Screening

Question 1

What is screening

Answer 1

A process which sorts out apparently well people who probably have a disease (or precursors or susceptibility to a disease) from those who probably do not
- not yet symptomatic

Question 2

Secondary prevention

Answer 2

Detective early disease in order to alter the course of the disease
Eg screening by mammography for breast cancer in order to treat it early

Question 3

Primary prevention

Answer 3

Prevent a disease from occurring
Eg screening to identify people with risk factors and reduce risk factor levels

Question 4

Purposes of screening

Answer 4

Reduce the risk of developing disease
Provide treatment
Provide information eg pre-natal screening for genetic disorders

Question 5

Sensitivity

Answer 5

The proportion of people with the disease who are correctly identified by the screening test
A/a+c

Question 6

Specificity

Answer 6

The proportion of people without the disease who are correctly excluded by the screening test
D/b+d

Question 7

Positive predictive value PPV

Answer 7

The proportion of people with a positive test result who actually have the disease
A/a+b

Question 8

Negative predictive value NPV

Answer 8

The proportion of people with a negative test result who do not have the disease
D/c+d

Question 9

How many conditions does the NHS screen for

Answer 9

36 health condition

Question 10

Prevalence

Answer 10

A+c/a+c+b+d

Question 11

Criteria for screening

Answer 11

Wilson and Jungner criteris

Question 12

Criteria for screening- the condition

Answer 12

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 13

Criteria for screening- the treatment

Answer 13

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 14

Criteria for screening- the test

Answer 14

7. A suitable test should be devised for the early stage
8. Test should be acceptable
9. Intervals for repeating the test should be determined (not a one off)

Question 15

Criteria for screening- risks and benefits

Answer 15

10. There should be an agreed policy on whom to treat
11. Costs should be balanced against the benefits

• the risks (both physical and psychological) should be less than the benefits

Question 16

Evaluation of screening

Answer 16

Ideally by RCT (individual or cluster)
Could use other methods but potential for bias

Question 17

3 well-recognised biases

Answer 17

Selection bias
Lead-time bias
Length-time bias

Question 18

Selection bias

Answer 18

People who choose to participate in screening programmes may be different from those who don’t
- may be at a higher risk eg women with family history of breast cancer more likely to attend
- may be at a lower risk eg women in higher socioeconomic groups (lower risk of cervical cancer) more likely to attend

Question 19

Lead time bias

Answer 19

Disease starts
1. Patient A diagnosed after screening
2. Patient B diagnosed when symptoms develop
Patient A- lives longer

Overestimation of survival duration due to earlier detection by screening than clinical presentation.

Question 20

Length-time bias

Answer 20

Because more aggressive diseases are asymptomatic for a shorter period, screening is more likely to detect slower progressing diseases, such as slow-growing tumours, which have a better prognosis, including longer survival.

Question 21

Types of screening

Answer 21

Population-based screening program
Opportunistic screening
Screening for communicable diseases]
Pre-employment and occupational medical
Commercially provided screening

Question 22

Population based screening

Answer 22

Thailand
National diabetes and hypertension screening

Question 23

Opportunistic screening

Answer 23

Prevention and control of substance abuse

Question 24

Screening for communicable diseases

Answer 24

Heaf test

Question 25

Pre-employment and occupational medical

Answer 25

Vision test for commercial drivers

Question 26

Examples of screening on the NHS

Answer 26

Abdominal aortic aneurysm (AAA)
Bowel cancer screening
Breast screening
Cervical screening
Diabetic eye screening
Newborn screening eg hearing, physical examination, blood spot
Screening in pregnancy

Question 27

What affects PPV and NPV

Answer 27

Prevalence
Lower prevalence:
PPV - decreases
NPV - increases