Your assay has changed - is it still 'fit for purpose'? What evaluation is required

A reliable laboratory assay is an essential tool for the diagnosis or surveillance of most animal diseases. Before routine use, assays should be appropriately validated to ensure that they have performance characteristics that provide reliable results and can be used for the intended purpose. It is inevitable that, over time, changes will need to be made to assay reagents, to the assay format, to test a different species or for implementation in a new laboratory. Whenever there is a change (whether it be components, application or location), it is essential to establish whether the new circumstances affect the biological basis and properties of the assay. If the modifications do not affect the biological basis of the assay, the changes might be considered minor and a verification study can be conducted to confirm that the performance characteristics have not been adversely affected. Major changes require a new validation to be carried out. A method comparability study, where original and modified assays are run concurrently to test the same sample panel, provides an extremely robust comparison. However, comparability studies are not always an option, especially for the introduction of a method to a new laboratory. Access to original validation data and suitable reference sample panels then becomes essential to provide evidence that the assay remains 'fit for the intended purpose'.