Retrograde extrapolation of blood alcohol data: an applied approach

Retrograde extrapolation is a mathematical process, based on sound scientific principles, that is used routinely in pharmacology, toxicology, and clinical medicine. This process may be applied to the situation of ethyl alcohol consumption with reliability when reasonable assumptions are made concerning absorption rates, elimination rates, and patterns of alcohol consumption, including drinking duration and volume consumed. By utilizing an established range of values for the elimination rate of alcohol of 0.015-0.020 g/dl/h, a relatively narrow range of extrapolated blood alcohol concentrations (BACs) can be determined in situations where the time frame in question is after peak alcohol absorption into the blood. A wider range of elimination rates of 0.01-0.03 g/dl/h may be applied and will satisfy the possibility of nonlinear kinetics within an individual; however, this wider range will have little practical effect on the predicted BACs. When the time point in question is prior to peak absorption, a wider range of predicted BAC values will result. The extent of this range will be influenced by the amount of information available concerning the temporal pattern of alcohol consumption. Reported drinking volumes are notoriously inaccurate and, in fact, are of little practical use. Given the parameters of body weight and time duration between initiation of drinking and determination of the BAC, the number of "drinks" consumed may be reliability calculated. Retrograde extrapolation is applicable in the forensic setting with scientific reliability when reasonable and justifiable assumptions are utilized.