6.1. Sweat Specimens
Drugs of abuse and their metabolites have long been known to be excreted in sweat. Quinine was detected in sweat in 1844, morphine in 1942, and amphetamines in 1972. The development and patenting of a sweat patch collection device by PharmChem Laboratories (Haltom City, TX) in the 1990s has allowed for the ready detection of drug use over a period of approx 1 wk of patch wear (50-54). The sweat patch is a simple Band-Aid®-like device consisting of a small 3 x 5 cm absorbent cellulose pad covered by a gas-permeable polyurethane membrane that allows water vapor to pass through while trapping in the absorbent pad any drugs and/or their metabolites excreted in sweat. The patch is held in place on the torso or arm by a special Band-Aid-like adhesive layer surrounding and covering the cellulose collection pad. After a wear period of approx 1 wk, the analysis of the sweat patch is relatively straightforward—drugs are eluted from the collection pad and the extract subsequently analyzed by immunoassay and GC-MS.
The criminal justice community has shown great interest in sweat-patch testing for drugs of abuse. The patch offers the primary advantage of constantly monitoring for any drug use over a period of approx 1 wk, obviating the need for multiple urinalyses to effectively monitor for any drug use over that period. The patch cannot be removed or tampered with without it being apparent to a trained technician. There has also been interest in the use of the sweat patch for federally regulated workplace drug-testing applications (4). However, given the invasion-of-privacy implications of an employer monitoring the off-duty behavior of an employee, the sweat patch would likely be used only as a last-chance agreement between an employee and employer after a prior failed drug test or other evidence of workplace drug use.
Sweat arises from both eccrine and apocrine sweat glands. The eccrine sweat glands are found on most parts of the body, whereas apocrine sweat glands are found primarily in the axillary, inguinal, and perineal areas. The apocrine sweat glands open directly onto the hair follicle and are less well studied and understood than eccrine sweat glands. Eccrine sweat-gland density varies widely, from 60/cm2 on the back to 600/cm2 on the sole of the foot. Glandular sweat production is approx 1-5 nL/min/gland. Insensible sweat amounts to 400-700 mL/d. Sweat is 99% water, originally isotonic with plasma, but water re-absorption makes it hypotonic. Sweat pH when resting is 5.8, but exercise increases sweat pH to 6.1-6.4.
The patch is applied to the torso or arm after precleaning the skin with alcohol wipes. The cleaning is designed to not only remove any possible surface contaminants but also to ensure an effective seal of the adhesive. The patch is worn for about a week, absorbing sweat and any drug and metabolites present in that sweat. The patch absorbs approx 300 ^L of sweat each day, or approx 2 mL/wk. After approx 1 wk, the patch is removed and sent to Pharm-Chem Laboratories for elution and analysis by immunoassay and GC-MS. Elution is performed using an aqueous methanol/acetate buffer. Cut-off levels for reporting a positive result are on the order of 25 ng/patch. The Administrative Office of the US Federal Courts has also administratively required that to report positive test results for cocaine or methamphetamine, their respective metabolites must also be present at their limits of detection.
The patch has been demonstrated to be sensitive and accurate, although its use has not been without challenges. There have been claims that the patch may be contaminated from exposure to drugs, both from the environment and from residual levels of drug in the skin from prior use (55,56). It has been demonstrated that under certain laboratory conditions, drugs applied in certain pH solutions to the outside of the patch can migrate through the polyurethane outer membrane into the underlying collection pad when the underlying pad is also soaked with certain pH buffers. However, these laboratory experimental conditions are not likely to occur in a real-world setting. In addition, it has been demonstrated that when alcohol-precleaned skin is doped with drug in solution, the preplacement cleaning procedure does not remove all of the drugs from the skin and can generate a positive test result. Again, these conditions are not likely to occur in a real-world setting. Although at least one federal court has recognized the possibility for such external contamination, these contamination challenges have been effectively rebutted in several subsequent cases. Further research would be welcome to define more accurately whether the patch may be prone to the possibility of contamination under more real-world drug-exposure conditions.
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