Public Health Screening

Question 1

What are the criteria for a screening programme?

Answer 1

1. Condition:
   - Important with known epidemiology with link between risk marker/latent stage and serious illness
   - Cost effective primary prevention interventions have been implemented
   - If carriers of mutation identified, natural history of people with this status are understood, including psychological implications
2. The test:
   - Simple,safe, precise, and validated
   - Distribution of test values in target population should be known and cut-off defined
   - Acceptable to target population (from sampling to result delivery)
3. Treatment:
   - Should be effective intervention with evidence that intervention at pre-symptomatic phase leads to better outcomes
   - Agreed evidence based policies covering which individuals should be offered interventions and the appropriate intervention offered
4. The programme:
   - Evidence from high quality RCTs that screening is effective in reducing morbidity and mortality
   - Complete screening programme is clinically, socially and ethically acceptable to all
   - Benefit gained by individuals should outweight any harms eg. over-diagnosis, false +ves, false reassurance, complications
5. Implementation
   - Mx of conditions fully optimised before screening is implemented
   - Adequate staffing and facilities for testing, diagnosis, Tx and programme Mx.

Question 2

Define sensitivity

Answer 2

Proportion with disease that are detected

= True +ve/disease total

Question 3

Define specificity

Answer 3

Proportion without the disease which are detected

= True -ves/non-disease total

Question 4

Define PPV?

Answer 4

Proportion of positive results that have disease

= True positives/Total positive results

Question 5

Define NPV?

Answer 5

Proportion of negative results that are free of disease

= True negatives / total negative results

Question 6

Define length bias

Answer 6

Screening is likely to detect individuals with longer pre-clinical phase than those whose disease is progressing more quickly. individuals with longer preclinical phase more likely to have better prognosis leading to over-estimation of beneficial effect of screening

Question 7

Define lead time bias

Answer 7

Survival will be longer among people with disease detected by screening during pre-clinical phase compared to people with diagnosis detected when they develop symptoms–> Overestimated beneficial effect of screening

Question 8

Define overdiagnosis?

Answer 8

Detection of disease through screening that would otherwise have not been diagnosed within the patients lifetime

Question 9

Three things that need to be considered when evaluating a screening programme?

Answer 9

Lead time bias, length bias, Overdiagnosis