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Background
Almost all offenders convicted of impaired driving are ordered to abstain from consuming alcohol as a condition of sentencing or probation. Despite the variety of alcohol testing methods (e.g., blood, breath, urine) available to monitor compliance with this condition, compelling offenders to remain sober has been an elusive goal and a notoriously difficult condition to enforce.

Existing blood, breath, and urine testing protocols are used infrequently and are not consistently applied because of significant staffing, resource and cost implications. Recent findings from a national survey of 890 probation officers in 41 states revealed that officers spend less than 10% of their time engaged in random testing of offenders. As such, the ability of officers to enforce this condition is limited, and, not surprisingly, offenders are able to engage in undetected drinking behavior.

In the past decade, alcohol testing technology has evolved, giving rise to a new generation of testing devices. Alcohol monitoring bracelets are now being employed and use transdermal alcohol monitoring to allow for continuous monitoring of offenders 24 hours a day, seven days a week for the duration of the supervision period.

The rapid proliferation of these devices has created a need among criminal justice professionals for information about the research on continuous transdermal alcohol testing and monitoring, and its role in dealing with offenders. The following provides a comprehensive review of existing research on transdermal testing.

Measuring Alcohol Consumption through Perspiration
Since 1930, it has been well known and scientifically established that ingested alcohol diffuses throughout water in the body and is present in various bodily substances, including blood, breath, urine, and sweat. Once alcohol has been ingested, most of it is metabolized in the liver, some is removed through exhaled air or breath, and some leaves the body unchanged in the urine. Only about 1% of ingested alcohol crosses the skin, either as sensible perspiration (sweat in the liquid phase) or insensible perspiration (constant, unnoticeable sweat in the vapor phase). This phenomenon, first studied and understood as early as 1936, and later investigated in other studies     , is known as transdermal excretion of alcohol or excretion through the skin.

Results obtained from the scientific research into measuring transdermal excretion of alcohol in the 1970s and early 1980s were also promising. It was concluded that the concentration of alcohol in the collected sweat rose with the amount of alcohol consumed and with the mean concentration of alcohol in the blood. Further research in the 1980s drew comparable conclusions, highlighting that the blood alcohol concentration at a specific point in time cannot be accurately estimated using sweat samples due to a time delay between absorption of alcohol in the blood and excretion through the skin. Due to this time delay, it is recommended that transdermal alcohol testing be regarded primarily as a screening method for detecting and monitoring episodes of alcohol use, rather than for determining precise levels of alcohol in the body at specific points in time.

In the 1990s, when the most recent generation of test devices, the transdermal alcohol bracelet, became available, earlier findings regarding the accuracy of transdermal testing were again corroborated (e.g., ).

After more than 70 years of research and 22 independent, peer-reviewed studies, it has been established that ingested alcohol can be validly measured in perspiration through the process of transdermal alcohol testing. Research about the dynamics of transdermal alcohol testing is still ongoing (e.g., ) and the dynamics of transdermal alcohol testing may vary both between subjects and within subjects. This means that some variation in repeated measures taken from a single subject can occur as the human body is not static, and that some variation in measurements from different subjects can occur, as no two people are alike. Such biological differences between and among individuals is not uncommon. For example, there are variations in blood partition ratios (used in blood alcohol testing) between individuals.

Studies over the past 10 years do conclude that transdermal alcohol concentrations reflect blood alcohol concentrations accurately, but with a measurable delay in absorption and elimination ;. As such, simultaneous breath or blood and transdermal alcohol readings should not be expected to produce similar results at a specific point in time.

“On average, the device shows discriminative validity as a semi-quantitative measure of alcohol consumption […]”. This means that this technique is a valid way of determining whether someone has consumed a small, moderate, or large amount of alcohol and to gauge compliance with orders of abstinence.

Collection and Testing of Transdermal Alcohol
Transdermal alcohol can be collected in the liquid phase (sensible perspiration) or the gaseous phase (insensible perspiration). Collection in the liquid phase may occur using a transdermal patch or an alcohol band-aid applied to the skin to trap ethyl alcohol eliminated in perspiration. Collection in the gaseous phase can occur using a wide variety of techniques to capture an air sample directly above human skin, or biological fluids.

Both liquid and gaseous perspiration samples can be analyzed or tested for ethyl alcohol using a variety of scientifically accepted techniques, including electrochemical sensors, colorimetric or integral, enzymatic, and chromatographic methods.

In the 1990s, technological advances led to the development of more sophisticated and practical methods of measuring transdermal alcohol by means of transdermal alcohol bracelets. These devices can be easily attached to an individual for extended periods and continuously collect insensible perspiration samples just above the skin. These samples are analyzed by an electrochemical sensor in the bracelet to estimate the concentration of alcohol in the body, and, thereby, provide an indication of alcohol use.

Comparing Alcohol Test Results
Results from transdermal alcohol testing can be compared to the results of other alcohol tests such as blood or breath. While alcohol pharmacokinetics (the manner in which alcohol is metabolized in the body) in humans may be complex, the principle of transdermal testing is easily understood and not different from the principles that govern breath testing.

There is a general consensus that blood analysis is the “gold standard” because it provides the most reliable measure of blood alcohol concentration (BAC) and because behavioral impairment is most strongly correlated with the level of alcohol in blood.

Breath alcohol concentration (BrAC) measurements are accepted as surrogate blood alcohol measurements because of the scientifically established correlation between the concentration of alcohol in blood and in breath. This proven correlation has permitted the use of breath as a reliable estimate of blood alcohol concentrations by the police, courts, and probation since the 1970s.

Transdermal alcohol testing relies on the same principle. Since alcohol is excreted unchanged wherever water is removed from the body (breath, urine, perspiration, and saliva), there also exists a correlation between the alcohol concentration in perspiration (i.e., transdermal alcohol concentration or TAC) and the alcohol concentration in the bloodstream (e.g., ).

It has been established that individual transdermal alcohol readings cannot be considered equivalent to blood alcohol concentrations. The main difference between blood or breath alcohol testing and transdermal alcohol testing is a time delay in the absorption, peak, and elimination of alcohol that occurs with transdermal testing. As noted previously, simultaneous breath or blood and transdermal alcohol readings should not be expected to produce similar results at a specific time.

However, rather than using this method to quantitatively estimate precise alcohol levels, research shows that transdermal alcohol testing may be validly used as a method to qualitatively identify drinking episodes.

Comparing Alcohol Test Protocols
Transdermal testing compares favorably with other test protocols. Blood and breath testing are invasive and require active participation by the offender. Conversely, the transdermal collection of sweat is both non-invasive and passive - offenders are not actively involved in delivering a sample; nor are officers involved to collect the sample.

Blood and breath testing have a higher cost per test whereas transdermal alcohol testing has a lower cost per test -- the ease of transdermal alcohol testing enables more tests in a given time period for the same cost. For example, instead of one test per week with a probation officer or physician, or instead of several breath tests per day at home with an electronic test protocol, transdermal testing can occur every hour throughout the day, at any location. Although breath testing can be used as a random protocol (e.g., multiple times daily, weekly), a high frequency of testing rarely occurs due to associated staffing and resource costs. Conversely, transdermal monitoring of sweat is a continuous protocol, making it very difficult for the offender to avoid detection for non-compliance.

Finally, while each of the test protocols described has the power to discriminate between the consumption of small, moderate, and large quantities of alcohol and gauge alcohol use, only blood and breath testing provide a precise alcohol concentration at a specific point in time.

Another technology worth mentioning is the actigraphy-based substance abuse screening. Research demonstrates that alcohol consumption disrupts sleep. Once baseline measures of normal sleep have been collected, sleep patterns are monitored using actigraphy for evidence of possible intoxication episodes. However, this approach is less than continuous because testing only occurs when sleep patterns occur and because possible intoxication episodes have to be confirmed using a corroborating source of evidence or, more precisely, biomarkers collected from the offender’s blood or urine (e.g., ethylglucuronide or EtG). The advantages of the continuous monitoring element (the continuously monitored sleep disruptions) may be compromised by the disadvantages of the non-continuous monitoring element (taking of blood or urine samples at predetermined points in time).

Conclusions from the Scientific Research
As discussed previously, the scientific conclusions regarding transdermal alcohol testing in general are:
 * Ingested alcohol can be validly measured in perspiration through the process of transdermal alcohol testing.
 * TACs reflect BACs accurately and reliably, but with a measurable delay in absorption and elimination. TAC readings can distinguish qualitatively between consumption of small, moderate or large amounts of alcohol; however, they are not intended to provide precise, quantitative estimates of alcohol consumption similar to evidential tests.
 * The current validity and the level of accuracy of transdermal alcohol testing permit it to be used as a screening tool to verify compliance with orders of abstinence.