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The Science Behind DUI Testing – Breath and Blood Tests

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When you have been charged with a Colorado DUI, it is important to work with an attorney skilled in successfully navigating these cases. One very important aspect is understanding the science behind the breath and blood tests that you were given.  

Criminal charges can be based on these tests meant to determine your blood alcohol content (BAC) which is used to indicate whether or not you’re able to drive a vehicle safely. There are many moving pieces to these tests, and depending on the circumstances, many possible defenses to these criminal charges. 

What is BAC? 

BAC is a measurement of how much alcohol is in your bloodstream. It’s usually expressed as a percentage, so a BAC of 0.08% would be an estimate that 0.08% of your blood is alcohol. It’s used (rightly or wrongly) to determine how impaired by alcohol you are.  In DUI cases, this is calculated by a breathalyzer device, blood tests, or both. 

Breathalyzer is just one brand of these types of machines, which use different methods to measure BAC, but it’s become the common name for all of these devices. 

A breathalyzer reading may be inaccurate and subject to challenge for several reasons: 

  • The presence of substances other than alcohol may influence breathalyzer results. 
  • Instruments testing for BAC must be regularly calibrated and maintained to ensure accurate results. Failing to do so can lead to inaccuracies. 
  • Certain health conditions, such as diabetes or hypoglycemia, can affect BAC readings. Medications and other substances in the body may also interfere with accurate measurement 

Your BAC is measured by one machine or another, and every type has an opportunity for error. 

How Do Breathalyzers Work? 

The principle behind breathalyzers is the concentration of alcohol in your breath is proportional to its concentration in your blood. Three standard methods used to come up with a BAC figure are:  

  • Infrared absorption: The device emits infrared light through a sample of your exhaled breath. Alcohol molecules absorb specific light wavelengths. The device measures the amount of absorbed light and calculates the concentration of alcohol in your breath. This is typically the method used by table-top breath machines at the police station. 
  • Fuel cell technology: There’s a chemical reaction between alcohol in your breath and a fuel cell in the device. If alcohol in your breath contacts the fuel cell, it creates an electrical current. Its strength is proportional to the amount of alcohol in your breath, and the breathalyzer estimates your BAC. This tends to be the method in hand-held, preliminary breath test (PBT) devices that are more commonly used by law enforcement at the site of being pulled over. 
  • Chemical testing: The BAC is determined based on how the alcohol (if any) in your breath interacts with chemicals in the device 

Each type has its pros and cons, and all are at risk of mistakes. 

Blood vs. Breath Tests 

Breath tests are generally less accurate than blood tests and depend on a trained law enforcement officer to use them properly. Blood tests are more invasive and take longer but are usually more accurate. Breath tests only measure alcohol, while blood tests generally measure a whole panel of other substances in addition to alcohol. 

Part of the blood sample that’s taken at the time of arrest isn’t tested as part of the process. A defendant can have it tested by an independent laboratory to determine if the first result is accurate. There’s no saved breath sample to re-test. Furthermore, the lab technician who performs blood tests is frequently more of an expert at it than a law enforcement officer responsible for doing breathalyzer tests along with many other things. 

Potential Legal Challenges 

There are many possible issues when it comes to breath and blood tests, including: 

  • Whether the breathalyzer was used and calibrated correctly 
  • Substances in the driver’s system or medical conditions can affect the breathalyzer’s BAC calculations 
  • The breathalyzer software may not be up to date 
  • The device’s battery’s charge level can impact results 
  • Blood test samples must be handled properly, including providing evidence the sample is yours and not mixed up with another driver’s blood 
  • Poor sample handling or time delay can cause the blood level to increase after it’s out of your body, resulting in an inaccurately high reading 

Which defense may be available to you depends on the facts of your case. 

Don’t Go It Alone    

For more than 24 years, Kevin Churchill has helped clients like you with DUI cases. If you’re charged with DUI, DUID, or DWAI in the Denver Metro area or anywhere in the Front Range, contact a Denver DUI defense attorney you can trust, Kevin Churchill, at (303) 832-9000.