S5) Population-based Screening

Question 1

How can we detect diseases?

Answer 1

• Spontaneous presentation
• Opportunistic case finding
• Screening

Question 2

Define diagnosis

Answer 2

Diagnosis is the definitive identification of a suspected disease or defect by application of tests, examinations or other procedures to definitely label people as either having a disease or not having a disease

Question 3

What is screening?

Answer 3

Screening is a systematic attempt to detect an unrecognised condition by the application of tests, examinations, or other procedures to identify healthy people who may be at increased risk of disease or condition

Question 4

Outline the screening process

Answer 4

Question 5

What is the purpose of screening?

Answer 5

To give a better outcome compared with finding something in the usual way (having symptoms and self reporting to health services)

Question 6

One of the criteria for implementing a screening programme is Condition.

Explain this

Answer 6

Condition: an important health problem (frequency/severity) with epidemiology, incidence, prevalence and natural history understood

Question 7

One of the criteria for implementing a screening programme is Test.

A screening test is going to make two types of error: false positive and false negative. What are these?

Answer 7

• False positive – refers well people for further investigation, putting them through stress, anxiety and inconvenience

- False negative – fails to refer people who have an early form of the disease, inappropriately reasurring them

Question 8

What are the four features of test validity?

Answer 8

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

Question 9

What is sensitivity?

Answer 9

Sensitivity is the proportion of people with the disease who test positive (aka detection rate)

Question 10

What is specificity?

Answer 10

Specificity is the proportion of the people without the disease who test negative

Question 11

What is positive predictive value?

Answer 11

Positive predictive value is the probability that someone who has tested positive actually has the disease

Question 12

What influences PPV?

Answer 12

Positive Predictive Value is strongly influenced by the prevalence of the disease

Question 13

What is negative predictive value?

Answer 13

Negative predictive value is the proportion of the people who test negative and actually do not have the disease

Question 14

What are the implications of false positive results?

Answer 14

• Offered diagnostic testing with all its anxieties and risks for an absent condition
• May lead to lower uptake of screening in future and greater risk of interval cancer

Question 15

What are the implications of false negative results?

Answer 15

• Not offered diagnostic testing when in fact they may have benefited from it
• False reassurance – may present late with symptoms as a consequence

Question 16

One of the criteria for implementing a screening programme is Intervention.

Explain this

Answer 16

• Effective intervention for patients identified through screening

- Intervention at a presymptomatic phase leads to better outcomes for the screened individual

Question 17

One of the criteria for implementing a screening programme is Screening programme.

Explain this

Answer 17

• Screening programmes have proven effectiveness in reducing mortality or morbidity
• Benefit gained should outweigh any harms e.g. overdiagnosis, overtreatment, false positives, false reassurance, uncertain findings and complications

Question 18

One of the criteria for implementing a screening programme is Implementation.

Explain this

Answer 18

• Clinical management and patient outcomes should be optimised
• Evidence-based information available to potential participants (informed choice)
• Public pressure should be anticipated

Question 19

Identify 2 advantages of screening for disease

Answer 19

• Detect many diseases at birth, with no risk to the child
• Avoid serious and permanent consequences for the child (presymptomatic treatment)

Question 20

Identify 4 disadvantages of screening programmes

Answer 20

• Specimens may need to be taken a second time
• False positives
• False negatives
• Complications could still arise

Question 21

One evaluation difficulty is lead time bias.

What is this?

Answer 21

• Lead time bias is when early diagnosis falsely appears to prolong survival
• Screened patients appear to survive longer, but only because they were diagnosed earlier

Question 22

One evaluation difficulty is length time bias.

What is this?

Answer 22

Length time bias involves the easy detection of slow growing, unthreatening cases rather than aggressive, fast-growing ones

Question 23

Another evaluation difficulty is selection bias.

What is this?

Answer 23

• Studies of screening often skewed by ‘healthy volunteer’ effect
• Those who have regular screening likely to also do other things that protect them from disease

Question 24

Give examples of 4 screening programmes in the UK

Answer 24

• NHS breast screening (BSP) programme
• NHS cervical screening (CSP) programme
• NHS abdominal aortic aneurysm (AAA) programme
• NHS bowel cancer screening (BCSP) programme