The tests need to be field validated and their limitations established so that appropriate screening strategies can be initiated and test results can be rationally interpreted. There is currently a lack of field-validated tests for most of the chemical and infectious agents of concern. Diagnostic tests can be characterized as definitive or presumptive. Definitive tests involve unequivocal identification of an agent or compound. For example, isolation of the organism by bacteriologic culture, followed by the use of confirmatory tests such as serotyping, gives positive identification of an agent.
Presumptive tests measure the animal's response to the presumed presence of an agent.
A diagnostic test should be as accurate as possible. It should also be kept in mind that, even with a perfect test, it may be of limited practical use if a more effective CCP exists at other stages of the food production continuum. In addition, a perfect test is of limited practical use if there is an inability for that information to be put to constructive use for controlling the pathogen on the farm. In short, preharvest food safety diagnostic tests need to be more than accurate, they must provide useful information that can be acted upon. There are four major points to consider when assessing the accuracy of a test.