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Explaining DUI blood alcohol concentration (BAC)

All drivers in Arizona, regardless of their own possible experience with DUI charges, should at least be somewhat familiar with the procedures that are followed when someone is pulled over by the police and there is reason to believe that they might be intoxicated. One of the key components of Arizona’s notoriously strict approach to drinking and driving relies on what is commonly referred to as the driver's BAC  (blood alcohol concentration) at the time of the traffic stop. But what does that really mean? How much does the average person know how the BAC is calculated and whether it is accurate evidence of being over the DUI limit?  And more importantly, are the results of these tests as infallible as we’ve been led to believe? 

The basics of BAC

In simplest terms, your blood alcohol concentration (BAC) is the percentage of alcohol in your blood. With each alcoholic beverage you consume, the proportion of alcohol to blood in your body changes. Your blood alcohol concentration rises, and the higher your BAC percentage is, the more likely you are to be impaired by the alcohol in your system. 

Testing methods

Your blood alcohol concentration can be measured in a few different ways. Arizona determines BAC using one or more of the following tests:

●      Breath Test - This is the most commonly used method of BAC testing, due in large part to the portability of the testing device (breathalyzer) and the immediate results. Testing consists of having the driver blow into the breathalyzer.

●      Blood Test - Widely considered by law enforcement to be the most accurate of the BAC testing methods, blood is drawn from the driver and tested in a lab. 

●      Urine Test - Considered to be the least reliable testing method of BAC, this test requires the driver to provide a urine sample to be tested.

BAC test results

The BAC percentage that is recognized as legally intoxicated is determined on a state level, so it can differ. In Arizona, you could be arrested for DUI if you are impaired by any substance to the slightest degree, You will be charged with a separate crime if your BAC test results are at or above 0.08%. This limit is lower for drivers of commercial vehicles (0.04%) and there is a zero tolerance policy for anyone under 21, meaning a reading of any percentage will result in a DUI arrest. 

Every state relies on BAC percentages to help identify drivers who are under the influence. While these tests are clearly a more reliable form of measuring intoxication than a field sobriety test, this does not mean that these tests are always without error. In fact, there are several factors that can skew the precision of the results of a BAC test that are not always taken into consideration. These factors can range from simple physical details, like how much the person drinking weighs to much more complex issues with the equipment used to draw and even analyze the samples.

A few examples of things that can alter an accurate result include:

●      Gender of the person drinking

●      Medications taken

●      Amount of food in the stomach at the time of drinking

●      Individual body type, weight, and age

●      Time of day

●      Type of alcohol consumed

 For blood tests specifically, there are studies which have shown that the accuracy of the test results can be off for a variety of reasons. These include the manner in which the blood is drawn, where and at what temperature the sample is stored, and the amount of time between the drawing of the sample and the analyzing of it. 

While BAC tests can be a useful tool for law enforcement, helping them determine when someone behind the wheel is a danger to other and themselves, these tests may not be unquestionable evidence of guilt that some believe them to be. If you have been charged with a DUI and feel that your BAC results may have been faulty, contact us today.


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