A prosecutor's estimate about your alcohol level earlier in time can raise fines, license consequences, and even make the difference between a weaker case and a conviction. That makes this a high-stakes issue, because a number pulled backward from a later test can look more precise than it really is.

Retrograde extrapolation is a scientific method used to estimate a person's earlier blood alcohol concentration, or BAC, by working backward from a later breath or blood test. The calculation assumes a rate at which alcohol was absorbed and eliminated from the body, then applies that rate to the time gap between driving and testing. In DUI cases, it is often used when the chemical test happened well after the traffic stop. The catch is that the estimate depends on facts that may be uncertain, including body size, food intake, drinking pattern, medications, illness, and whether the person was still absorbing alcohol.

In North Carolina, that can matter in a prosecution under G.S. 20-138.1 for impaired driving, especially when the State tries to prove BAC at the actual time of driving rather than at the time of testing. Expert testimony offering this kind of opinion may be challenged under Rule 702 of the North Carolina Rules of Evidence, as amended in 2011, if the assumptions are shaky.

It can also spill into a related personal injury claim after a crash. In North Carolina's contributory negligence system, being found even 1% at fault can block recovery, so an aggressive BAC estimate may be used to shift blame and cut off compensation.