Viral filtration (VF) using nanofilters removes endogenous and/or adventitious viruses from biologic drug-substance manufacturing processes (1). The gold particle test (GPT) is performed as part of postuse integrity testing — to complement postuse leakage testing — for cellulose filters such as Planova 20N filters from Asahi Kasei Corporation. First, a proprietary gold-colloid solution matched to the filter type (e.g., 20N) is filtered through the test article. That filter’s pore-size distribution can be assessed using spectrophotometric absorbance readings of the integrity-test…