6: Screening

Question 1

What is screening?

Answer 1

• A systematic attempt to detect an unrecognised condition
• Through application of tests, examination etc
• Can be applied rapidly (and cheaply)
• Distinguish between apparently well persons who probably have the disease, and those who probably do not

Question 2

What are the three ways in which disease is detected?

Answer 2

1. Spontaneous presentation - patient to doctor with symptoms
2. Opportunistic case finding - presents with symptoms, doctor takes opportunity to look for other diseases
3. Screening - label as screen positive, then diagnosis

Question 3

What is diagnosis?

Answer 3

Definitive identification of a suspected disease.
Tests, exams, other procedures
Treatment will follow

Question 4

What are the 4 areas of criteria for implementing a screening programme?

Answer 4

Disease/condition
Test
Treatment
Programme

Question 5

Describe screening criteria concerning the disease/condition

Answer 5

• Important health problem
• Need to understand epidemiology and progression
• Need an early detectable stage
• Must have considered primary prevention measures

Question 6

Describe screening criteria concerning the test

Answer 6

• Simple and safe
• Precise and valid
• Acceptable to the population
• Distribution of test values in the population must be known
• Define agreed cut off for test positives
• Agree who to test further

Question 7

Describe screening criteria concerning the treatment

Answer 7

• Must have effective evidence based treatment
• Early treatment must be advantageous - don’t just bring forward diagnosis date
• Agreed policy who to treat
• Optimise clinical management before participation in programme

Question 8

Describe screening criteria concerning the programme

Answer 8

• Proven effectiveness - RCT data
• Quality assurance for whole programme
• Other options besides screening considered
• Be able to justify parameters to the public
• Benefits to outweigh physical and psych harm - offer counselling

Question 9

What is the calculation for sensitivity?

Answer 9

True positive/true positive + false negative

Question 10

What is the calculation for specificity?

Answer 10

True negative/ false positive + true negative

Question 11

What is the calculation for PPV?

Answer 11

True positive/true positive + false positive

Question 12

What is the calculation for NPV?

Answer 12

True negative/ false negative + true negative

Question 13

What is sensitivity?

Answer 13

The detection rate - the proportion of people with the disease who test positive

Question 14

What is specificity?

Answer 14

The proportion of people who do not have the disease who test negative

Question 15

Do sensitivity and specificity ever vary?

Answer 15

No, they are functions of the test and should be the same in different populations

Question 16

What is the positive predictive value?

Answer 16

The probability that someone who tested positive has the disease

Question 17

How is PPV influenced by prevalence?

Answer 17

If it is a high prevalence condition, the PPV will be high, so only screen in high prevalence populations

Question 18

What is the negative predictive value?

Answer 18

The proportion of people who test negative who do not have the disease

Question 19

What is prevalence? In terms of positives/negatives

Answer 19

True positives+ false negatives/whole population

Question 20

What are some advantages of screening?

Answer 20

Early detection may improve outcome

True negatives reassure patients

Question 21

What are some disadvantages of false positives?

Answer 21

• Invasive diagnostic tests
• Anxiety
• May lead to lower uptake of future screening
• Increased risk of interval cancer
• Low PPV - many false positives

Question 22

What are some disadvantages of false negatives?

Answer 22

• False reassurance
• Not offered diagnostic tests from which they may benefit
• Disease, although present, is not diagnosed
• May present late - poor outcomes

Question 23

Name three difficulties in evaluating effectiveness of screening programmes

Answer 23

• Lead time bias
• Length time bias
• Selection bias

Question 24

What is lead time bias?

Answer 24

Screen patients appear to survive longer, only because they were diagnosed earlier.
Patients live the same amount of time, but longer knowing they have the disease

Question 25

What is length time bias?

Answer 25

Screening is better at picking up slow growing, unthreatening cases than aggressive ones Diseases detected by screening are more likely to have a good prognosis, may never have caused a problem.

Question 26

What is selection bias in relation to screening?

Answer 26

Skewed by the healthy volunteer effect. Those regularly screening are more likely to engage in health behaviours protecting them from disease. Similar to healthy worker bias - RCT would help deal with bias.

Question 27

Describe some critical perspectives on screening programmes

Answer 27

Alteration of the usual doctor-patient contract - usually sick patient presents to the doctor Must be based on good quality evidence Some are very complex - Limitations include potential harm as well as benefit, can't guarantee protection, increased emphasis on promoting informed choice.

Question 28

What are 4 sociological critiques of screening

Answer 28

Structuralist Surveillance Social constructionist Feminist

Question 29

Describe structural critiques of screening?

Answer 29

Victim blaming - are all equally able to take responsibility for their own health Individualising pathology - not addressing the underlying material cause

Question 30

Describe surveillance critiques of screening

Answer 30

Wider apparatus of social control

Question 31

Describe social constructionist critiques of screening

Answer 31

Health and illness practices can be seen as moral - screening seen as responsible. Obligation to go - not going is deviant and irresponsible

Question 32

Describe feminist critiques of screening

Answer 32

Is it more targeted at women than men?

Question 33

Give some examples of screening programmes in the UK

Answer 33

- AAA --> all men USS >64 years old - Bowel cancer --> faecal occult blood --> colonoscopy (2 yearly, 60-69) - Breast cancer --> mammogram --> FNA (60-69, 2 yearly) - Cervical cancer - Down's - PKU

Question 34

Describe the uptake of screening in lower socioeconomic groups

Answer 34

- Less likely to attend screening - May have difficulty mobilising resources required to attend - May be more likely to have a negative definition of health --> manage as a series of crises, don't see the need for preventative services.