Principles of Diagnostic Assay Validation for Plant Pathogens: A Basic Review of Concepts

Diagnostic test results are used for many purposes and are heavily relied upon to prevent movement of pathogens from one country or region to another; to clear plants as free from disease for commerce; and to surveil against natural, accidental, or unintentional pathogen introductions and new and reemerging diseases. Diagnostic test results are also used by plant producers to make critical management decisions. The level of confidence in each assay is defined by a set of metrics that describe the performance of the assay under defined conditions. Collectively, these metrics are called "validation" of the assay. There are hundreds of diagnostic plant disease assays used every day in the United States that either are not validated or are validated in an ad hoc way. An endemic pathogen in one region can be an exotic pathogen in another. Therefore, this multitude of diagnostic assays for which performance criteria are not standardized impairs communication about confidence in the test outcome. We propose to create a framework for standardizing assay validation language and definitions within the United States across existing plant diagnostic networks. The long-term goal is to have operable standards, understanding the "trueness" of assay results, and sustained communications between diagnostic laboratories that use and those that develop plant disease diagnostic assays.