I have written previously in this blog about the unreliability of retrograde extrapolation in drunk driving cases.
This is an important topic because state experts in drunk driving cases often will come into court and try to convince the jury that the driver’s blood alcohol at the time of the driving was higher than at the time of the test.
This practice is very common in OWI Causing Death cases because the blood evidence is usually several hours old.
The following list of five reasons retrograde extrapolation is unreliable is from Dr. Stefan Rose from the state of Florida:
- The most important fact for any retrograde extrapolation is for the subject to be in the pure elimination phase of ethanol kinetics. This is nearly impossible to know with certainty because it is impossible to know with certainty that the driver is fully absorbed. There are large population differences in how long full absorption of alcohol may take.
- The best way to determine the above is for multiple, timed blood samples to be taken from the subject in the time interval from crash (stop) to blood draw of interest. That only happens in clinical controlled trials. It never happens with a drunk driving case where there is only one blood draw or two breath tests taken nearly simultaneously.
- The next method is to determine when the drinking episode began and ended, what alcohol beverages were consumed (number of drinks, alcohol per drink and time of each drink), what the absorption time for each drink was (impossible to predict) and what the cumulative absorption time for all the drinks was. The police almost never ask these questions and therefore the state expert will not have this information available to them.
- Finally, what is the elimination rate of alcohol for that subject at that time? Can not be predicted, only measured, as above. Again, there are large population differences and elimination rates can be as low as about .009 to as high as about .40 at the extreme ranges.
- Therefore State witness is not able to provide a scientifically reliable estimate of the BAC at any given point in time. Only the BAC result of the blood draw can be used.
Bottom line is this; retrograde extrapolation in a drunk driving case is only guess work and in criminal cases state witnesses should never be allowed to guess!