Following the initial observations by Chen et al., leading manufacturers of biopharmaceuticals have determined that endotoxin (LPS) can be “masked” in biopharmaceutical formulations typically containing phosphate, polysorbate, and sugars (1). Being in a masked state, endotoxin is undetectable by the commonly used factor C detection methods such as the Limulus amebocyte lysate test (LAL). This phenomenon has also been denominated low endotoxin recovery (LER). As previously shown by Reich et al. (2), endotoxin masking is time dependent, meaning that…