight variations can cause the machine to overstate blood alcohol levels.

Most people don't think about forensic errors except in high-profile cases - e.g. when DNA disproves old forensics to identify a wrongful conviction. More frequently ignored but just as corrosive to justice are routine errors tolerated in lower-level workaday cases like DWI.

7 comments:

whitsfoe said...

It doesn't matter in Williamson County Grits. This weekend, according to the Statesman, they're just gonna force anyone suspected of a third DWI to give blood to determine blood/alcohol ratios. I can only imagine what would happen if someone tried to refuse. Those blood sucking bad boys in Wilco...

Anonymous said...

It used to be, here in Alberta, breath tests were only used for screening. The testers also did not give a numeric value, only Pass, Warn, and Fail. Failing got you a blood test. Failing the blood test got a conviction. It worked much better. They changed it some years ago claiming the blood tests cost too much. :(
It should be noted, however, that DWI here is Driving While Impaired, and is not limited to substance abuse.

Anonymous said...

Who is Williamson Co. going to get to draw blood? Look at the legal implications involved; person drawing blood not knowing proper technique, possiblity of not knowing the anatomy of the body and not knowing that some peoples veins do not run the same paths as the normal routes taken by other peoples anatomy. Then what about contamination? What is that blood is contaminated, which is a great possiblity and infection also. A person who draws blood should be a medical person who knows anatomy and knows that some veins have to be felt and until you learn that veins tend to roll the person drawing that blood could cause undo pain and that is totally wrong.

If I were Williamson County, and DA John Bradley, I would certainly give this a lot more thought before I let this be done. No one will ever draw my blood unless I see a certificate stating they have passed all the legal requirements and are legally and have knowingly accepted the responsibility of drawing blood. This is from a medical person and you can find this in an medical book you want to look at, taking blood from someone involves a lot more than just sticking a needle in someones vein, it is harder than those in Williamson County tend to know.

Anonymous said...

When you drink alcohol the blood in the arteries has more alcohol than blood in the veins.

The law doesn't specify any difference but to be fair in an "equal protection" sort of way... it should be taken from everyone the same way.

Anonymous said...

Anonymous 11:46 AM please read the comment above yours. I am a Registered Nurse and I know a few things about how to draw blood. I also, when a person is intoxicated, there is more alcohol in the blood, but what I am trying to impart is; there is a way to draw blood and a way to not draw blood in a safe manner. You have to be trained and know the antomy to know where the veins run and you can not always see them. A trained person can feel and tell a vein is there by touch, where an untrained person will only cause pain to someone who perhaps is heavy set, they are particularly hard to draw blood on and especially women. Also people who are Black are a hard stick in some instances and a trained person can feel where the vein is and be able to stick that vein also.

Your response makes no sense whatsoever and if you would like to reference your source of knowledge, I would be glad to reference mine. Mine begins with 4 years of college, over 20 years in the operating room and emergency room, so please be more specific when you try to dispute a known fact!

Anonymous said...

Hey 5:02, it wouldn't surprise me to see Williamson County just punch someone in the nose to get their blood. They don't need needles.

Anonymous said...

When my wife gives blood for test, the RN thanks she knows everything so she sticks my wife everywhere, the wife told her several time to stick her here.After a few words the RN listens to her.As for me I went to Herman Hos. Emergency Room and they drew my blood and placed it on my chart.I was in the fast track section of the ER and this lady came in and they did the same thing.To make a long story short nether tube of blood was labeled,the RN came back in and looked at both charts and we both where watching her and blood was switched.It was done by acc.I know.So just because you have 4yrs of college and 20 yrs of OR and ER does not make you a good stick.So you can get off your high horse.And everybody knows there is alcohol in the blood.