The study also attempted to characterize the prevalence and concentration distributions of EPH, PSEPH, and PPA in samples that produced positive amphetamine screening results. Approximately 27,400 randomly collected human urine samples were obtained from Navy and Marine Corps members. These samples were simultaneously screened for amphetamines by using the DRI and Abuscreen online immunoassays at a cutoff concentration of 500 ng/ml. All samples that screened positive were confirmed for amphetamine (AMP), methamphetamine (MTH), 3,4-Methylenedioxyamphetamine (MDA), 3,4-Methylenedioxymethamphetamine (MDMA), EPH, PSEPH, and PPA by means of gas chromatography/mass spectometry (GC/MS). The DRI AMP immunoassay identified 1,104 presumptive amphetamine positive samples. Only 1.99 percent of these sample confirmed positive for the presence of AMP, MTH, MDA, or MDMA. In contrast, the online AMP reagent identified 317 presumptive amphetamine positives with a confirmation rate for AMP, MTH, MDA, or MDMA of 7.94 percent. The presence of EPH, PSEPH, or PPA was confirmed in 833 of the 1,104 samples that failed to confirm positive for AMP, MTH, MDA, or MDMA. All of the 833 samples contained PSEPH. When compared with the entire screened sample set, PSEPH was present in approximately 3 percent, EPH in 0.9 percent, and PPA in 0.8 percent of the samples. The findings show that cross-reactivities for EPH, PSEPH, and PPA are greater than reported by the manufacturer of these reagents. Although these reagents may produce fewer false positives due to PSEPH when using a 1,000-ng/ml cutoff, at a 500-ng/ml cutoff, a substantial number of false positive screening results were obtained. The distribution of concentrations shows that very large concentrations of EPH, PSEPH, and PPA are common. Likewise these compounds are commonly present at concentrations that may indicate excessive use of the drugs. 2 tables, 1 figure, 17 references