Host cell protein (HCP) levels in biologics are on the critical path for assessing product quality, and they pose a serious risk to drug safety. The challenge is to accurately quantify a complex mixture of HCP impurities, which vary in properties and abundance depending on cell line, media, and process parameters. HCP immunoassay analysis is based on polyclonal antibodies raised against HCPs from nontransfected cell lines. How well a particular HCP assay recognizes all proteins depends on how well the…