Screening

Question 1

Define overdiagnosis

Answer 1

Correct diagnosis of a disease, but the diagnosis is irrelevant because the disease will never cause symptoms within the patient’s lifetime

Question 2

Define overtreatment

Answer 2

Unnecessary treatment which does not improve health

Question 3

What is the popularity paradox?

Answer 3

A person treated as the result of a positive screening result believes that they are in the group who have truly benefited from the screening programme, making the screening more popular (despite overdiagnosis meaning that many don’t benefit/ are harmed by this)

Question 4

What is the Wilson + Jungner Criteria?

Answer 4

Set of defined principles and practices for screening for disease

Question 5

What principles are outlined by the Wilson + Jungner Criteria?

Answer 5

1. Condition must be important health problem
2. Treatment available
3. Facilities available
4. Disease has latent stage
5. Test available
6. Acceptable test
7. Natural history of disease understood
8. Agreed policy on whom to treat
9. Economical balance
10. Case-finding should be a continuous process, not just a “once and for all” project.

Question 6

Who makes decisions regarding screening decisions in the UK?

Answer 6

National Screening Committee (Public Health England)

Question 7

How are test results classified?

Answer 7

True positive
True negative
False positive
False negative

Question 8

What metrics are used to define how good a test is?

Answer 8

Sensitivity
Specificity

[Nb. these cannot tell you the probability that you have a disease if the test is positive, as that depends on the prevalence of the disease]

Question 9

What is sensitivity in regards to test results?

Answer 9

Proportion of actual positives, which are correctly identified as such (e.g. people who have the disease that are correctly identified by the screening programme)

Question 10

What is specificity in regards to test results?

Answer 10

Proportion of negatives, who are correctly identified as such (e.g. those who do not have specific disease are correctly identified as having no disease by screening programme)

Question 11

Which metric needs to be higher in screening?

Answer 11

Specificity

Question 12

What is the positive predictive value (PPV)?

Answer 12

Proportion of subjects with positive test results who are correctly diagnosed

Question 13

What is the negative predictive value (NPV)?

Answer 13

Proportion of subjects with a negative test result who are correctly diagnosed

Question 14

What are the common types of bias specific to screening research?

Answer 14

Healthy screenee (idea that people who attend screening programmes are more healthy and generally proactive about their health anyway) 
Lead time (idea that survival time looks longer due to earlier diagnosis from screening programme but no extension of life) 
Length time (screening can appear to increase survival time by simply detecting disease that develops more slowly but not actually resulting in longer life)

Question 15

What is a true positive?

Answer 15

Test correctly identifies disease

[Nb. can include overdiagnosis so not all of these people necessarily benefit]

Question 16

What is a true negative?

Answer 16

Test correctly identifies individual of being free from disease being screened for

Question 17

What is a false positive?

Answer 17

Test incorrectly identifies healthy person as having disease

Question 18

What is a false negative?

Answer 18

Test incorrectly identifies person with disease as being free from disease being screened for

Question 19

How is sensitivity calculated?

Answer 19

True positives / (true positives + false negatives)

Question 20

How is specificity calculated?

Answer 20

True negatives / (true negatives + false positives)

Question 21

How is the positive predictive value calculated?

Answer 21

True positives / (true positives + false positives)

Question 22

How is the negative predictive value calculated?

Answer 22

True negative / (true negatives + false negatives)

Question 23

What is the Healthy Screenee bias?

Answer 23

People who attend screening programmes are likely to live longer because they are generally healthier and are more proactive about their health, as opposed to those who do not attend who are more likely to exhibit risky health behaviours (e.g. smoking, poor diet etc.)

Question 24

What is the Lead Time bias?

Answer 24

Useless screening can appear to increase survival time by simply detecting the disease earlier but not actually resulting in a longer life (difficult to test!)

Question 25

What is the Length Time bias?

Answer 25

Screening is better at detecting disease that develops more slowly but slow developing disease generally means you live longer - therefore screen detected disease is likely to have a better prognosis even if it results in no difference in treatment

Question 26

What did the neuroblastoma screening programme teach us about the efficacy of screening programmes?

Answer 26

Seemingly increased survival rate for children with neuroblastoma however no change in death rate - was a result of overdiagnosis in which screening programme was identifying subclinical cases that normally resolve themselves with no symptoms