Lecture 35 - Screening

Question 1

What is screening

Answer 1

Screening is the widespread use of simple test to detect disease in an apparently healthy (asymptomatic) population

Question 2

Screening programme

Answer 2

It is an organised system that uses a screening test among asymptomatic individuals to identify early cases of disease and improve outcomes

Question 3

Diagnostic test vs screening test

Answer 3

A screening test empathises cost and safety, detecting possible disease cases for further testing, while a diagnostic test focuses on definitive diagnosis. A screening test is not a diagnostic text or a screening programme

Question 4

Why is early detection through screening beneficial

Answer 4

Early detect aims to limit the consequences of disease through easy diagnosis and treatment, often improving prognosis

Question 5

Key concepts of screening

Answer 5

• Aims to improve outcome, reduces mortality
• Screening programmes vs case finding (opportunistic screening)
• All screening programmes do harm; some can do good as well
• Screening is a pathway not a test

Question 6

Screening programme pathway

Answer 6

The pathway includes health promotion, invitation, screening procedure, diagnosis, treatment or recall

Question 7

Examples of NZ screening programme

Answer 7

• Antenatal screening for Down syndrome
• Breast screen Aotearoa
• National bowel screening programme
• National Cervical screening programme
• Newborn Metabolic screening programme
• Universal newborn hearing screening programme

Question 8

When should we screen?

Answer 8

• Is the disease appropriate
• Is the test appropriate
• Would a programme be effective
• Consider benefits vs harms of screening

Question 9

What factors determine whether a disease is appropriate for screening

Answer 9

1. The seriousness of the disease
2. Ability to alter course of the disease
3. Prevalence of pre-clinical disease
4. Lead time

Question 10

Seriousness of disease

Answer 10

Screening is resource-intensive
So makes sense to screen for disease with potential severe consequences

Question 11

Prevalence if pre-clinical disease

Answer 11

More efficient when high prevalence of preclinical disease
- Positive Predictive value

Question 12

Lead time

Answer 12

Lead time is the extra time gained by detecting a disease early through screening before symptoms appear

Question 13

ability to alter course of disease

Answer 13

1. Disease is not detectable - first biological onset
2. Screening may be of benefit - ideal to screen
3. Usually diagnose - no benefit
   Screening has to improve the length and/or quality of someones life

Question 14

Intrinsic test properties - Is the test appropriate

Answer 14

Can measure accuracy with sensitivity and specificity

Question 15

Sensitivity

Answer 15

Sensitivity is the proportion of people with the disease who test positive. High sensitivity reduces false negative
True positive/ (true positive + false negative)

Question 16

Specificity

Answer 16

Proportion of people without the disease who test negative. High specificity reduces false positive.
True negative/ (false positive + true negative)

Question 17

Which is the best, sensitivity or specificity

Answer 17

Increasing sensitivity may reduce specificity and vice versa. The balance depends on the consequence of false negatives vs false positives

Question 18

How do you choose which one to maximise

Answer 18

• consider consequences of missing. cases versus false alarms
  Sensitivity:
• Detecting as many cases as possible important
• Costs or risks of next step not too high
  Specificity:
  Costs or risks of next step high

Question 19

Predictive values

Answer 19

Measure test performance in a particular population - What proportion of people who test positive/negative do/don’t have disease

Question 20

Positive predictive value (PPV)

Answer 20

Proportion of people who test positive and have the disease. Depends on the prevalence
True positive / (true positive + false positive)

Question 21

Negative Predictive Value

Answer 21

Proportion of people who test negative and don’t have the disease. Depends on the prevalence
True negative / (false negative + true negative)

Question 22

Screening programmes effectiveness - having the resources

Answer 22

Facilities and systems:
- Manage participation
- Cost and accessibility
- Quality control and monitoring
Treatment:
- Capacity to treat true positives
Cost effectiveness:
- Many people over long period
- Cost vs Benefit
Evaluation of screening programmes:
- Crucial to determine if screening programme actually leads to benefit

Question 23

Benefits of screening programmes

Answer 23

• Potential for early detection and interventions
• Reassurance (true negatives)
• Improved health of population

Question 24

Harms of screen programmes

Answer 24

• Increase in health inequities from unequal participation or treatment
• Physical - from complications and etc
• Psychological - from anxiety from waiting , distress from procedures and etc
• False positives - period of stress and uncertainty
• Financial - to individual
• Lead time bias
• Length

Question 25

Lead time Bias

Answer 25

Lead time bias refers to the appearance of increase survive time simply due to earlier detection. Over diagnosed and / or over-treatment - may increase morbidity without reducing mortality

Question 26

Length bias

Answer 26

Length bias occurs because screening is more likely to detect slower growing, less aggressive cases of disease, leading to a better prognosis than in faster growing cases, skewing results.