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(i) Standard solutions of buprenorphine (B) and three metabolites; (ii) immunoassay (IA) reagents designed for the analysis of B and/or its metabolites; and (iii) clinical urine specimens collected from patients (under B-treatment), constitute the B-System for fundamental study of parameters critical to the two-step test strategy, an analytical approach designed for a high-volume testing environment. The cross-reacting characteristics of IA reagents were examined using standard solutions of B and its metabolites. Resulting data were used as the basis for selecting target analytes suitable for the preliminary and the confirmatory test steps. Test data derived from IA and GC-MS analysis of clinical urine specimens (with natural distribution of B and its metabolites) were quantitatively correlated. Correlation parameters were examined: (i) to verify whether the analyte-pair targeted by the IA and GC-MS test steps has been properly selected; and (ii) to decide on appropriate cutoffs for the two test steps. In conclusion, this study has demonstrated that the most effective analyte(s) that should be targeted in the GC-MS determination step vary with the IA selected in the preliminary test step. All analytes that generate significant responses to the IA reagent should be targeted in the GC-MS test step.