Interpretations of Sweat Patch Test Results

Controlled Dosing Studies

In order to clarify the pharmacokinetics of the drugs identified in the patch such as heroin; cocaine; methamphetamine and codeine; various controlled dosing studies have been carried out. Lately, a few detailed and well designed controlled-dosing studies have been published on codeine, cocaine, tetrahydrocannabinol (THC) and methamphetamine by the National Institute of Drug Abuse (NIDA). The studies were subjected on drug users only. Therefore, a pre study washout period was carried out on them where sweat patch was monitored. Sweat patch analyses from this segment of studies established some degree of probability of further elimination of drugs into the sweat after the regular use has been stopped.

As confirmed through the studies, even some hours after dosing, considerable amount of administered drugs do not appear in sweat; and during the first day of dosing, majority of the detected drug element in sweat gets eliminated. However, a few frequent users of drug showed week long elimination in sweat after their latest use, but concentrations in the patch were below the criteria for test to be declared positive. Thus, usage of drugs a week before sweat patch application, potentially results in a positive result. In order to obtain evidence regarding the claims of subject towards drug use, the concentrations of metabolite and drug prove to be a fruitful guidance. Figure 8-2 demonstrates results from early controlled studies on cocaine proving that increase in doses of cocaine increases the concentrations detected in the sweat patch.

Figure 8-2       Cocaine dose vs. amount detected in the patch.

Figure 8-3 demonstrates that modest amount of cocaine is eliminated in sweat during the initial hours right after the drug usage and the drug detected in the sweat patch is mostly eliminated during the first 24 to 48 hours. This was experimented when the sweat patch is worn for varying time intervals after the drug is used.

Figure 8-3       Sweat patch cocaine concentration after controlled dosing.

As shown in the figure, the decrease in concentration levels after some days of patch wear is deduced to be because of the reabsorption of the drugs back into the skin from the sweat patch.

Figure 8-4 demonstrates the concentration differences between the urine and sweat patch after controlled dosing with cocaine. The sweat patch test, for one week wear period, remained positive, where as the urine tests were positive only for two days after dosing. As per the figure, days 3 to 7 resulted in negative for the urine tests, the usage of drugs being undetected, unlike the sweat patch experiment, which showed positive.

Figure 8-4       Urine and sweat patch cocaine concentrations after controlled dosing.

Figure 8-5       Drug detection rates for cocaine, opiates, and marijuana in the sweat patches and urine.

Field Studies

In case of criminal justice and treatment settings, various field studies have confirmed the usefulness of sweat patch testing over the urine drug testing (Figure 8-5 and 8-6). From the place of work population, no parallel comparison data can be known on the use of urinalysis and sweat patch testing. Studies have proved that compared to the periodic urine testing, sweat patch testing is far better in drug use detection. This is because the sweat patch monitors the drug regularly, as long as the patch is worn, whereas, the urine test needs to be done daily and requires limitations and dilution controls in order to be successful at isolated occasions of drug use, or else it would fail to detect. Yet, the sweat patch is less sensitive to detect use of marijuana as compared to urine test due to the elimination of cannabinoids at lower concentrations in sweat than in urine.

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