Qualitative Detection of Opiates in Sweat by EIA and GC-MS

Sweat was collected with the PharmChek sweat patch, and drugs were eluted from the collection pad of the patch. A solid-phase enzyme immunoassay (EIA) using microtiter plates was modified for the analysis of opiates in sweat. After opiate administration, sweat contains primarily parent opiate (heroin, codeine) and lipophilic metabolites (6-monoacetylmorphine [6-MAM]). The immunoassay was determined to have a cross-reactivity with codeine of 588%, with hydrocodone of 143%, with diacetylmorphine of 28%, and with 6-MAM of 30% relative to 100% for the morphine calibrators. The optimum cutoff concentration for this modified assay was determined by receiver operator characteristic analysis using 215 patches from 95 subjects to be 10 ng/mL morphine equivalents. At this cutoff concentration the assay had a diagnostic sensitivity of 86.9% and a diagnostic specificity of 92.8% versus gas chromatography-mass spectrometry (GC-MS), which was the reference method. The positive predictive value at a prevalence of 50% was 86%. The intra-assay precision at 10 ng/mL was 7.8%, and the interassay coefficient of variation (CV) was 39%. Analysis of spiked patches around the cutoff gave a percent positive threshold of approximately 50% between 10 and 15 ng/mL and a 95% confidence level for a positive result by the EIA between 20 and 25 ng/mL. Eighteen possible adulterants that could be injected into or under the patch were studied. Two (tile cleaner and detergent) can cause false-positive responses in the immunoassay. Two adulterants reduced response to spiked drug (Visine eye drops and Ben Gay ointment), which could cause a false-negative response. All results were confirmed by GC-MS. The clinical sensitivity and specificity for detecting drug use by analyzing sweat collected from human subjects following known doses of codeine (0, 30, and 60 mg orally) or heroin (20 mg intravenously) were 76 and 100%, repectively.