Demonstrating disease freedom-combining confidence levels

Part of the requirements for demonstrating disease freedom usually will be that sufficient testing be done to give a specified confidence of detecting the disease if it were present at a specified level. Often, this requirement is translated into a fixed testing regime that must be followed (an inflexible approach that might not be the most economic or practical solution).

A more flexible approach is to specify the capabilities of the various tests that can be used to detect the disease, and let the party hoping to demonstrate disease freedom decide upon the testing regime. The question then arises as to how to combine information that can come from a variety of sources over a period of time to give an overall level of confidence.

Two methods are given. The first, an exact method based on multiplying probabilities, would be more appropriate for a survey of an area in which no disease is thought to be present. The second method (more appropriate for a herd-assurance program within an infected area) is a point-based system that takes into account the different sensitivities of the methods used to detect disease and the change in prevalence over time. It allocates points for each test done proportional to the sensitivity of the test and the prevalence at the time of testing.