Court makes it easier for California DUI defendants to challenge breath tests

Today the California Supreme Court ruled unanimously that defendants in drunk driving cases can challenge accuracy of the state's breath test machine.

Motorists accused of drunk driving can now present evidence that the ratio used by the machine's computer program to calculate the amount of alcohol in their blood that the machine doesn't take into account the driver's own partition ratio, temperature, sex or medical condition. They may also challenge the precision of the machine.

In People v. McNeal, S157565 (July 9, 2009), the Court held that a person charged under the state's "generic" DUI statute (loss of normal use...) has the right to challenge the accuary of the breat test machine, but that a person charged with a per se violation does not because the state mandated the 2100:1 partition ratio in the statute. Mr. McNeal was charged with violating both the generic statute and the per se statute. When he attempted to blow into the machine, it took five blows to produce two valid samples, both reading .10.

The jury convicted on the generic charge but hung on the per se charge.

"Evidence casting doubt on the accuracy of the breath-to-blood conversion ratio is just as relevant as other evidence rebutting the presumption of intoxication from a breath test result, such as evidence that the defendant had a high tolerance for alcohol or performed well in field sobriety testing." Justice Carol A. Corrigan, California Supreme Court.

While we in Texas take for granted our right to challenge the assumptions behind the state's breath test machine, there are plenty of folks accused of DWI that don't have the right to do so. In California the jury is instructed that if a breath test taken within 3 hours of the traffic stop indicates a blood alcohol concentration of .08 or higher, then the jury may infer that the driver was over the legal limit at the time of driving.

I find it very troubling that in some jurisdictions around this country, juries are instructed that they must accept the results of a chemical test in a drunk driving trial. In the end, the state's breath test machine is just another witness; albeit one we don't have the ability to cross examine.

See also:

• California Supreme Court says DUI breath test accuracy can be weighed (San Jose Mercury-News, July 9, 2009)

Pseudo-science and the breath test

According to the DWI statute, a person is intoxicated if they have an alcohol concentration 0f .08 at the time of driving. The alcohol concentration can be measured one of three ways -- by breath, by blood or by urine. If measured by urine, the limit is .08 grams per 67 liters of urine. If measured by blood, the limit is .08 grams per 100 mL of blood. Finally, if measured by breath, the limit is .08 grams per 210 liters of breath.

According to the statute, these three methods of determining alcohol concentration are identical (and that is the last we will speak about urine). If the concentration measured by blood and by breath are the same, that means for every 1 part of blood in your body you have 2100 parts of breath. This is also known as the partition ratio.

The problem is, no one runs a test to determine what the partition ratio is for the citizen accused of driving while intoxicated. There is, in fact, no scientific evidence to support the 2100:1 assumption. Estimates of actual partition ratios vary from 1100:1 to 3400:1! If a person's actual partition ratio is higher than 2100:1, the Intoxi-liar will overestimate his breath alcohol concentration.

This lack of a scientific standard could also lead to a very troubling result -- it is possible for a person to have an alcohol concentration of under .08 by one method of testing but have an alcohol concentration of over .08 by the other method! All the more reason you should NEVER submit a breath sample if you've been arrested for DWI.

Burping, belching or regurgitating prior to blowing in the Intoxi-liar may also produce a false reading due to the presence of mouth alcohol in the sample. The "slope detector" in the machine is supposed to prevent mouth alcohol from producing an inaccurate result -- but if the slope detector program is so reliable, why are breath test operators instructed to keep the test subject in their "continual presence" for 15 minutes prior to the test? Other sources of mouth alcohol are pockets around the base of the teeth due to gum disease and bits of food stuck between teeth or in dental work.

If you've been arrested for DWI, whatever you do, DON'T BLOW IT!