A backward estimate of what a person's blood alcohol level was at an earlier time.

"Backward estimate" matters because the test usually happens after the stop, crash, or arrest, not at the exact moment someone was driving. Retrograde extrapolation takes a later breath test or blood result and works backward using alcohol absorption and elimination rates. "Blood alcohol level" means the amount of alcohol in the body, usually reported as BAC. "Earlier time" is the key fight: the state may use it to argue a driver was over the limit when the vehicle was moving, while the defense may challenge the math, the timing, or what the person drank and ate.

In real cases, this estimate can be shaky if the facts are thin. A missed timeline, one last drink right before driving, vomiting, injuries, cold exposure, or delayed testing can throw off the calculation. After a wreck on icy roads or in bad visibility, officers and prosecutors may lean hard on timing. If retrograde extrapolation shows up in your case, get the test records, dispatch times, body-cam, hospital records, and a full drinking timeline fast.

In Alaska, AS 28.35.030 (2024) sets the DUI limit at 0.08 and ties chemical testing to a four-hour window after driving. That makes timing critical, and retrograde extrapolation can affect both a DUI prosecution and any related injury or insurance claim about fault.