Do False Positive Readings Occur With Ignition Interlock Devices?

Posted by Doug Murphy | May 01, 2020 | 0 Comments

After being arrest for DWI, many judges now require--and the law requires in certain situations--an ignition interlock device to be installed in any vehicle you drive as a condition of your bond while your case is pending.  In the event you are convicted of driving while intoxicated, you can expect a series of hoops to jump through before you can regain your ability to drive in the State of Texas.  One of these requirements is known as an ignition interlock device.

The ignition interlock device is purportedly designed to prevent an intoxicated person from starting their vehicle and driving.  It requires a driver to blow into a device installed in their vehicle. The device tests their blood alcohol concentration and will prevent the vehicle from starting if it registers any degree of alcohol on the driver's breath.

A failed interlock device result could have serious repercussions that impact your ability to drive. If you are on a bond condition, a violation could lead to a bond revocation that sends you back to jail. If you are on probation, a violation could lead to a probation revocation that sends you back to jail. Unfortunately, it is possible for these interlock failures to occur due to a false-positive result.

False Positives

The ignition interlock device is similar to the portable breath tests used by police during traffic stops in that they measure the amount of alcohol in a driver's system. However, the same factors that can lead to a false positive on a portable breath test or a breathalyzer can also be problematic for an interlock device.

In most cases, false-positive interlock device readings occur based on a substance in the driver's mouth at the time they are giving a breath sample. These machines are carefully calibrated to read tiny samples of alcohol within the exhaled breath of the driver. If that person had even trace amounts of alcohol or a substance known to give false readings, it could result in a surprisingly high false-positive test result.

These false positives can occur even when you have not consumed any alcohol whatsoever. A number of foods and other substances have been known to cause false results with breathalyzer testing. The same substances can also lead to false results with an ignition interlock device. Examples of these substances include:

  • Yeast bread
  • Honeybuns
  • Ripe fruit
  • Chewing gum
  • Hot sauce
  • Vanilla extract
  • Kombucha
  • Lip balm
  • Vinegar
  • Energy drinks

Other substances, like mouthwash and cold remedies, include a small amount of alcohol. The interlock device is not sophisticated enough to distinguish between that and a positive result stemming from the consumption of an alcoholic beverage.

Your health condition might also play a role in this false positive reading. Ignition interlock devices can confuse acetone in your bloodstream caused by diabetes as alcohol. Acid reflux could also leave a false positive.

In the unfortunate event you have a false positive on your ignition interlock, what should you do?  

1. You should immediately drink cold water.  

2. You should also swish and rinse out your mouth with cold water to make sure you do not have any food or other items in your mouth.  

3. Drink more cold water.  

4. Then you should blow back into the ignition interlock device. If you blow negative, that will undo the false positive.  

If the ignition interlock shows another false positive, you should immediately have your urine tested for EtG (Ethyl Glucuronide) & EtS (Ethyl Sulfate) to demonstrate you have not been drinking.  EtG and EtS are metabolites your liver produces when you consume alcohol.  These metabolites stay in your body long after the alcohol wears off.  EtG will stay in your body up to 2-4 days and EtS up to 5-7 days.  Urines tests can be conducted at any local hospital, small emergency clinics, Lab Corp., Quest Diagnostics, or AnyLab location.  A negative EtG/EtS urine test can help you defeat a false positive, but you must act quickly. 

Fighting Back Against False Positives in Houston, TX

Judges and prosecutors will aggressively pursue revocation of bond or probation based on an interlock device.  A Board Certified DWI Defense  Attorney could help you identify the cause of your false reading and explain to the judge and/or prosecutor what actually happened.

It is helpful to remember that your best protection against these false tests is to avoid a conviction entirely. A seasoned and Board Certified Houston DWI defense lawyer could assist you with beating your charges.

About the Author

Doug Murphy

Doug Murphy is one of only two Texas lawyers Board Certified in Criminal Law by the Texas Board of Legal Specialization, and also in DWI Defense by the National College for DUI Defense, accredited by the American Bar Association and the Texas Board of Legal Specialization.


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