Motorists arrested for DUI / DWI, driving under the combined influence of alcohol and drugs, or any drinking and driving offense are offered an evidential breath test or a blood test. (When motorists are arrested for DUID – driving under the influence of drugs – they will be offered a blood or urine test.) Clients arrested for DUI / DWI often ask, after the fact, whether they did the right thing by selecting the breath test, or whether the blood test would have been better. Thankfully, criminal defense lawyers who focus on DUI / DWI defense have developed strategies to attack the results of both blood and breath testing.

When someone is arrested for drunk driving, the breath machine they are tested on varies from jurisdiction to jurisdiction. No matter which machine forms the basis for the DUI / DWI charge, all breath testing in drinking and driving cases is an indirect measurement of blood alcohol content (BAC). Breath testing devices all engage in a conversion process, whereby a blood alcohol level is predicted on the basis of certain scientific assumptions, which may or may not be applicable to the person being tested, arrested, and prosecuted for DUI / DWI, drunk driving, or driving under the influence of alcohol and/ or drugs (DUID). These scientific assumptions include drivers’ blood/breath partition ratio, their breath temperature, and many other factors.

Additionally, since breath testing in a DUI / DWI always takes place after the time of driving (which is, of course, the relevant time in a drunk driving case), the prosecution attempts to look backwards and give an opinion about the allegedly drunken driver’s alcohol level at the earlier time of driving based upon the later breath test results.

There are significant problems with this type of speculation. Alcohol levels change over time, and the manner in which they change is subject to much variation. Stomach contents, the amount of alcohol consumed, the length of the drinking period, body weight, gender, elimination or “burn-off” rate, and other personal metabolic and medical factors all interfere with the assumptions made by the prosecution in a DUI / DWI arrest.

To get around these problems in DUI / DWI investigations, many states use a blood or breath standard, meaning that it is a violation of the “per se” charge when the person gives a blood sample or a breath sample that exceeds the legal DUI / DWI limit. There are many challenges to breath testing in DUI / DWI cases. Some of the defenses assume the truth of the results, but challenge whether the driver was under the influence of alcohol or exceeded the legal limit at the time of driving. Other attacks relate to the accuracy breath test results in the first place. A criminal defense attorney will typically explore both avenues on behalf of his or her DUI / DWI client.

The first place to look when challenging a DUI / DWI breath test is the functioning of the breath test machine itself. The type of machine will vary from state to state, and from county to county within a given state. No matter what machine is used, the machine must be working properly to give a reliable result. This means ensuring that the accuracy checks, calibration records, usage logs and maintenance history are all properly documented and don’t reveal any problems.

An experienced New York DUI / DWI defense lawyer will know just how to investigate each area of potential problems. When these problems are revealed, they may result in the breath test results being excluded or may be used to attack the weight of the evidence. In either event, a criminal defense DUI / DWI lawyer will know how to use these issues to the driver’s best advantage.