Host cell proteins (HCPs) can be present in significant amounts in biological products through copurification with a recombinant protein-drug substance. Purified active ingredients developed for human use must be free of all contaminants — including residual HCPs — to minimize the incidence of immunogenicity against a drug product or its trace contaminants (1).   HCP assays allow for monitoring host-related impurities during product development and process development. Most are in the enzyme-linked immunosorbent assay (ELISA) format for detecting…