This is the scientific term for the ability to look at someone's alcohol level at the time of testing, and look backwards in time to determine what the alcohol level was at the time of driving. This can sometimes be important to attempt to show that at the time of driving the person was not at BAC which would be illegal.  This can be proven based on many factors such as number of drinks, type of drinks, size and weight of the Defendant, time since last drink, food consumed and other factors.  This analysis may become less important however in per se cases where the Maryland DUI driver has blown a number in excess of .08.