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Proponents of the so-called “standardized” field sobriety tests (SFSTs) have long pointed to field studies which indicate a high correlation between performance on the tests and actual blood alcohol concentrations (BAC).  A new study now calls those conclusions into question.

Originally, the National Highway Traffic Safety Administration (NHTSA) paid a private group, the Southern California Research Institute, to conduct studies to find which among the various field sobriety tests used by police were most effective and to develop a standardized 3-test battery.  SCRI subsequently reported to NHTSA that a battery of walk-and-turn, one-leg-stand and nystagmus provided a strong correlation with breath test results.

Confronted with questions about those conclusions, NHTSA later commissioned the same researcher who had conducted the original studies, Marcelline Burns, to  corroborate the accuracy of her own findings.  Burns accompanied a small number of San Diego police officers conducting actual DUI investigations in the field.  After administering the SFSTs, the officers were asked to guess whether suspects had blood alcohol  concentrations (BAC) over or under .08%.   Burns reported a 91% correlation between SFSTs and BAC over-under estimates, arguably validating the battery of tests she had helped create.

A subsequent scientific article called Burns’ conclusions into question.  In Hlastala, Polissar and Oberman, “Statistical Evaluation of Standardized Field Sobriety Tests”, 50(3) Journal of Forensic Sciences 1, the raw data used in the validation study were obtained from NHTSA through the Freedom of Information Act.  The methodology used was then reviewed and the data subjected to statistical analysis.

The methodology was found to be seriously flawed in a number of respects.  For one thing, many of the suspects had very high BACs, making estimates of whether a suspect was over .08% obvious regardless of SFST performance.  For another, there was no attempt to isolate the influence of SFST performance from other factors:  officers estimated BACs after the field sobriety tests, but they also took into account earlier observations, such as erratic driving, slurred speech, odor of alcohol, flushed face, admissions as to amount of alcohol consumed, etc.

The most glaring defect in Burns’ corroborative study was that “all police officers  participating in the study were equipped with NHTSA-approved portable breath testing devices”.  In other words, the San Diego officers already had the results of portable breath tests when they were asked to estimate the BACs later obtained at the station!

After reviewing the flawed methodology, the raw data was then statistically analyzed.  The conclusions:

If we consider three ranges of MBAC [measured blood alcohol content], 0.00% to 0.04%, 0.04% to 0.08%, and 0.08% to 0.12%, the officers’ EBAC [estimated blood alcohol content] overestimated the MBAC 76%, 67% and 48% of the time, and underestimated it 14%, 26% and 28% of the time. 

In other words, officers relying upon field sobriety tests were far more likely to overestimate  BACs than underestimate — particularly with those suspects having low BACs. 

(T)he utility of the SFST depends very much on how intoxicated an individual is.  Accuracy (and specificity) are low when individuals are close to 0.08% MBAC, but if the individuals are quite intoxicated, such as above 0.12%, then accuracy is high.

In other words, in borderline cases involving persons at or under the legal limit, officers were very poor at estimating levels over .08% based upon SFSTs.  And it is these cases, of course, that are critical.  Suspects with high BACs are relatively easy to single out without the help of field tests; it is for the closer cases, particularly those who are innocent (below .08%), that the SFSTs are designed.  And it is with these very cases that the tests apparently fail. 

Put another way, accuracy in using field sobriety tests is high when they are not needed — and low when they are. 
 

DUI BLOG

If you are arrested for DUI and a breath test shows a blood alcohol concentration (BAC) of .08% or higher, you are guilty. It does not matter, of course, whether you are a man or a women: the laws do not discriminate.

Maybe they should…

Researchers at the University School of Medicine in Trieste, Italy, found that the stomach lining contains an enzyme called gastric alcohol dehydrogenase that breaks down alcohol — and that women have less than men. To determine the relative effects of the enzyme, they gave alcohol both orally and intravenously to groups of alcoholic and non-alcoholic men and women. They found that women reached the same levels of blood alcohol as men after drinking only half as much; with weight differences taken into account, they found that women reached BAC levels illegal in a DUI case after drinking 20 to 30 percent less alcohol than men.

The scientists’ conclusion: legislatures may need to consider sex differences in drunk driving laws when defining safe levels of drinking for driving motor vehicles. Frezza and Lieber, "High Blood Alcohol Levels in Women: The Role of Decreased Gastric Alcohol Dehydrogenase Activity and First-Pass Metabolism", 322(2) New England Journal of Medicine 95 (1990).

Yet another study has found that women have lower "partition ratios" of blood to breath. What kind of ratios? Well, all breath machines in DUI cases measure the amount of alcohol in a person’s breath. But the what we really want to know is the amount of alcohol in the person’s blood. So how do we get that? Simple: a small computer in the breathalyzer multiplies the amount of alcohol it detects in the breath sample by 2100 times.

This is based upon the theory that, on average, there are 2100 units of alcohol in the blood for every unit of alcohol in the breath. (Note: that’s an average — but it varies from person to person.) According to the study, women have a significantly lower partition ratio. Jones, "Determination of Liquid/Air Partition Coefficients for Dilute Solutions of Ethanol in Water, Whole Blood and Plasma", Analytical Toxicology 193 (July/August 1983). And the lower the ratio, the higher the reading — even though the true BAC does not vary. Example: a woman with a true BAC of .06% and a ratio of 1500:1 (rather than the presumed 2100:1) will get a reading on the machine of .09% — above the legal limit. Put another way, the breath machine will show an average man accused of drunk driving to be innocent — but a woman with the same blood alcohol level to be guilty.

And then there’s the problem of birth control….

Scientists in Canada have found that "women taking oral contraceptive steroids (O.C.S.) appeared to eliminate ethanol significantly faster than women not taking O.C.S." Papple, "The Effects of Oral Contraceptive Steroids on the Rate of Post-Absorptive Phase Decline of Blood Alcohol Concentration in the Adult Woman, 15(1) Canadian Society of Forensic Science Journal 17 (1982). That means that women will reach peak BAC faster, and return to lower levels more quickly. This, of course, can create serious problems in a DUI case when attempting to estimate BAC at the time of driving based upon a breath test administered one hour later. Making the problem worse, researchers have also discovered that women who were taking birth control pills or who were pregnant had higher levels of acetaldehyde on their breath, due to the decreased ability to metabolize the enzyme as the level of sex steroids increases.

So what?

Well, most breath machines use infrared analysis in measuring the breath sample of a DUI suspect. But these machines don’t really measure alcohol, rather they measure any compound which contains the "methyl group" in its molecular structure. And acetaldehyde is one of these compounds. Result: a higher "blood alcohol" reading on the breathalyzer. Jeavons and Zeiner, "Effects of Elevated Female Sex Steroids on Ethanol and Acetaldehyde Metabolism in Humans", 8(4) Alcoholism: Clinical and Experimental Research 352 (1984).

It’s always a problem when the law, in its infinite wisdom, assumes that all of us are exactly the same.
 

DUI BLOG

As my recent posts have indicated, there is a growing awareness in many states of the unreliability of blood-alcohol testing — and a growing willingness to shutdown testing statewide.  Now this from Michigan…

DUI Blood Cases Could Face Scrutiny After Judge’s Ruling

Lansing, MI.  May 11 – Blood tests in drunken-driving cases statewide will face more scrutiny, experts say, after a Mason County judge ruled that the state crime lab’s test results "are not reliable."

In a ruling signed Friday, 79th District Court Judge Peter Wadel refused to admit blood-alcohol results in a drunken-driving case. He said the crime lab – which conducts blood and other forensic tests in cases from around the state – does not report an error rate, or margin of error, along with blood-alcohol results.

Police routinely report a single number for blood-alcohol content in drunken-driving cases. But East Lansing attorney Mike Nichols, who is handling the case in Mason County – which includes the city of Ludington along Lake Michigan – said there are no absolutes in science.

"Everyone says a blood test is so accurate. Well, it’s not," Nichols said. "That’s what this judge has ruled."

Not including a range of possible results, Nichols said, ignores the uncertainties in the collection, handling, analysis and reporting process.

A blood-alcohol level of 0.08 percent is the threshold in Michigan for being charged with drunken driving. But Nichols said when someone’s blood-alcohol is determined to be 0.10, for example, it could actually be higher – or lower – than 0.08.

The Mason County case is being watched by attorneys across Michigan and the country.

Washington-based attorney Ted Vosk, who consults with defense attorneys and prosecutors about the importance of calculating error rates, praised Wadel’s ruling.

If police and prosecutors don’t acknowledge scientific uncertainties, Vosk said, innocent people will be convicted and guilty people will go free.

"And we won’t know which are which," he said…

Is it possible that law enforcement and the courts will finally stop ignoring science in DUI cases?

(Thanks to John Kruzelock.)
 

DUI BLOG