Every 40 minutes, about .01 of blood alcohol content (BAC) is metabolized by the liver and eliminated from the body. So, what happens if someone is arrested and tested later at the police station for BAC? Or, what if someone involved in an accident is rushed to the hospital where it is later realized that the individual is under the influence? How do they know what the level was at the time of driving?
There is a process called retrograde extrapolation by which the BAC, at the time a person was actually behind the wheel, can be estimated by using a mathematical equation. One has to understand the average levels of absorption and metabolism of alcohol over a period of time to perform this calculation.
Usually, the rate of alcohol metabolism is about .015 to .020 grams per deciliter each hour. The equation for this is g/dl/h. But, it is not a perfect equation as it cannot take into consideration the variances between individuals.
Not only are there differences between blood volumes (men generally have 58% water content whereas women usually have 49%), but there are also differences in rates of sweating, body temperature, the type of drink consumed, and how much of what kind of food was eaten with it.
One reason why weight is usually used in calculations for BAC charts, and not the volume of water in the body, is simply that it is easier to measure. The other is because some of the alcohol gets temporarily stored in the fatty tissues, as the body tries to get the alcohol out of the blood. Later, it will be re-released into the blood and metabolized by the liver and processed through bodily secretions. Liquid volume does not take this into account.
For the time being, it seems that there may never be a perfect test for the measuring of alcohol content in the body. For now, the blood test is the most accurate measure we have, with the breathalyzer being the best for on-site testing, as a relatively accurate estimator of BAC.