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Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a serine protease that plays an important role in the regulation of serum low-density lipoprotein (LDL) cholesterol by downregulation of LDL receptor, and as such is considered a novel target in cholesterol lowering therapy. In support of the drug development program for Evolocumab, a fully human IgG2 antibody that targets PCSK9, a quantitative ELISA to measure free PCSK9 in human serum was developed. PCSK9 serves as a biomarker of pharmacological response during treatment, and measuring levels of the free ligand post-dosing was of interest as an aid to establishing the pharmacokinetic and pharmacodynamic properties of the therapeutic. Given the complexities associated with the measurement of free ligand in the presence of high concentrations of circulating drug, it was important to challenge the method with experiments designed to assess ex vivo conditions that have the potential to affect the binding equilibrium of drug and ligand within test samples during routine sampling handling and assay conditions. Herein, we report results of experiments that were conducted to characterize the assay in alignment with regulatory guidance and industry standards, and to establish evidence that the method is measuring the free ligand in circulation at the time serum was collected. A robust supporting data package was generated that demonstrates the method specifically and reproducibly measures the free ligand, and is suitable for its intended use.