Understanding Breathalyzer Test Accuracy in Missouri DUI Cases

Driving under the influence (DUI) cases in Missouri often begin with traffic stops. Police may ask a driver they suspect of being under the influence of alcohol or drugs to perform field sobriety tests, or they may conclude that a driver is impaired based on observations of their behavior or the odor of alcohol. 

These methods leave the decision to arrest someone for DUI up to an officer’s interpretation of what they see. Breath testing is supposed to provide a more scientific basis for a DUI arrest by measuring a driver’s blood alcohol content (BAC) level. 

Breathalyzer devices have a far-from-perfect track record on reliability. They need careful maintenance and calibration to ensure that they provide accurate results. Challenging breathalyzer results is a common DUI defense strategy. 

If you are facing DUI charges in Missouri, an experienced DUI defense lawyer can evaluate the evidence against you and help you fight the charges.

How Do Breathalyzers Work?

When a person becomes intoxicated, alcohol shows up in their breath, blood, and urine. Unless a suspect agrees to provide a blood sample, police usually need a warrant to draw a blood sample since it requires an invasive procedure. They usually also need a warrant to collect a urine sample because of privacy. Collecting a breath sample is neither invasive nor private, though, so police do not need a warrant. This has made breath testing by far the most common way to test BAC.

After a person blows into a breathalyzer, the device can measure the breath alcohol concentration (BrAC) almost immediately. Several techniques involving chemical or electrical reactions are possible for measuring BrAC. The device converts BrAC to BAC by applying a partition ratio based on research into how alcohol moves from the digestive system to the blood and from there to the lungs.

BAC is a measurement of the percentage of alcohol in a person’s blood by volume. A BAC of 0.01% means that a person has 0.01 grams of alcohol per 100 milliliters of blood. Missouri’s “legal limit” for most drivers is 0.08%.

Missouri DUI laws identify two separate offenses, both of which may go by the name “DUI” or “DWI”:

• Driving while intoxicated: Operating a motor vehicle while impaired by alcohol or drugs.
• Driving with excessive blood alcohol content: Operating a motor vehicle with BAC of 0.08% or more, or 0.04% or more for commercial drivers.

Can I Refuse Breath Testing in Missouri?

Missouri’s “implied consent” law states that anyone who drives in the state consents to providing a breath sample if a police officer suspects them of DUI. Police cannot compel you by force to give a breath sample, but refusal to submit to breath testing after an arrest may result in the revocation of your driver’s license. Prosecutors may use your refusal as evidence against you in court.

Police in Missouri use two different kinds of breathalyzer tests. People may use the term “breathalyzer” to refer to both types of tests, which can be confusing:

• Preliminary breath test (PBT): Police may use a handheld device during a traffic stop to test a person’s BrAC. These devices are not reliable. 

Their results are not admissible as proof of intoxication, but they are admissible as proof that an officer had probable cause to arrest a driver. The implied consent law does not apply to this type of test as long as the driver is 21 or older.

• Breathalyzer test: After an arrest for DUI, police may test a driver’s BAC using a non-portable device at the police station. These results are admissible as proof of intoxication. This type of test is subject to the implied consent law.

What Factors Might Affect Breathalyzer Accuracy?

A wide range of factors may affect the accuracy of breathalyzer testing.

Device Calibration and Maintenance

Breathalyzer devices are complex machines that perform sensitive measurements. Without careful calibration, they can give inaccurate readings.

The Missouri Department of Health & Senior Services (DHSS) sets standards for approved devices, including routine maintenance and calibration. Police departments must keep documentation showing that each device meets the state standards.

Operator Error

The DHSS also sets standards for breathalyzer device operators. This includes checklists to ensure that each test is performed correctly. Errors that may affect accuracy include:

• Failure to prepare the device properly before administering the test;
• Failure to instruct the driver about how to submit a breath sample; and
• Failure to monitor the driver to ensure they follow the instructions.

External Factors

Conditions that are not related to how much alcohol a person has consumed may result in inaccurate readings. These may include the following:

• Residual alcohol in the mouth: If a driver has recently consumed something containing alcohol, it could result in a false positive. This could even occur with mouthwash.
• Medical conditions: Certain conditions, such as gastroesophageal reflux disease (GERD), diabetes, or ketoacidosis, can create false positives.
• Environmental factors: Factors like the room’s temperature or radio frequency interference from nearby electronic devices could affect test results.

How Can I Challenge Breathalyzer Results in a Missouri DUI Case?

Challenging breathalyzer results can be an important defense strategy in DUI cases.

• Calibration and maintenance records: Maintenance logs can show whether the device met state standards at the time of a defendant’s breath test. Courts have dismissed cases for failure to perform required maintenance on a device.
• Improper administration of the test: Evidence that an officer or technician administered a breath test incorrectly can lead to suppression of the test results. Without BAC evidence, a court might dismiss the case.
• Alternative explanations for high BAC readings: An attorney can raise medical defenses and offer scientific arguments showing that the results are inaccurate.
• Independent blood tests: Drivers in Missouri can request blood testing by a medical professional of their choosing at their own expense. The result of that test could be different from the breath test.

Talk to a DUI Lawyer to Learn More About Your Rights

DUI is a serious charge in Missouri that can have substantial impacts on your life. Sloan Law Office provides skilled and compassionate legal advocacy for people facing DUI charges in the Kansas City area. 

We can help you understand your options, analyze the state’s allegations against you, and fight for your rights. Contact us today to schedule a confidential and free consultation to discuss your case.