Court Ruling Recognizes Unique Human Element in DUI Cases

Defense attorneys have always had the opportunity to rebut the accuracy of breathalyzer readings. Now that path of attack can include physiological variables giving criminal defense attorneys whose clients are charged with drunk driving a new defense strategy.

The Arizona Supreme Court ruled this week that those charged with DUI may present evidence to jurors that there are variables among individuals that make it impossible to precisely convert a breath reading to someone’s specific blood-alcohol content. One prominent factor to consider includes body temperature and it can be crucial in determining whether a person is legally impaired. There are also arguments that environmental factors and breathing patterns can affect readings.

State law presumes a BAC reading of zero to 0.049 shows the driver is not under the influence of alcohol. There is no presumption either way for a reading of 0.05 to 0.079, with a jury able to consider “other competent evidence’ to determine if the person is guilt. Only at 0.08 and above is someone presumed to be legally intoxicated.

Testimony from a defense expert revealed that hematocrit — essentially the concentration of red blood cells — can affect blood-alcohol concentrations by up to 5 percent, plus or minus. Even a one degree variance in the temperature of a person’s breath from the setting on the breathalyzer could mean a difference up to 8 percent in its BAC reading.

The high court, without comment, upheld the appellate court ruling that those variables are relevant to the question of accuracy of breath tests.

The ruling still requires a defense attorney to provide specific evidence of how his or her client’s physiology differs from the presumptions the machine makes. That goes to the relationship between alcohol on the breath and a person’s presumed BAC level. The Legislature has decided that, as a matter of law, there is a ratio of 2,100 parts of alcohol on the breath to 1 part of alcohol in the blood.

This means that if prosecutors use breathalyzer readings to try to gain a separate conviction on a charge of driving while impaired, then defendants can argue that the ratios do not apply to their clients.