Ligand binding assays have been widely used to quantify the amount of biological drug molecules in pharmacokinetic studies. The reagent screening and assay development of LBAs to measure PK have traditionally used a semi-empirical approach. However, the increased usage and development of therapeutic antibodies presents an opportunity to develop and use a systemic approach to increase efficiency, quality, and robustness of the LBA PK assay. For some therapeutic antibodies, the presence of a circulating soluble target could interfere with the PK assay performance and requires consideration to ensure total drug levels can be measured, that includes both free and target-bound drug. Identification of reagents with a high binding affinity specifically for the mAb regardless of presence of soluble target and a low matrix interference are key qualities for the optimal reagent. However, the most robust PK assay cannot be developed with such reagents if the optimal PK format for these reagents has also not been selected. Therefore, screening and selection of critical reagents concurrently with the determination of the most robust assay format are key components in developing a robust PK assay capable of measuring free and bound mAb levels in serum.