Retrograde Extrapolation in DUI Prosecutions

As a DUI defense attorney, one of the most common questions I hear is: "How can they know what my BAC was when I was actually driving?" This question is especially important when there's a significant time gap between the traffic stop and when a breath or blood test is conducted. The answer lies in the science of retrograde extrapolation — a method used to estimate a person’s BAC at an earlier point in time.

What Is Retrograde Extrapolation?

Retrograde extrapolation is a process used by forensic toxicologists to estimate what a person’s BAC was at the time of driving, based on a known BAC result obtained later (usually via breath, blood, or urine test).

This calculation attempts to “work backwards” using several key factors:

• The time elapsed between the stop and the BAC test

• The rate at which alcohol is metabolized by the body (typically 0.015% to 0.020% per hour)

• The timing and amount of alcohol consumed

• The individual's weight, sex, and whether they had food in their stomach

Despite numerous challenges, the California Supreme Court has recognized the validity and admissibility of expert testimony on retrograde extrapolation. (See, e.g., People v. Clark (1993) 5 Cal.4th at p. 993; People v. Thompson (2006) 38 Cal.4th 811, 826 (maj. opn. of Baxter, J.); id. at p. 834 (dis. opn. of Werdegar, J.).) It has cautioned, however, that the fact that such extrapolations can be speculative goes to the weight rather than the admissibility of such testimony.

Example Scenario

Imagine someone is pulled over at 11:00 PM but their breath test isn’t administered until 12:00 AM. Their BAC is recorded at 0.09%. If they hadn’t consumed any alcohol after being stopped, it’s possible to estimate that their BAC was higher an hour earlier — possibly over the legal limit at the time they were driving.

On the other hand, if they had just finished drinking shortly before being pulled over, their BAC might have been lower at the time of driving, since alcohol takes time to absorb into the bloodstream and affect the central nervous system. This is called the absorption phase, and it’s one reason why retrograde extrapolation is not an exact science.

Why It Matters in a DUI Case

BAC at the time of driving is what matters under the law—not necessarily what your BAC is when tested later. Prosecutors often rely on retrograde extrapolation to prove someone was over the legal limit while driving. But these estimates can be challenged based on:

• Incomplete or incorrect assumptions about drinking history

• Individual metabolism variability

• Lack of information about when alcohol was consumed

• Time delays in testing

As defense attorneys, we scrutinize these assumptions carefully. If the prosecution’s extrapolation is based on flawed or missing data, we can argue that there’s reasonable doubt as to your actual BAC when you were behind the wheel.

Challenging BAC Evidence

A strong DUI defense often includes:

• Cross-examining the toxicologist or arresting officer about the basis for their estimates

• Consulting with expert witnesses to challenge the prosecution’s calculations

• Investigating whether your BAC was still rising (not yet peaked or plateaued) at the time of the test

Final Thoughts

If you’ve been charged with DUI in Northern California, it’s critical to understand that BAC results are not always as straightforward as they seem. The law focuses on your BAC at the time of driving — not necessarily at the time of testing. At the Davis W. Hewitt Law Office, we use every available tool to challenge questionable science and defend your rights.

Contact me today for a free consultation. Let’s review your case and determine the best path forward.