Public Health

Question 1

What is screening?

Answer 1

A process which sorts out apparently well people who probably have a disease from those who probably do not

Question 2

What is the sensitivity of a screening process?

Answer 2

The proportion of people with the disease that are correctly identified by the screening test
(no. of people who test positive and have the disease/no. of people that have the disease)

Question 3

What is the specificity of a screening process?

Answer 3

The proportion of people without the disease who are correctly excluded by the screening test
(no. of people without the disease that test negative/no. of people without the disease)

Question 4

What is the PPV of a screening process?

Answer 4

Positive predictive value - the proportion of people with a positive test result that actually have the disease
(no. of people that have the disease and test positive/no. of people that tested positive)

Question 5

What is the NPV of a screening process?

Answer 5

Negative predictive value - the proportion of people with a negative test result that don’t have the disease
(no. of people that don’t have the disease and test negative/no. of people that tested negative)

Question 6

Why are the PPVs of some diseases so different?

Answer 6

Predictive values are dependant on the underlying prevalence of a disease (sensitivity and specificity are not)

Question 7

What are most screening criteria based on?

Answer 7

The Wilson and Jungner criteria

Question 8

What are the 1-3 criteria for a screening process? (Condition)

Answer 8

1. The condition sought 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

Question 9

What are the 4-6 criteria for a screening process? (Treatment)

Answer 9

4. There should be an accepted treatment for patients with recognised disease
5. Facilities for diagnosis and treatment should be available
6. Adequate health service provision should be made for the extra clinical workload resulting from screening

Question 10

What are the 7- 8 criteria for a screening process (Testing)

Answer 10

7. There should be an suitable and acceptable test devised for the early stage
8. Intervals for repeating the test should be determined (not a one off)

Question 11

What are the 9-10 criteria for a screening process? (Risks and benefits)

Answer 11

9. There should be an agreed policy on whom to treat
10. The costs should be balanced against the benefits
    Additionally: The risks both physical and psychological, should be less than the benefits

Question 12

How are screening processes evaluated, and what are the main biases?

Answer 12

Ideally evaluated using RCT
- selection bias
- lead-time bias
- length-time bias