Retrograde extrapolation is the process of counting backwards in time in order to establish a blood alcohol level at the time of driving. This method is not an accepted science and the top scientist in the world state that this method is unreliable.
In certain cases it may be to your advantage to use retrograde extrapolation. Let's assume that you recently consumed alcohol just prior to driving and approximately an hour and a half goes by before you are tested. Based on these facts it is most likely that your blood alcohol was lower at the time of driving rather at the time of testing. In CA, there is a presumption that if your blood test is done within 3 hours of the time of driving then you BAC reading is deemed to be your BAC at the time of driving. However, you can rebut this with affirmative evidence through the testimony of an expert to show that at the time of driving your BAC was actually lower than at the time of testing. This is done through retrograde extrapolation.
The word "Extrapolate" means to make a guess at a value that is outside a known range. The meaning alone should not evoke confidence in the process.
BASIC PRINCIPLES: In order to to a retrograde extrapolation you must have BAC measurements at 2 different points in time. A dangerous assumption must also be made and that is that you BAC is on the "downslope."