Detection of methamphetamine in sweat by EIA and GC-MS

Sweat was collected with the PharmChekTM sweat patch and drugs were eluted from the collection pad of the patch. A solid phase, enzyme immunoassay using microtiter plates was modified for analysis of methamphetamine in sweat. After methamphetamine administration, sweat contains primarily parent methamphetamine. The immunoassay was determined to have crossreactivity relative to 100% for the methamphetamine (MA) calibrators; to 144% for methylenedioxymethamphetamine (MDMA); to 30% for d-amphetamine; to 21% for methylenedioxyamphetamine (MDA); and to 8% for I-methamphetamine. The optimum cutoff concentration for this modified assay was determined by receiver operating characteristic analysis to be 10 ng/mL amphetamine equivalents. At this cutoff concentration the assay had a diagnostic sensitivity of 84.5% and a diagnostic specificity of 93.2% versus gas chromatography-mass spectrometry (GC-MS). The positive predictive value at a prevalence of 50% was 86%. The intra-assay precision at 10 ng/mL was 9.9% (coefficient of variation, CV) and the interassay CV was 13%. Analysis of spiked patches at plus or minus 25 and 50% around the cutoff gave a percent positive threshold of approximately 50% at a cutoff of 10 ng/mL and a 95% confidence level for a positive result by the EIA between 15 and 20 ng/mL. Of 18 potential adulterants that might be injected into or under the patch, two (tile cleaner and cough syrup) caused a false-positive response by immunoassay. All results were confirmed by GC-MS. The clinical sensitivity and specificity of the overall analysis system (sweat collection and analysis) were 85 and 100%, respectively, using known methamphetamine dosing of volunteers (10, 20, and 25 mg) as the reference standard.