You claimed:

And FWIW, a properly conducted breath test is every bit as accurate as a properly conducted blood test.

That is a false statement. I responded to it. If for no other reason than the blood test is, in most states, definitional for...um...the blood alcohol concentration. To even consider if a breath sample is correct for determining the BAC, they compare it to blood tests.

You can do the research for why there is a difference found between the two. Much of it may be because the test was performed incorrectly. But, even when studies are done by experts (actual doctors and doctorates), there is variation. Some is statistical, some may have other sources. Hyperglycemia is one possibility of error. While many defense attorneys might claim "diabetes" as a cause for the high reading on the breath they have a problem because of real world observation. It is low blood sugar that might cause impairment observed by an officer while it is high blood sugar that might give a false reading on the breath test. But, it is the combination of the two that gives rise to driving under the influence. Hypoglycemia alone can give a higher breath test result than the BAC would be. GERD is another. I know, that was the reason for the caveat of "properly conducted" and why the suspect is watched for 15 minutes and why two samples are taken at different times. But, even then, the procedure only reduces the chance of error, it does not eliminate it.

Then we get to the basic statistic, 1:2100. The claim is that such a ratio favors the person as the actual ratio is around 1:2300--thus making a rounding down of the BAC. But, that is what I talked about in the first post in regard to the "middle sigmas" of population. The ratio varies among people. (For those other than Citicop as he knows, that is the calculation the machine makes to translate concentration in the breath to concentration in the blood.) The ratio on large samples seem to be distributed as a normal bell curve. The numbers on the curve may differ--depending on the size of the study and the accuracy of the instruments and perhaps other facts. But, no matter which study, it is a bell curve.

In a bell curve, the middle two sigmas cover 68.2% of the population. The middle four covers 95.5% and the middle six covers 99.7% of the population. That's not argument, that's just math. One good study has the middle two sigmas (With the median [technically the modal] being 2280:1.) range from 2038 to 2521. The next two from 1797 to 2763. With the next two from 1555 to 3005. That variation is the statistical reason why the breath test is simply not as accurate as the blood test when both are properly done.

A lot of the studies seem from the '80s and '90s in regard to other physiological issues. I assume the more modern machines have adapted to resolve some of them, like the need to take temperature of the breath. Each particular type of machine also has bias of its own based on testing method.