Not a magic math trick that can tell exactly how intoxicated someone was at the moment of driving, retrograde extrapolation is an estimate of a person's earlier blood alcohol level based on a later chemical test and assumptions about how alcohol was absorbed and eliminated.

That estimate can be useful, but it is only as reliable as the facts behind it. The calculation depends on variables like body weight, drinking pattern, food intake, timing of the last drink, and whether the person was still absorbing alcohol when tested. Bad advice often treats it like hard science when, in reality, small missing details can swing the result. In a DUI case, prosecutors may use it to argue a driver was over the legal limit earlier even if the breath or blood test happened later. Defense lawyers often challenge the assumptions, the testing timeline, and the qualifications of the state's expert.

In Hawaii, that timing issue matters because DUI charges under HRS §291E-61 can turn on alcohol concentration measured within a set time after driving, and delayed testing can become a battleground. In crash cases, retrograde extrapolation may also affect an injury claim by shaping arguments about negligence, causation, or even punitive damages. If a seriously injured person was treated at a hospital such as Tripler Army Medical Center before testing occurred, the gap between driving and test time can become a major point of dispute.