Being pulled over in California for a suspected DUI can be reason enough for concern for any motorist. But breath test results may not be accurate for individuals on the Keto diet because of the way fat is broken down by the liver. When the body is in what's known as ketosis, a false positive could be registered on a Breathalyzer under the right circumstances.
The breakdown of fat that occurs on the keto diet produces acetone as a byproduct, which can be released via the breath as isopropyl alcohol. While inexpensive DIY breath test models may not be able to tell the difference between ethanol and isopropyl alcohol, police typically use fuel cell-based Breathalyzers in their cars. These more sophisticated devices claim to be able to tell the difference between these two types of alcohol, but this may not be the case if someone consumes alcohol while their body is in ketosis.
The body sometimes converts acetone to isopropanol, which may then be oxidized like ethanol by a fuel cell Breathalyzer, although some manufacturers claim their devices are ethanol-specific to prevent inaccurate readings. There is an account of an incident involving a man on a low-carb diet who had trouble with a breathalyzer ignition lock on his vehicle. Devices used in vehicles often shut cars off at blood alcohol content levels far lower than DUI limit. So, it could be possible for a sober person on the Keto diet to produce a low-level BAC that prevents a car from starting.
A criminal defense attorney isn't likely to use a Keto diet defense alone for someone who fails a field sobriety test. Preliminary breath test results are often inadmissible in court anyway. But a lawyer may question the reason for pulling someone over in the first place. Certain medications and health conditions could also produce inaccurate results.