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Forensic Toxicology · Tools
Back-calculate blood alcohol concentration from the time of testing to the time of driving.
C_driving = C_measured + β × t
Back-calculation to the time of driving
As reported by the evidential test.
0.015 %/h is the common mid-range figure. The reported population range is 0.010 to 0.025.
Estimated BAC at time of driving
0.0725% w/v
C_driving = C_measured + β x t = 0.05 + 0.015 x 1.5 = 0.0725 %
Back-calculation is only valid if the subject was past peak absorption and in the elimination phase for the whole interval. If they were still absorbing, the true figure at the earlier time was lower, not higher.
Retrograde extrapolation is contested in court for good reason. It is invalid during absorption, and the population spread in β means the honest output is a wide range rather than a single value. Many jurisdictions restrict or exclude it. Present the range and the assumptions, never a single number.
Where a breath or blood sample is taken hours after driving, the measured value is lower than the value at the wheel because alcohol has been eliminated in the interval. Adding back the eliminated amount is retrograde extrapolation.
The method is only valid if the subject was past peak absorption for the whole interval. Since the population range for the elimination rate spans 0.010 to 0.025 percent per hour, this tool reports the range across that span rather than pretending to a single answer.