Breath Alcohol Testing

When a driver is arrested for driving under the influence, the police will seek to test the driver’s alcohol content.  The driver may refuse.  However, refusal will result in the automatic revocation of the driver‘s license and a charge of aggravated DWI.


The most popular testing method is by breath testing via a breathalyzer. A breathalyzer is the common term for the machine that uses a breath sample to determine the amount of alcohol in a person’s blood.  The resulting alcohol score is referred to as BAC (breath alcohol content) and this BAC card is what the state will attempt to enter into evidence against you if your case proceeds to trial.

Accuracy of Breath Alcohol Testing

There are a number of potential problems with the accuracy of the breathalyzer.  Though these will not keep the breath score out, the accuracy of the instruments is a common ground for cross examination of the state‘s witnesses, including the officer.  In addition, the defendant may if he or she is financially able, want to hire an expert to address the accuracy issues.  This can be quite effective with juries though much less so in a bench trial as with a DWI first offense in New Mexico.

Partition Ratio:

There are a number of issues with the accuracy of a breathalyzer. The first problem is the use of a “partition ratio.”  When the breath sample is collected by the breathalyzer, the amount of alcohol detected is multiplied by 2100. The number 2100 is the “partition ration,” which represents the assumption that the body contains 2100 units of alcohol in the blood for every one unit of alcohol found in the driver’s breath.

However, testing has shown that the number of units of alcohol found in the blood can actually range from 900 up to 3500. In essence, a driver with a partition rate of 900 taking a breathalyzer would show a breath alcohol content much higher than their true blood alcohol content, due to the fact that their body is different than the scientific average.

Timing of the Test:

The second problem is the timing of the breath testing. The body absorbs alcohol from 45 minutes to two hours after eating. If the driver has food in his or her stomach, the absorption of the alcohol is slowed even more. Additionally, the alcohol in the body is absorbed at differing timelines. In other words, each part of the body can have a different amount of alcohol than others. Therefore, if the breathalyzer testing is done too soon after the driver stops drinking, his or her breath alcohol content reading could be significantly higher than their true blood alcohol level.

False Positives:

Third, the biggest problem with breathalyzers, and the focus of most legal challenges to breathalyzer testing, is the fact that they do not test for just alcohol. Breathalyzers do not actually test for the amount of alcohol in the driver’s body. A breathalyzer tests for and then measures anything in the breath that contains a certain methyl group within the chemical makeup. There are a number of compounds found in the human body that naturally contain the methyl group that a breathalyzer measures, which can erroneously be recorded as alcohol. As a result, a driver’s breathalyzer test could be over the legal limit, even if their true blood alcohol level is much lower.

Failure to Properly Administer the Test

Finally, the breathalyzer testing may not be properly administered or the machine may not be properly calibrated. A failure to properly administer the test will in fact keep the breath score out.

Breathalyzer machines must be properly calibrated and maintained on a regular basis in order to continue providing accurate results. The police department will generally keep records of such calibration and maintenance, but if those records are not available it will call into question the breathalyzer machine’s accuracy.

In addition, there are certain procedures under New Mexico law governing the administration of the breathalyzer test. For example, the driver must provide two breath samples within fifteen minutes of one another following a twenty minute deprivation period.

Consult With an Attorney

These issues should be addressed by your attorney.  Some of the relevant information should be passed on by you so your attorney is aware of potential issues.  An attorney experienced in DWI defense will be alert to opportunities for attacking the BAC card with the ultimate goal of keeping the card out of evidence altogether.

(505) 242-5958