Ethanol is primarily metabolized in the body by alcohol dehydrogenase to acetaldehyde. This is a very low Km reaction, so at most alcohol concentrations achieved after social drinking, it is very often assumed to obey a near zero order type of elimination kinetics.
In reality however, ethanol elimination kinetics, like every other drug follows a zero- to first order type of elimination profile. The above concentration-time curve illustrates the elimination kinetics from very high to very low ethanol concentrations.
During the zero order phase, there is a relatively fixed rate of ethanol elimination (NB. not elimination rate constant), but becomes clearly exponential at much lower concentrations.
All this might be merely of academic interest if not for the fact that ethanol concentrations are very often back-extrapolated to a prior time (e.g. time of an accident or assault) because of forensic investigations. If the wrong model is used, the back-extrapolation can be quite erroneous.
Most often the back-extrapolations have been simplistically done assuming a zero order model, applying a fixed rate of elimination. We can see from the plot below, that the use of a zero order model, when the measured concentration is relatively low (less than 0.1 mg/mL) can lead to significant over-estimates of a prior concentration.
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Alcohol dehydrogenases (ADH) are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD+ to NADH). alcohol dehydrogenase
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