Appropriateness of in vitro potency tests as a measure of vaccine or reference stability.

A proposed definition of a stability indicating assay is "a validated quantitative analytical procedure that can detect changes over time in the pertinent properties of the product" (Federal Register/Vol. 75 No. 180/Friday, September 17, 2010/Proposed Rules). In vaccines intended for veterinary usage, the potency assay has traditionally been used as a measure of stability. Some potency assays may be acceptable as stability indicating assays, whereas other potency assay will not meet the criteria for stability indicating assays. For example, an ELISA potency test may or may not detect degradation products depending on the specificity of the antisera. With time, the ELISA may overestimate the antigen as partial degradation occurs or if an aggregated or particulate antigen dissociates. Specific assays parameters and attributes that are required for a potency assay to be indicative of serial or reference stability are discussed.