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Estimating peak alcohol concentration

One of the hazards of estimating peak alcohol levels based on a drinking pattern is over-simplification and unrealistic reliance on the absolute number.

Many organizations and agencies have produced wallet-sized cards with predictions of what blood alcohol concentration (BAC) a person of a certain weight will achieve after consuming a specific alcohol dose. It is well known that age, sex, and weight can make appreciable differences in the peak alcohol level reached. Women, because of their relatively low body water, will reach a higher alcohol level than men, even when compared to men of the same weight. Stomach contents also will result in a lower and a prolonged time to reach peak BAC. See Factors affecting absorption.

Predicting the exact time of peak absorption is thus very difficult.

Conservative estimates are statistically more accurate

When an expert witness is asked to determine the time of peak absorption in a given circumstance, the entire range of peak absorption times must be considered. [See Graph 4.]

Graph 3

Graph 3

Using a more conservative time frame will most likely make the expert’s assumption of peak absorption time true in an unknown defendant’s situation, since the statement statistically will include the largest portion of the population (e.g., 95% to 99%). For example, stating that peak absorption has occurred after 15 minutes may be true for a portion of the population, but a more valid statement may be that peak absorption has occurred after 1.5 hours (on an empty stomach), since that would be true for 95% of the population.

Blood alcohol (BA) levels can change or plateau

The movement of alcohol in the body and the systematic excretion and metabolism is a dynamic process. Nonetheless, most states have enacted laws that allow a presumption of the driver being at the same BA level at the time of driving as at the time of the test, provided the test is given within a mandated number of hours after driving. This presumption has little scientific basis. Although in some circumstances the amount of absorption and elimination may be the same (with the net result of a plateau for a period of time), it is just as likely that the alcohol level may be rising or falling over that same period of time as well.

Many experts develop graphs that show the change of alcohol over time. [See Graph 5.] Such graphs are meant to aid the jury in visualizing how alcohol rises for a period of time, peaks, and then decreases over time. The first portion of the curve is alternately called the “absorption” or “rising” side of the curve. The peak is alternately called “maximum BAC” and the latter portion of the curve is called the “elimination,” “post absorptive” or “falling” part of the curve.

Graph 4

Graph 4

Alcohol levels change over time. Extrapolation and prediction of alcohol levels is difficult when only the consumption pattern or a test result is known. However, if one knows the consumption pattern and the test result at a particular time, it can be reasonably stated that at a specific time before the test, the subject’s BA was the same level, lower, or higher than the test result. If no reliable details on drinking pattern are available, however, any of these three alternatives is equally possible.

To further complicate the theory of extrapolation, alcohol levels may remain the same over a period of time. [See Graph 6.] If alcohol levels are the same over a period of time, the amount of alcohol entering the system from the stomach and small intestine is precisely the same as the amount the liver is eliminating. Plateau times can vary, but are often greater than an hour after drinking.

Graph 5

Graph 5

Alcohol curve may fluctuate

The alcohol metabolism curve is often depicted as linear; a straight rise over time, followed by a peak and a linear fall back to zero. In reality, the curve may contain fluctuations. During the metabolism process, short-term fluctuation and spiking in the “linear” curve and curve irregularities are common.

“Zig-zag” effect or “steepling”

The “zig-zag” effect can be seen most readily in studies where many blood or breath samples are taken in 2 to 10 minute intervals. Studies in which subjects are tested every five minutes, for example, show fluctuations, as much as 0.03% in less than 10 minutes.

When the BAC is plotted against time, the BAC curve no longer has a smooth line, but rather a “saw-tooth” pattern. [See Graph 7.] If specimens are taken 30 to 60 minutes apart, the effect is not as obvious. The “zig-zag” effect (sometimes called “steepling”) is present in both the rising and falling side of the BA curve. One postulate is that these short-term fluctuations are more pronounced during absorption because of the fluctuation of stomach emptying and the irregular opening of the pyloric valve, whereas fluctuations during elimination may be primarily a function of analytical testing errors.

Graph 6

Graph 6

The concept of the “zig-zag” effect is an important concept when trying to determine an alcohol level at a time before or after a chemical test result. If there is no predictable linear regression, then predicting blood alcohol levels in the future or past may be erroneous.

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