All recombinant protein biotherapeutics must be tested for the presence of residual host-cell protein (HCP) impurities (1–3). The most common analytical method for doing so is a polyclonal sandwich immunoassay. Polyclonal anti-HCP antibodies are selected to recognize the broadest population of HCPs possible. The immunogen and analytical standard are produced from a blank-run fermentation that mimics the production run but lacks the specific biotherapeutic protein. Because of the large number of impurities present in harvested cell-culture fluid (HCCF) that might…