Time spent in qualification of assays for any application can pay off substantially in the long run. Designing assays for qualification of sera have to be based in the specific application. There may be more than one assay required to manage the risk for a process. Characteristics may include parameters such as cell attachment, population doubling time, viral infectivity, etc. It may be worth running through some sort of fault analysis, such as a process FMEA, to make sure all the critical points are identified. Design several assays and see which has the best assessment qualities for the specific application. Ideally it would be one short assay, but that is not always achievable. The variability in cell culture processes can make it challenging to create a single assay for multiple applications. Building up the statistical confidence in the assay is also a key function in minimizing risk. A potential pitfall is in creating assays that are very sensitive. It really needs to fit the application. If 90% of the samples pass the test and are all effective in the application – that is great. There is no value in being more discriminating when it is not necessary.