The first phase of the strategy involved using a generic screen of the anti-IDs with a biotinylated mAb as a capture and an anti-mouse IgG-HRP as a detector (Figure ). This initial screen allowed for determination of both the effect of matrix and soluble target interference on the ability of the anti-IDs to bind to the mAb. A scoring system was used to rank-order the anti-IDs that assigned a numerical value to four different parameters based upon the effects of matrix and soluble antigen interference (Figure , D; Supplementary Tables ,3). The use of this scoring system allowed for identification of anti-IDs that had the lowest cross-reactivity to human serum and interference from soluble antigen. Identifying the anti-IDs with the top scores decreased the number of pairs of anti-IDs to use in a labeled screen and allowed for a more targeted investigation into the optimal anti-IDs and PK assay format. The top five performing anti-ID's based on their scores and a generic detection reagent were chosen to move to the second stage of screening.