Occasional challenges, regarding the interpretation of sweat patch results being positive, arose when the sweat patch testing was introduced to the treatment sectors and criminal justice. There were two major challenges to the patch results. First one was the infectivity of the skin from environmental drugs not being completely removed from the skin by cleansing measures, before application of sweat patch. The second challenge was the claims for false positive patch test results that occurred due to environmental drugs entering into the absorbent pad of the sweat patch from the outer membrane, while it is worn, causing external contamination of the patch.
Figure 8-6 detection of amphetamine, cannabinoids, cocaine, and opiates in discrete urine specimens and the sweat patch.
Some laboratory experiments show 2 possible mechanisms of positive sweat patch test results from exposure to external environment, as documented and reviewed in literature. Yet, the authors of this chapter had reservations that the experimental conditions for laboratory tests conducted do not correspond to the circumstances favorable in practical life settings. Despite of the claims, nothing in actual sweat patch wear settings has been observed that would demonstrate unintentional skin infection or environmental exposure as a reason for positive sweat patch test results.
External Contamination of the Sweat Patch
As observed, a few drugs, such as cocaine and methamphetamine, when placed on the outside of the sweat patch, penetrated through the external (polyurethane) membrane into the absorbent pad, getting detected. On the other hand, it should be kept in mind that the observation has been conducted under particular and extraordinary laboratory testing conditions. The sweat patches were kept in a warm and humid sealed environment, upon the Petri dishes, during the experiment which demonstrated drug penetration through the membrane of sweat patch under the mentioned controlled experimental conditions. Also, the absorbent pad of the patch was saturated with a buffer of an acidic pH than usual sweat is, in the experiment held at laboratory conditions where the patch was not even worn by ambulatory subjects under practical situation.
Moreover, the drugs were in contact to the membrane in buffer, ensuring a unionized form of drug (pH 8 or above for cocaine and much more for methamphetamine) that could easily penetrate the membrane of the patch. BE could be used as a biomarker as under various pH conditions, it did not penetrate through the outer shell of the sweat patch.
Neither the real world patch wear, nor the laboratory experimental scenarios demonstrate such external contamination that could end up with positive test results for anyone wearing the sweat patch, outside the experimental conditions. Though, ‘positive but false’ sweat patch results regarding external contamination claims have been made for subjects in hypothetically practical situations, scrupulous examination does not prove the absence of drug use. To minimize the possibility of positive but false test results, the authors claimed false positive swear patch results for the treatment of subjects using cocaine, wearing the sweat patch regularly and suggested use of higher 75 ng/patch (25 ng/mL) cutoff instead of the present 25 ng/patch (10ng/mL). Hence, it is concluded that contamination of the environment wasn’t a significant issue and it has been observed that many other groups have dealt with the question in concern, i.e. the permeability of sweat patch.
A possible source of external contamination is the presence of drug in the environment where the subject who is using the patch is, and that can be transmitted to the external layer of the sweat patch. The problem of external contamination exists because of the possibility of the presence of drug contamination in the atmosphere that increases the chances of its transfer to the outer skin of the patch. For instance, currency has been proven to contain drug contamination, yet the transfer of that drug to skin or other surfaces has not been established. Studies investigating the transmittance of pesticides from contaminated carpets or methamphetamine from infected surfaces have established it to be impossible or limited in transfers to human beings.
The US Court’s reporting standard calls for the presence of cocaine metabolite, BE and the methamphetamine metabolite, amphetamine, to show positive for these drugs, in order to deal with the external contamination matter. To report a positive methamphetamine, Proposed Revisions 2004 of SAMHSA requires the presence of amphetamine. The test results use the presence of metabolite as an additional assurance to their report, but this may not be completely certain as there is a possibility of presence of metabolite impurities in the drugs and at times, the parent drug breaks down within the sweat patch. Still, a number of studies reflect the stability of cocaine in the sweat patch while it is worn, with a slight chance of conversion to BE.
In order to obtain approval of the sweat patch for the detection of cocaine, the study data was submitted to the FDA. It included the details of the subjects wearing the patch being indirectly exposed to vaporized cocaine in an unaired room, and also victimized to dermal exposure. It was then concluded by the study authors that subjects wearing patches and being exposed to accidental cocaine present in the atmosphere would not report a false positive result as the patch is insensitive to the cocaine present in the atmosphere. The patch was given clearance by the FDA on review of the provided data and conclusions of professionals and the results were later documented in black and white.
In conclusion, SAMHSA, after reviewing the claims and arguments on the possibility of external contamination of the sweat patch, reported in its 2004 Proposed Revisions that “on the basis of available information, the department considers the statement that under normal circumstances, the external absorption of any drugs via the external shell into the patch is impossible”.
Contamination of the Donor’s Skin: Dermal Exposure, Incomplete Removal, and Prolonged Dermal Residues after Drug Use
Incomplete removal of drug deposits in or out of the donor’s skin due to environmental contamination can lead to positive sweat patch results. Even though the skin is to be meticulously cleaned using alcohol wipes before applying the sweat patch onto the donor’s skin, it has been stated that the complete removal of drugs from environmental exposure cannot be achieved.
Some experiments are documented where the subject’s skin was contaminated artificially with cocaine and then tested with sweat patch which resulted positive. However, these do not reflect the circumstances of routine life, hence regarded irrelevant. Initially, it was derived from an experiment that a skin in alcoholic solution was contaminated with cocaine, which was easily removed while cleansing was done. Moreover, the skin was cleaned with alcohol and then contaminated with cocaine exposed to alcohol solution, rather than the simple skin contact with powdered cocaine, just like an environmental exposure. The experiments were not ideal and did not represent reality, as agreed by the study authors: ‘The drugs were applied in alcohol solution, although unlikely to occur in real world…’ I t was observed by other researchers during similar experiments regarding skin contamination where the sweat patches did not meet the standards to be accounted as positive. A few results for cocaine were, at the highest level of contamination, below the reporting cutoff but above the limit of quantitation. BE, although determined, yet it wasn’t found in any observation.
One of the most impermeable biological membranes ever known is the human skin, stratum corneum, as referred by the dermatologist. It is the most outmost layer that protects the skin, hence making it difficult for drugs to penetrate through. In the above mentioned laboratory tests, the skin was first cleaned with alcohol before the experiment as the alcohol severely affects the protective layer, making it permeable and allowing the drug to get absorbed through.
The skin acts as a transitory tank for some drugs such as pesticides and steroids, as these drugs have been drenched into the outer (stratum corneum) and the lower layers of the skin and are not easily eliminated by cleansing. Therefore, sweat patches applied after the drugs get impregnated into the skin layers, result positive for drug traces for quite a few days. Yet, these are mere experiments that do not reflect the actual reality facing the donor wearing the patch. The drug users wearing the patch are not anticipated to clean their skin with chemical solvents and then apply the solution with dissolved drug in it. Also, the stratum corneum (the outer layer of skin) is restored speedily within few hours with 60 percent of the protective layer, whenever it gets removed.
A study was conducted to verify whether the skin holds back drugs (cocaine), for which the cocaine was administered and the results suggested that the skin does not work as storage for cocaine. This is because the cocaine was not detectable after 6 hours in the skin and neither was BE found up till 72 hours after dosage. On the other hand, a study also found BE after cocaine dosage in unwashed and shabby stratum corneum, but with negligible concentrations about few nanograms per milligram (0.2-2 ng/mg) (concentrations of cocaine fluctuated in low dose about 0-0.8 ng/mg and 0-3 ng/mg after high dosage). Stratum corneum of about 1 milligram was successfully collected from area that was covered by almost 2 sweat patches. Such drug levels of skin would not be enough to produce a positive test result, even if it is fully transferred to a sweat patch.
A study in 2004 reported, “After systematic administration, we have in recent times noted that skin does not act as a reservoir for cocaine and that the cocaine follows a parallel disposition pattern in the skin as compared to plasma, disappearing as quickly from the skin as it does from the plasma. Therefore, after transdermal absorption, the extended disposition of cocaine in plasma or skin is improbable”.