Most recombinant monoclonal antibodies (MAbs) are produced by mammalian cells. Because biopharmaceuticals derived from mammalian tissue culture carry the risk of adventitious virus contamination, regulatory agencies expect risk-mitigation strategies to include validation of purification unit operations for their ability to clear viruses (1). Guidelines from the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) describe how to prove viral clearance in downstream purification processes using an orthogonal approach (2). Viral log10 reduction values (LRVs) are…