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The SCRAM device works by measuring the gas alcohol concentration over the skin. Alcohol is delivered to the skin via blood flow. Alcohol then diffuses through the perfused tissue layer, the epi- dermis and the stratum corneum and then into the gas above the skin1. The stratum corneum is made up of densely packed cells and represents the ma- jor barrier to alcohol diffusion. Thus the diffusion process is a “diffusion-limited” (depends on the re- sistance to diffusion) system which varies consid- erably depending on the physiological (or patho- logical) properties of the skin.
In addition to passive diffusion, detectable perspiration contributes a conductive component to the process. In the case of perspiration, alcohol dissolved in sweat contained in the sweat glands is carried to the surface by the convective liquid movement to the surface. Under normal circum- stances, perspiration represents only a small com- ponent of the transdermal skin flux. However, under conditions of exercise (increase in body heat) or hyperthermia, the increase in sweat pro- duction to help in body cooling will enhance the rate of transdermal alcohol exchange.
The diffusive alcohol exchange occurs due to a net movement from a region with high concentra- tion to a region with a lower concentration. When the blood alcohol concentration (BAC) is higher than that at the surface of the skin, there is a net alcohol flux from the blood to the gas above the skin. During the alcohol absorption phase, when theBACisincreasing,alcoholdiffusesfromthe blood toward the skin surface.
During the elimination (burn-off) phase, the opposite occurs as alcohol diffuses from the gas above the skin toward the blood. Because the ex- change is diffusion-limited in both directions, the shape of the transdermal alcohol concentration (TAC as denoted by AMS) curve is distorted rela- tive to the shape of the BAC curve. The TAC curve is generally flatter (decreased peak height) and the rates of increase and decrease of TAC are reduced relative to the same values for the BAC curve (Anderson & Hlastala). This curve distor- tion complicates the interpretation of TAC curves.
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DWI JOURNAL: Law & Science — April 2006 — Page 3
Figure 1 illustrates the distortion of the TAC curve relative to a hypothetical BAC curve given the inter-individual variation in skin diffusion properties.
Generally,theprocessofauthenticatingan alleged drinking episode requires two important assumptions. First, that the height of the TAC curve is directly related to the BAC curve and is determined by adjusting with a fixed correction factor. The practice of converting TAC curve by a constant multiplier assumes that the shape of the TAC curve is identical in form to the BAC curve. This assumption is flawed because the shapes are quite different (as shown in Figure 1). If the sub- ject has an average TAC curve, then correction by a constant factor would be appropriate. However, if the subject has a near maximum TAC curve, then the peak BAC would be overestimated using an average correction factor. Similarly, if the sub- ject has a near minimum TAC curve, then the peak BAC would be underestimated using the av- erage correction factor.
Second, that the rate of TAC decline is decline is equal (or at least similar) to the rate of BAC de- cline (burn-off rate). Under normal circumstances neither of these assumptions can be true due to the variations in skin diffusion properties among the normal population, thus leading to false posi- tives (apparent readings of supradermal gas alco- hol concentration above 0.02 gm/dl when the BAC is actually lower than 0.02 gm/dl). The impact of
errors in these assumptions have not yet been adequately evaluated. Because of the normal physiological variation in skin diffusion proper- ties, the SCRAM device can yield either false posi- tives or false negatives.
The method used by AMS to determine whether an alleged drinking event is a “Confirmed Drinking Event” is subjective at best. A data string showing the alleged event is re- viewed by employees of AMS. The criteria used for determining whether the data truly indicates a drinking event is unclear and not specifically defined. This determination ostensibly relates to the examination of the TAC rate of decline with the assumption that this rate directly correlates with the BAC burn-off rate. However, the ability to make this determination depends critically on the assumption that diffusion properties of the specific subject’s ankle skin are the same as an
average ankle. If AMS were to develop a set of specific crite-
ria for judging an alleged drinking event, it would make the determination “objective” rather than “subjective,”legitimizingtheinterpretationand minimizing the possibility of false-positive deter- minations of drinking events. Such an approach would also speed up notification to the subject when a positive drinking event is identified so that he or she can obtain exculpatory data.
Another concern relates to the resolution of determination of the “burn-off” rate of the TAC curve. The data points are obtained every 30 min- utes (rather than continuously). In the presence of measurement or random noise, it becomes diffi- cult to accurately measure the rate of decline of the TAC curve. A systematic analysis of the ef- fects of measurement noise (error) has not yet been published.