Screening and surveillance-principles and practice.

Health screening can only be applied to populations, not individuals. For it to be effective, the initial screening test must be acceptable and reasonably accurate, the disease must be treatable with better outcomes when treated early and the harm and cost associated with screening must not outweigh its benefits. Robust evidence is therefore required before systematic screening is implemented. Surveillance implies the testing of people at high risk of disease and is therefore distinct from screening in both scale (smaller) and intensity (greater). In both cases, however, clear information must be provided to potential participants so that they can weigh up the balance of benefit and harm before deciding on whether or not to engage in the process.