Screening
Content
Knowledge of risks and benefits of screening programmes and information available to patients to aid decision making
Principles of screening (Wilson’s criteria) and applying the concepts of primary, secondary and tertiary prevention;
Application to current and intended programmes of screening and recall systems
Wilson–Jungner criteria for appraising the validity of a screening programme
1. The condition being screened for should be an important health problem
2. The natural history of the condition should be well understood
3. There should be a detectable early stage
4. Treatment at an early stage should be of more benefit than at a later stage
5. A suitable test should be devised for the early stage
6. The test should be acceptable
7. Intervals for repeating the test should be determined
8. Adequate health service provision should be made for the extra clinical workload resulting from screening
9. The risks, both physical and psychological, should be less than the benefits
10. The costs should be balanced against the benefits
In addition the WHO criteria (2003) specify:-
“All other options for managing the condition should have been considered (for example, improving treatment and providing other services), to ensure that no more cost-effective intervention could be introduced or current interventions increased within the resources available”.
Screening Test
(SNnOUT) Sensitivity means that if the test is negative then the condition is unlikely.
(SPpIN) Specificity means that if the test is positive the the condition is likely
The ideal test is both sensitive and specific
Lead Time Bias
Lead time is the length of time between the detection of a disease and its usual clinical presentation and diagnosis.
Lead time bias results in an apparent increase in survival due to a health condition, such as cancer being detected at an early stage, when there is no actual effect of early detection on survival. The patient just lives for a longer period with the diagnosis.
When evaluating the effectiveness of the early detection and treatment of a condition, the lead time must be subtracted from the overall survival time of screened patients to avoid lead time bias. Historically this has been a flaw in the evaluation of cancer screening programmes, and in particular breast cancer screening programmes, resulting in inflated claims of effectiveness.