Population Based Screening

Question 1

What is screening?

Answer 1

A systematic attempt to detect an unrecognised condition using tests, examination or other provedures

Distinguishing between people who probably have a disease (or precursor) and those who probably do not

Question 2

What are the 5 criteria used by the national screening committee to decide if a screening programme should be undertaken?

Answer 2

1. Condition
2. Test
3. Intervention
4. Screening Programme
5. Implementation

Question 3

For a screening programme to develop, what must we know about the condition?

Answer 3

Important health problem (variable frequency/severity)

Epidemiology

Natural history and disease progression

Question 4

For a screening programme to develop, what factors must be considered regaring the test to be performed?

Answer 4

Complexity

Safety

Precise & Validated

Agreed cut off level

Agreed policy for further investigation

Question 5

What is a key consideration when considering the proposed intervention following a screening programme?

Answer 5

Evidence that invervention leads to better outcomes at a pre-symptomatic phase

Clear policies as to who will be offered interventions and those who will not

Question 6

What genereal principles of the screening programme must be clear to the national committee when deciding to develop a new screening programme?

Answer 6

Proven effectiveness in reducing morbidity or mortality (RCT data)

Benefit gained by individual outweighs any harms (risk of false positives)

Opportunuty cost should be balanced against providing healthcare elsewhere

Question 7

Following implementation of screening programme, what must occur to ensure it is fit for purpose?

Answer 7

Quality Assurance

Information available to patinets (Informed Choice)

Adeuate staffing and facilities

Question 8

What are the 4 features of test validity?

Answer 8

Sensitivity

Specificity

Positive Predictive Value

Negative Predictive Value

Question 9

Define sensitivity

Answer 9

The proportion of people with the disease who test positive

(a.k.a. detection rate)

Question 10

What is the formula used to calculate sensitivity?

Answer 10

True Positives

_____________________________

True Positives + False Negatives

Question 11

Define specificity

Answer 11

Proportion of the people without the disease who are test negative

Question 12

What is the formula used to calculate specificity?

Answer 12

True Negatives

_____________________________

True Negatives + False Positives

Question 13

Define positive predictive value

Answer 13

Probability that someone who has tested positive, actually has the disease

Question 14

What strongly influenes the positive predictive value?

Answer 14

Prevalece of the disease

Question 15

What is the formula used to calculate the positive predictive value?

Answer 15

True Positives

_____________________________

True Positives + False Positives

Question 16

Define negative preductive value

Answer 16

Proportion of the people who are tested negative who actually do not have the disease

Question 17

What is the formula used to calculate the negative predictive value?

Answer 17

True Negatives

_____________________________

True Negatives + False Negatives

Question 18

What are the implications of a false positive result?

Answer 18

Patients offered invasive testing which they do not need

People becomes patients when they are not ill

Unnecessary anxiety

Risk of lower uptake of screening in future

Question 19

What are the implications of a false negative result?

Answer 19

Affected patients not offered diagnostic testing

False reassurance - may persent late with symptoms

Question 20

What is lead time bias?

Answer 20

Screened patients appear to survive longer

Just diagnosed earlier!

Question 21

What is length time bias?

Answer 21

False conclusion that screening is beneficial but actually:

Screening detects slow growing, less aggressive cases

Diseases detectable through screening may have more favourable outcome (may never have caused a problem)

Question 22

What is selection bias in the context of screening programmes?

Answer 22

‘Healthy Volunteer’ Effect

Those engaging in screening programmes, often also engage in other health promotion activities e.g. diet/exercise

Question 23

What are some negative views of screening programmes held by the public?

Answer 23

Morally obliged to take part - removes automony/choice

Social control - constant surveillance