In a Florida DUI case, timing matters. Prosecutors often rely on breath or blood alcohol results taken hours after a traffic stop or crash. But how do they determine what your blood alcohol content (BAC) was at the time you were actually driving?
That’s where retrograde extrapolation comes into play.
This scientific method can help the defense or prosecution—depending on how it’s applied. At Fletcher + Fletcher, we’ve seen both sides try to use retrograde extrapolation, and we know how to challenge its assumptions and leverage it in our clients’ favor.
In this blog, we’ll explain:
1. What retrograde extrapolation is and how it works,
2. When it can be used as a defense,
3. The process for admitting this type of expert testimony, and
4. Why having an experienced DUI attorney in St. Petersburg matters in these cases.
What Is Retrograde Extrapolation?
Retrograde extrapolation is the scientific process of estimating a person’s BAC at an earlier point in time, based on a known BAC result taken later.
The theory is grounded in the physiology of alcohol metabolism.
After a person stops drinking, their BAC rises, peaks, and then begins to decrease at a predictable rate—typically between 0.015 to 0.020 grams per deciliter per hour.
By working backward from a known BAC level, and considering factors like:
· Time of the test,
· Time of the incident,
· Body weight,
· Gender,
· Whether food was consumed, and
· Drinking pattern,
an expert can estimate what the BAC likely was at the time of driving or the alleged offense.
Example of Retrograde Extrapolation in a DUI Case
Let’s say a driver is involved in a crash at 9:00 PM but is not given a breath test until 11:00 PM. The breath test shows a BAC of 0.08%.
Using retrograde extrapolation, a defense expert might testify that:
· At 9:00 PM, the driver was still absorbing alcohol,
· The BAC at the time of the crash was actually lower, possibly even below the legal limit, and
· The prosecution’s reliance on the later test result does not reflect impairment at the time of driving.
This strategy can introduce reasonable doubt about whether the driver was impaired at the relevant time, which is the crux of a DUI charge.
Is Retrograde Extrapolation Admissible in Florida Courts?
Yes—but it’s not automatic. The admissibility of retrograde extrapolation depends on the use of a qualified expert and the court’s application of the Frye standard or Daubert standard (depending on the timing and context).
Here’s how it gets admitted:
A. Expert Qualification: The person performing the extrapolation must have expertise in toxicology, pharmacology, or forensic chemistry.
B. Scientific Reliability: The methodology must be scientifically accepted and reliably applied in the case.
C. Disclosure Requirements: The defense (or prosecution) must provide advance notice of expert testimony and allow the other side to review the expert’s findings.
Judges in Pinellas County have discretion over whether to allow this evidence. A seasoned DUI defense attorney must know how to qualify the right expert and lay a foundation for their testimony that complies with Florida evidentiary rules.
When Retrograde Extrapolation Helps the Defense
Retrograde extrapolation is often used by the State, but when used strategically by the defense, it can help:
1. Challenge delayed breath or blood results that don’t reflect BAC at the time of driving,
2. Explain post-driving consumption, if the driver drank after an accident but before the test,
3. Discredit the prosecution’s timeline and highlight the uncertainty in their case,
4. Reinforce the argument that the BAC was under 0.08% while operating the vehicle.
For example, if your breath test was conducted long after the stop and you were metabolizing alcohol at a normal rate, your BAC at the time of driving may have been well within legal limits, even if the test shows otherwise.
Why Local Experience Matters in DUI Cases Involving Expert Testimony
Not all DUI cases are alike—and neither are the judges or prosecutors. When dealing with scientific evidence, including retrograde extrapolation, it’s critical to work with a lawyer who:
✔ Knows how Pinellas County courts treat expert testimony,
✔ Understands which judges are more skeptical of extrapolation evidence,
✔ Has relationships with respected local experts in toxicology,
✔ Can anticipate how the State Attorney’s Office will present its own extrapolation.
At Fletcher + Fletcher, we’ve successfully challenged unreliable BAC estimates and used expert analysis to secure not guilty verdicts and dismissals in DUI cases across St. Petersburg and the surrounding area.
Facing a DUI Charge Based on Delayed BAC Testing? Let’s Talk.
If your DUI arrest involved a blood or breath test taken hours after the incident, you may have a strong defense based on retrograde extrapolation. Don’t let assumptions about your BAC go unchallenged.
Contact Fletcher + Fletcher now for a free consultation.
Let’s use the science to fight for your future.
