What Happens If a Breath Test and Blood Test Show Different Results?
If you were arrested for DUI and later discovered that your breath test and blood test showed different results, you may be wondering which one matters, or whether those conflicting numbers can affect your case.
This situation is more common than many drivers realize. Breath and blood tests are both used in California DUI cases, but they measure alcohol differently and are conducted under very different circumstances. Because of that, the results do not always perfectly match.
When there is a noticeable difference between the two, it can raise important questions about the reliability of the evidence and how your case should be evaluated.
Why Breath and Blood Tests Can Produce Different Results
Breath and blood tests are not identical forms of testing.
A breath test estimates blood alcohol concentration by measuring alcohol in a person’s breath and using a formula to calculate an approximate BAC. A blood test, by contrast, directly measures alcohol concentration in a blood sample.
Because these tests rely on different methods, discrepancies can occur for several reasons, including:
- The amount of time between each test
- Natural changes in alcohol absorption and elimination
- Equipment calibration or maintenance issues
- Individual physiological differences
In many DUI cases, the breath test is taken first, followed by a blood draw later. Since BAC can rise or fall over time, timing alone can significantly affect the results.
Timing Often Plays a Major Role
One of the biggest reasons breath and blood test results differ is simple: they are rarely taken at the exact same moment.
Alcohol levels are constantly changing in the body. Depending on when you last consumed alcohol, your BAC may still be rising, or it may already be declining.
This means:
- A later blood test could show a higher result if your BAC was still rising
- A later blood test could show a lower result if alcohol was already being metabolized
This timing issue can become highly relevant in DUI cases because the legal question is not just what your BAC was later, but what it was while you were actually driving.
Which Test Does the Prosecutor Rely On?
In many California DUI cases, prosecutors may rely more heavily on the blood test, particularly if it was obtained after arrest and analyzed through a laboratory.
Blood tests are often viewed as more direct because they measure alcohol concentration from an actual sample rather than estimating it through breath.
However, that does not mean blood test results are automatically beyond challenge. Blood evidence still depends on proper collection, storage, handling, and analysis.
Can Different Results Help Your Defense?
Conflicting test results can sometimes create opportunities for the defense, particularly when the discrepancy raises questions about reliability or timing.
A difference between breath and blood results may lead to questions such as:
- Was one of the tests inaccurate?
- Was the breath machine properly calibrated?
- Was the blood sample handled correctly?
- Was the driver’s BAC changing significantly between tests?
In some cases, these issues can help create doubt about whether the prosecution can accurately prove impairment or BAC at the time of driving.
Common Issues That Can Affect Test Accuracy
Differences in test results may also stem from procedural or technical issues.
Potential factors include:
- Improper breath machine maintenance
- Failure to follow required observation periods before breath testing
- Problems with blood sample storage or contamination
- Delays in testing or analysis
Even small procedural issues can become important when test numbers do not align.
The Importance of Reviewing the Full Timeline
When breath and blood results differ, context matters. A number alone rarely tells the full story.
A DUI defense attorney will often want to examine:
- When you were stopped
- When each test was administered
- What and when you last ate or drank
- Whether testing procedures were properly followed
This timeline can be critical in understanding why the results differed and whether those differences help or hurt the prosecution’s case.
Different Results Do Not Automatically Mean the Case Gets Dismissed
While conflicting breath and blood results can raise useful issues, they do not automatically result in a dismissal or dropped charges.
Instead, these discrepancies become part of the larger evidentiary picture. They may weaken the prosecution’s case, strengthen negotiation leverage, or create arguments for trial depending on the facts.
Every DUI case is unique, and the significance of differing test results depends heavily on the surrounding details.
Why Legal Analysis Matters in DUI Testing Cases
Chemical testing is often one of the most technical aspects of a DUI case. When results conflict, it can create scientific and legal issues that are not obvious to the average person.
An experienced DUI lawyer can evaluate whether:
- The discrepancy is meaningful
- Testing protocols were properly followed
- The prosecution can reliably prove your BAC at the time of driving
This kind of analysis can be critical in determining the strongest defense strategy.
San Diego DUI Defense Attorney
If your breath and blood test results showed different numbers after a DUI arrest, it is important to understand what those differences may mean for your case.
San Diego DUI defense attorney Rick Mueller has extensive experience analyzing chemical test evidence, identifying procedural issues, and challenging questionable DUI evidence.
Every detail matters in a DUI case, and when test results conflict, those details can become especially important. Speaking with an experienced attorney can help you better understand your options and next steps.
What Happens If a Breath Test and Blood Test Show Different Results?