Session 8 - Screening

Question 1

What is the purpose of screening and what does a positive screen and a negative screen indicate?

Answer 1

Purpose of screening is to detect disease at an early stage to enable better outcomes and treatment i.e. catch before the symptoms are present

Positive screen - high risk of developing the disease (need to have further diagnostic tests)

Negative screen - low risk of developing the disease

Question 2

What are the problems with screening (i.e. why do we not use it for every disease)?

Answer 2

• It can turn some people who test positive into ‘patients’ which can be significant
• Can have false negatives (missing individuals who are at high risk)
• Expensive financially and in terms of resources

Question 3

Outline the 5 areas of the National Screening Programme (explaining what needs to be met for a screening programme to go ahead)

Answer 3

1. Condition
   - Must be important (severity or frequency)
   - All other interventions must be considered
   - If genetic screening, need to understand the meaning and implications of the results
2. Test
   - Simple, safe, precise and validated
   - Must have an agreed ‘cut off’ point
   - Agreed policy on further diagnostic tests of those who test positive
3. Intervention
   - Need to have evidence that intervention at an early stage can improve the outcome of the disease (compared to usual care)
   - Should be agreed which individuals should be offered interventions
4. Screening programme
   - Proven effectiveness in reducing mortality or morbidity
   - Evidence that it is clinically, socially and ethically acceptable
   - Benefits must outweigh any harms (e.g. over diagnosis, false positives etc.)
   - Cost should be balanced in relation to spent on medical care as a whole
5. Implementation
   - Needs a management and monitoring programme to ensure quality assurance
   - All other interventions must be considered
   - Adequate staffing and facilities for the programme
   - Decisions should be scientifically justifiable to the public

Question 4

Explain the 3 reasons why it is hard to evaluate the successfulness of a screening programme

Answer 4

1. Lead time bias
   - Early diagnosis falsely appears to prolong survival (i.e. patients live for the same amount of time but just know they have the disease for longer)
2. Length time bias
   - Screening programmes are better at picking up slower growing and unthreatening cases, compared to fast aggressive cases
   - Diseases that are detectable through screening may never have the individual a problem (i.e. curing people that don’t need curing!)
3. Selection bias
   - Studies can be skewed by the ‘healthy volunteer’ where those that have regular testing are more likely to do other things that protect their health (could use randomised control trial to help with this)

Question 5

Explain the 2 types of error that exist with screening and why they are harmful

Answer 5

False positive - when someone incorrectly tests as positive
- Harmful because of the anxiety that comes with thinking that you will develop a disease

False negative - when someone incorrectly tests as positive
- Harmful because someone may not to present to a GP with symptoms due to thinking that they have tested negative for the disease

Question 6

Outline sensitivity and specificity and how it’s calculated

Answer 6

Sensitivity or detection rate: proportion of people with the disease who test positive
- Ability to identify true positives (high sensitivity is good)
Calculated by a / (a+c)

Specificity: proportion of people without the disease who test negative
- Ability to identify true negatives (high specificity is good)
Calculated by d / (b+d)

When a test is applied to different populations, it will have the same sensitivity and specificity

Question 7

Outline positive predictive value and negative predictive value

Answer 7

Positive predictive value (PPV) - probability that someone who has tested positive actually has the disease
Calculated by a / (a+b)

Negative predictive value (NPV) - probability that someone who has tested negatively doesn’t actually have the disease
Calculated by d / (c+d)

*Both PPV and NPV vary based on the population that you’re testing and the prevalence of the disease within the population e.g. high prevalence means a higher PPV

Question 8

What is the definition of screening?

Answer 8

Screening is the process of identifying healthy people who may have an increased chance of a disease or a condition

Question 9

Explain uptake and coverage in terms of screening

Answer 9

Uptake: proportion of those who are invited, that take up the invitation

Coverage: proportion of the population that have been screened in a given time period (more about the organisation)

Question 10

Outline some of the factors that affect screening uptake

Answer 10

• Type of screening i.e. invasive vs non-invasive
• Perceived threat of the disease and associated morbidity
• Awareness of the health benefits
• Convenience
• Level of reminders/organisation