The breath test is the most common test that people are asked to take once they are taken to the police station (or jail), but it is also highly susceptible to error. If the test is not administered correctly it will not accurately measure your blood alcohol concentration and your lawyer may be able to persuade the judge to throw it out.

Below is a list of factors that can make a breath test unreliable.

Calibration of the Machine

The machine must be accurately calibrated. If it is not, it will not provide an accurate reading. Think of a bathroom scale. If it's not set to zero, the correct weight is not going be displayed. The same is true for a breath test machine; after all, it is a machine

Residual Alcohol in the Mouth.

The main reason for the test’s potential inaccuracy is that, while it is supposed to measure how much alcohol is in your lungs, any residual alcohol in the mouth will result in a mistakenly high reading. In other words, the breathalyzer cannot distinguish between alcohol in the mouth (which does not reliably indicate intoxication) from alcohol in the lungs (which does). If there is any food trapped between the teeth, it will absorb alcohol and inflate the test result, as will any blood that is present in the mouth at the time the test is taken.

Belching, Hiccupping or Vomiting Prior to a Test

Time is the most important factor here. A person should not be tested for at least 15 minutes after belching, hiccupping or vomiting, as this increases the amount of alcohol on the breath. The officer is required to constantly observe you to ensure that you have not belched, hiccupped or vomited within 20 minutes of taking the test. Constant observation is a rule that the officer must follow. If it is not followed, the results of the test may be called into question.

The Temperature of Your Breath

The breath test works on the assumption that your breath is 34 degrees centigrade. Studies done with this equipment have shown that the real average breath temperature for people who have been arrested on a DWI is closer to 35.5, with some as high as 37. This alone would mean that the result of the test would be between 10 and 20 percent higher than it really is. While most breath test machines have the ability to measure your breath temperature, it’s not a feature that is added to the breath test machines in Texas.

How Fast Your Body Eliminates the Alcohol

Everyone has a different metabolism, but the breath test assumes that everyone is the same. This means that a person whose body gets rid of alcohol slower will have a higher BAC than someone with a faster metabolism, even after having the same amount to drink. Breath testing also assumes that the person is “post absorptive.” This means that the test assumes that the person is no longer absorbing alcohol into the blood. If you have a slow metabolism, you might still be absorbing alcohol by the time you take the test. If so, it will read your BAC as significantly higher than it actually is. If you know or suspect that you have a slower than average metabolism, you may eventually argue that your metabolism skewed the result of your test.

Other Chemical Compounds in Your Mouth

Strictly speaking, the breath test does not detect alcohol (ethyl alcohol). It detects part of the alcohol molecule called the methyl group. The significance of this is that the breathalyzer will also register other compounds besides alcohol, other compounds that are commonly found in human breath. If any of these compounds are present, it will result in an inflated and inaccurate BAC reading.

The amount of air in your lungs and the partition ratio

As you saw with the breath temperature, the breath test machine utilizes several assumptions. One of those is the amount of air in your lungs. A breath test machine is measuring the amount of alcohol in your lungs, not your blood. Therefore, in layman’s terms, the breath measurement is converted into a blood measurement. Based on questionable testing, it has been decided that 2100 ml of lung (alveolar) air, will contain the same amount of alcohol as 1 ml of blood. This is generally referred to as the blood/breath ratio.

There are several problems with the presumptions used for testing. The theory behind the testing is based on the assumption that alcohol moves from the blood to the alveolar sacs of the lungs, and is then exhaled. However, that has never been tested, because it’s impossible to collect a pure sample of “alveolar’ air without collapsing the lungs. If the exchange takes place at some other location in the airway, the calculation would be different. More recent testing has established that exchange actually takes place somewhere else, which suggests the assumptions underlying breath testing are flawed. This is an extremely complex subject, and requires an attorney to have a firm grasp of the science, and the current literature.

Another assumption used in breath testing is the “partition ratio”. The breath test machine is programmed to assume that for every one part of alcohol in the subject’s breath, there is another 2,100 parts in the blood. That assumption is based on norms, which will vary for different people. Studies have shown the partition ratio in healthy adults can range from 900:1 to 3,400:1. A decrease in the partition ratio can drastically decrease the result. Again, the attorney must understand this concept, and be apply to explain it before a jury.

Maintenance and other records

The breathalyzer is a machine, and like all machines it has to be serviced and maintained. There are some things that must be done, or the result is not admissible in court. While most times the technical supervisors keep up with those things, you don't know unless you check.

Like most machines, they also break down from time to time. That can be important, especially if it's right before or after your test. You also need to see how the machine has tested has other people. I recently looked at the records for a machine, and saw a test done a couple of days earlier where there two samples were way off - which means something went wrong. That's something you would want to follow up on, and see if the problem was fixed.

All of this requires work. You have to get the records, and spend the time going through them. Like records concerning the blood testing, these records are not usually sent to the prosecutor, so you have to ask for them. If you don't ask, and you don't look, you will never know if there's a problem.

If you been arrested for DWI in Waco, McLennan, Falls, Bell or Bosque Counties, and would like FREE - No strings attached - written evaluation of your case fill out the Do I have a Defense evaluation form.

If you have are looking to hire a lawyer to make sure all of this done, give us a call at 254-296-0020, or fill out the contact form on this page. You can also get our FREE book, The DWI Survival Guide which explains in detail the law concerning driving while intoxicated, and the defenses you can raise. Simply fill out the form on this page.

Walter Reaves
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Criminal Defense Attorney Walter Reaves has been practicing law for over 35 years.