Only few harvests need to be tested (validation). c Old cells may be from production runs (as post production cells) or from a separate culture kept in continuous culture for a long period and prepared for this analysis of 'limit of cell age' only (as late expanded cells). Extensive testing performed as part of the qualification of the MCB regarding absence of latent virus, inducible by cultivation on production conditions. d A very large sample volume for testing would be required for statistics of a sufficiently sensitive detection of low virus titers.