You'll usually see this phrase in a police report, prosecutor's charging papers, a toxicology report, or hear it from an officer or expert saying they can "work backward" from a later breath or blood test. What they mean is an estimate of what a person's blood alcohol concentration, or BAC, was at an earlier point in time by using a test taken later and applying assumptions about how alcohol is absorbed and eliminated by the body.

That estimate can matter a lot because a test is often taken well after a traffic stop or crash. If someone drank shortly before driving, their BAC may have still been rising when police first made contact. Retrograde extrapolation tries to fill in that gap, but it depends on facts that are not always clear: body size, food intake, timing of drinks, metabolism, and the length of time between driving and testing. A shaky estimate can create room to challenge the reliability of the state's evidence.

In Washington DUI cases, that matters because RCW 46.61.502 (2024) makes it illegal to drive with an alcohol concentration of 0.08 or higher within two hours of driving. In a crash case, that same reconstruction can affect liability, causation, and settlement value if alcohol use is being used to argue fault or reduce damages.