One of the more confusing phone calls I receive goes something like this:
“Hieu, I blew a zero.”
Or…
“I passed the breath test, but they arrested me anyway.”
People are understandably shocked.
After all, most of us have been taught that DUI is all about blowing over a 0.08% blood alcohol concentration.
So how can someone pass the breath test and still end up in handcuffs?
The short answer is simple:
Because passing a breath test doesn’t necessarily mean the officer believes you weren’t impaired.
DUI Isn’t Always About Alcohol
Many people think DUI only applies to alcohol.
It doesn’t.
California’s DUI laws prohibit driving while impaired by:
- Alcohol
- Marijuana
- Prescription medication
- Illegal drugs
- Certain combinations of medications
- Even legal medications if they impair your ability to drive safely
In other words, someone can register a 0.00% alcohol level and still be arrested for DUI if the officer believes drugs or another substance caused impairment.
The Breath Test Only Measures Alcohol
This is one of the biggest misconceptions.
The roadside breath test doesn’t detect:
- THC
- Cocaine
- Methamphetamine
- Xanax
- Pain medication
- Muscle relaxers
- Sleeping pills
It only estimates alcohol.
If the officer suspects another substance is involved, the investigation usually doesn’t end with the breath test.
Instead, it often continues with a blood test.
Why Did the Officer Think I Was Impaired?
This is where things become much more subjective.
An officer may point to things like:
- Poor driving
- Bloodshot eyes
- Slurred speech
- Slow responses
- Difficulty balancing
- Confusion
- Field sobriety tests
Some of these observations are legitimate.
Some aren’t.
I’ve seen reports where perfectly innocent explanations existed.
Someone involved in a traffic collision may appear confused because they suffered a concussion.
Someone may perform poorly on field sobriety tests because of age, knee injuries, back problems, or anxiety.
Someone may have naturally red eyes from allergies or fatigue.
Those facts don’t automatically prove intoxication.
Medical Conditions Can Mimic DUI Symptoms
Many medical problems resemble impairment.
For example:
- Diabetes
- Low blood sugar
- Head injuries
- Inner ear disorders
- Vertigo
- Neurological conditions
- Fatigue
- Panic attacks
An officer arriving at the scene may not know the difference.
Sometimes what looks like intoxication is actually a medical emergency.
What Happens Next?
If the officer still believes you are impaired, the investigation usually continues.
That often means:
- A blood draw
- Drug testing
- A Drug Recognition Expert (DRE)
- Additional questioning
- A review of prescription medications
Sometimes the blood results support the arrest.
Sometimes they don’t.
I’ve seen cases where the laboratory results created more questions than answers.
What If My Blood Test Comes Back Negative?
This surprises many people.
A DUI arrest doesn’t automatically become valid just because it happened.
If the blood test comes back showing no alcohol and no impairing drugs, prosecutors have to decide whether they can actually prove impairment beyond a reasonable doubt.
Sometimes they can’t.
I’ve seen cases rejected after the lab results came back.
Other times prosecutors proceed based on the officer’s observations, although those cases can present significant proof problems.
Every case is different.
What About Marijuana?
Marijuana cases are particularly challenging.
Unlike alcohol, California doesn’t have a legal THC limit for DUI.
There is no equivalent of “.08.”
Instead, prosecutors generally try to prove that the driver’s ability to operate a vehicle was actually impaired.
That often leads to disputes between:
- the officer,
- laboratory testing,
- expert witnesses, and
- the defense.
These cases can become highly fact-specific.
Passing One Test Doesn’t End the Investigation
Many people think passing the preliminary alcohol screening device means they are free to leave.
That’s not necessarily true.
The breath test is simply one piece of the investigation.
If the officer believes other evidence points toward impairment, they may continue investigating.
Whether that investigation ultimately supports a criminal conviction is another question entirely.
The Arrest Is Not the End of the Story
One mistake I see people make is assuming:
“If I got arrested, the officer must have been right.”
That’s not how our criminal justice system works.
An arrest is based on probable cause.
A conviction requires proof beyond a reasonable doubt.
Those are very different standards.
An officer can honestly believe someone was impaired and still be wrong.
That’s why we have courts.
Questions Worth Asking
If you passed the breath test but were arrested anyway, some important questions include:
- Why did the officer believe you were impaired?
- Was there another substance involved?
- Were field sobriety tests administered correctly?
- Was there a medical condition?
- What do the blood results actually show?
- Did body-worn camera footage support the officer’s observations?
- Were proper procedures followed during the investigation?
Those questions often tell us much more than the breath test itself.
Final Thoughts
Passing the breath test doesn’t automatically end a DUI investigation.
The breath machine only measures alcohol.
If law enforcement believes drugs, medication, or another factor impaired your driving, they may continue investigating and even make an arrest.
That doesn’t mean the arrest was correct.
It simply means the investigation isn’t over.
The important question isn’t whether you passed the breath test.
It’s whether the prosecution can ultimately prove that you were actually impaired while driving.
