What Is The Accuracy Of Alcohol Screening Machines And Preliminary Breath Test Devices?

There are different types of machines that detect or measure alcohol that has absorbed into the body. A passive alcohol sensor picks up molecules in the air and gives the officers an idea that someone may have been drinking, and/or confirm that they are indeed detecting the odor of alcohol. The San Rafael Police Department has these sensors, but most departments do not.

The field sobriety tests are voluntary. An officer will likely ask the individual to take a preliminary alcohol screening (PAS) test. Most officers use a version called Alco-Sensor 4, which I have in my office. Once the mouth piece is put in and they pull it out of the plastic, it will start giving messages and beeping, and eventually the subject has to blow into it. This is not the evidential test that is used to convict people. However, the D.A. will look at the results of this test when evaluating and negotiating a DUI case.

This PAS test is voluntary, and in most cases, I do not recommend that people take it. The Alco-Sensor 4 has a fuel cell in it. It is accurate. However, it is not as accurate as the evidential breath test machine, which is usually a Draeger machine. The Draeger machine is different because it tests using two different methods; one that uses fuel cell technology, and one that uses infrared technology. There is less of a margin of error with this machine. In addition, once the Draeger machine is used after the arrest, there will have been more time for full absorption of the alcohol, so it’s more likely that there will be an accurate reading. The preliminary test is usually done right after someone has been pulled over, so the individual may still be absorbing alcohol. If someone is still absorbing alcohol, then the actual result can be a false positive (i.e. slightly or significantly higher than the actual alcohol level at the time).

When we enter a trial situation, the district attorney will call a forensic toxicologist to testify about the testing, the accuracy of the machines, and the machine that was used in the case. Typically, they will say that there is a margin of 10 percent error. When it comes to blood testing, most forensic toxicologists will testify that the tests have a smaller margin of error, but if someone blows a 0.08 on that machine with a margin of error 10 percent, it could be higher, lower, or somewhere in between.

When defending a DUI case, I am looking for reasonable doubt. Machines are machines, and none are perfect. Consider, for example, what happens with a scale; each time someone steps on it, it gives a different number. The same can be said for breath test machines, because there are a lot of factors involved. It is not an exact science. Since no machine is perfect, we can show reasonable doubt and thereby avoid having all 12 jurors decide that the defendant is guilty.

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