L14 & 15: Screening

Question 1

What is screening?

Answer 1

Presumptive identification of unrecognised disease or defect using rapid tests, examination or other procedures to identify ‘well’ people who have the disease from those that do not

Question 2

What is meant by an apparently well person?

Answer 2

Person doesn’t have any symptoms or concerns about being unwell but they actually are ‘unwell’ or at risk of becoming unwell

Question 3

What is the difference between clinical care and screening?

Answer 3

Clinical care→ person presents with symptoms or concerns
Screening→ person is asymptomatic
Greater responsibility for care required in screening than in clinical care

Question 4

What is a positive predictive value?

Answer 4

Person who receives a high risk screening test result will want to know ‘what proportion of people who are high risk subsequently confirmed as having the disease or defect?

Question 5

What is the negative predictive value?

Answer 5

Person who receives low risk screening test result will want to know ‘what proportion of people with low risk are subsequently confirmed as not having the disease or defect?

Question 6

How is a screening programme made?

Answer 6

Programme needs to satisfy 5 criteria

1. The condition
2. The test
3. The intervention
4. The screening programme
5. Implementation

Question 7

How is the condition chosen?

Answer 7

• Need to be important health problem
• Have an understanding of epidemiology, incidence, prevalence and natural history
• Cost effective primary prevention interventions need to be implemented
• If condition due to mutation, history of people with status should be understood including the psychological implications

Question 8

How is the test chosen?

Answer 8

• Needs to be a simple, safe, precise and validated - screening test
• Distribution of test results needs to be known and agreed cut off level must be defined
• Needs to be acceptable to target population
• For those who test positive, need to have a policy on further diagnostic investigations and choices for them
• If test is for mutation or set of genetic variants the method for selection need to be reviewed regularly

Question 9

What is meant by the intervention?

Answer 9

Effective intervention for identified patients
Evidence that intervention at pre-symptomatic phase leads to better outcomes for the screened individual compared to normal
Evidence based policies on who should receive intervention and what the appropriate intervention i

Question 10

What is meant by the screening programme?

Answer 10

Needs to be proven to be effective in reducing mortality and morbidity
Evidence it is clinically, socially and ethically acceptable to health professional and public
Benefits should outweigh any harms (over diagnosis, false positives, false reassurance, uncertain findings, complications etc…)
Opportunity cost of the screening programme should be economically balanced in relation to expenditure on medical care as a whole

Question 11

What is meant by implementation?

Answer 11

All other options for managing condition should have been implemented
Programme needs to be quality assured
Adequate staff and facilities available
Evidence based information available to potential participants
Public pressure should be anticipated- decisions scientifically justifiable to the public

Question 12

What types of error do all types of screening test unavoidably make?

Answer 12

False positives→ refer well people for further investigations
False negatives→ fail to refer people who actually have the early form of the disease

Question 13

How is the effectiveness of screening determined?

Answer 13

Need to look at the:

• Sensitivity
• Specificity
• Positive predictive value
• Negative predictive value

Question 14

Define sensitivity?

Answer 14

Proportion of cases the test correctly detects

Question 15

Define specificity?

Answer 15

Proportion of non cases which the test correctly detects

Question 16

Define PPV?

Answer 16

Positive predictive value

Proportion of positive tests that are cases

Question 17

Define NPV?

Answer 17

Negative predictive value

Proportion on negative tests who are non cases

Question 18

How do you work out the sensitivity of the test?

Answer 18

True positive / (True positive + False negatives)

Question 19

How do you work out specificity of the test?

Answer 19

True negatives / (False positive + True negatives)

Question 20

What is a false negative?

Answer 20

Test result negative

Disease actually present

Question 21

What is a false postive?

Answer 21

Test result positive

Disease absent

Question 22

How is the positive predictive value calculated?

Answer 22

True positive / (True positive + False positive)

Question 23

How is the negative predictive value calculated?

Answer 23

True negative / (True negative + False negative)

Question 24

What is the effects of screening high and low prevalence populations?

Answer 24

↑prevalence of disease - PPV↑ (NPV↓) - the more common a disease the more likely a ‘high risk’ result will be confirmed as correct
↓prevalence of disease - NPV↑ (PPV↓)- the less common a disease the more likely a ‘low risk’ result will be confirmed as correct

Question 25

How are screening programmes evaluated? What are the problems with evaluation?

Answer 25

Need to be based on good quality evidence
Need evidence that early detection is better
Can be bias→ lead time bias, length time bias, selection bias

Question 26

What is lead time bias?

Answer 26

Early diagnosis falsely appears to prolong survival
→ diagnosed earlier so appear to live longer
Actually live same length of time but longer knowing they have the disease

Question 27

What is length time bias?

Answer 27

Better at picking up slow growing, unthreatening cases than aggressive, fast growing ones
Detected disease are more likely to have a favourable prognosis→ may never have caused a problem
False conclusion that screening is beneficial to lengthening lives

Question 28

What is selection bias?

Answer 28

Studies of screening skewed by the ‘healthy volunteer’ effect
Those who have regular screening are likely to also do other things that protect them from screening
(Randomised controlled trials help prevent this)

Question 29

What is informed choice in screening?

Answer 29

Making sure patients make the choice themselves of whether to enter a screening programme or not
Important for individual patients even in the case of evidence based and endorse national screening programmes
Patients need to understand the benefits, harms and risks of screening/not screening

Question 30

Define uptake?

Answer 30

Proportion of those invited who take up the invitation to participation

Question 31

Define coverage?

Answer 31

Proportion of the eligible population who have been screened within a given time period

Question 32

What factors contribute to inequalities in screening?

Answer 32

Affluence, deprivation, ethnic diversity

Question 33

What are some of the factors that affect uptake to screening programs?

Answer 33

• Feel healthy so don’t see the need
• Fear of outcome
• Lack of time to get test done
• Acceptability of the test- invasive vs non invasive
• Convenience
• Forget to have it done
• Accessibility- disability, language barrier, ethnic minority

Question 34

What developments are taking place in screening?

Answer 34

More targeted screening → focus on those at higher risk
Possible new population screening programme
New biomarkers
Genomic revolution: Polygenic risk scores
Artificial intelligence