NACDL - Retrograde Extrapolation: A Scientifically Flawed Procedure (DWI)

Retrograde Extrapolation: A Scientifically Flawed Procedure (DWI)

If a driver’s blood-alcohol concentration (BAC) is below the legal limit when the driver gives a breath or blood sample, this fact will not stop the prosecution from trying to obtain a DWI conviction. At trial, the state may attempt to use retrograde extrapolation to convince the jury that the BAC exceeded the legal limit earlier when the police stopped the driver on the highway. But retrograde extrapolation is an unreliable tool.