Comparison of the one- and two-step double antigen sandwich TRF assays. The S/Co values of all sera (n=131) analyzed in this study, using the one-step (x axis) and two-step (y axis) assay formats are included in this comparative analysis. Each circle in this plot represents one sample. The sera represented members of the in-house (red), PRB 931 (blue), WWRB 302 (green), PCA 201 (magenta) and PRB 108 (olive) panels. Filled circles represent anti-HIV positive and empty circles represent anti-HIV negative samples, as determined by the commercial tests described in the text. The dashed horizontal and vertical lines represent the cutoffs (at S/Co=1) for the two assays. Commercial panel members #1 (panel WWRB 302), #12 (panel PRB 108) and member #s 11 and 25 (panel WWRB 302), discussed in the text are indicated by ‘a’, ‘b’ and ‘c’, respectively.