Lecture 26: Screening; A Special Type Of Prevention Strategy

Question 1

What is an example for primary prevention screening?

Answer 1

Screening women for alcohol intake to prevent breast cancer

Question 2

What is an example of secondary prevention screening?

Answer 2

Breast cancer screening (detects early stage of disease and aims to prevent more serious disease

Question 3

What is the screening test like?

Answer 3

Less expensive, less invasive (so effective on large population)

Question 4

What’s the diagnostic test like?

Answer 4

It’s called a gold standard
Often invasive
Expensive
Impractical for large populations

Question 5

What happens after the gold standard diagnostic test?

Answer 5

Disease positive get the intervention or treatment

Disease negative get rescreened after a specific period

Question 6

What happens to test negative after first screen?

Answer 6

They get rescreened after a specific period

Question 7

What are the 4 screening criterion?

Answer 7

1. Suitable disease
2. Suitable test
3. Suitable treatment
4. Suitable screening time

Question 8

Two features of a suitable disease?

Answer 8

1. An important public health problem

2. Knowledge of the natural history of the disease (or relationship of risk factors to the condition)

Question 9

Is a suitable disease common or uncommon?

Answer 9

It can be either- if it’s effective and easy, uncommon screenings can occur (eg babies screened for pku)

Question 10

Why do we screen for uncommon disease too?

Answer 10

Because if we have an early detection and intervention we can get a better outcome

Question 11

Why is knowledge of the disease important for screening?

Answer 11

So we know it’s detectable early with a detectable risk factor/disease market and needs an increased duration of preclinical phase (after biological onset, before clinical diagnosis)

Question 12

Examples of screening?

Answer 12

Breast cancer, cervical cancer, diabetes

Question 13

What are the key aspects of a suitable test?

Answer 13

1. Reliable
2. Safe
3. Acceptable
4. Simple
5. Cheap
6. Accuracy

Question 14

What is accuracy?

Answer 14

The ability of a test to indicate which individuals have the disease and which don’t

Question 15

Two measures of accuracy?

Answer 15

Specificity

Sensitivity

Question 16

What two types of tests are used?

Answer 16

Screening test

Gold standard test

Question 17

What is the gold standard?

Answer 17

Is in effect the ideal diagnostic test (example: colonoscopy). Is expensive and invasive

Question 18

What is a general screening test?

Answer 18

(A less expensive diagnostic test) eg faecal occult blood test (for colon cancer)

Question 19

Describe the gate frame screening SQUARE

Answer 19

Top left: true positive
Bottom left: false negatives
Top right: false positives
Bottom right: true negatives

Question 20

What is sensitivity?

Answer 20

The ability to identify correctly those who have the disease (a) from all individuals who have the disease (a+c)

The likelihood of a positive test in those with the disease

Question 21

What is specificity?

Answer 21

The ability of the test to identify correctly those who do not have the disease (d) from all individuals who don’t have the disease (b+d)

Question 22

Sensitivity calculation?

Answer 22

True positives/all with disease x100

A/(a+c) x100

“X% of those with the disease will test positive in the new test”

Question 23

Specificity calculation?

Answer 23

True negatives/all without the disease
= b/(b+d) x 100

“X% of those without the disease will have a negative result in the new test”

Question 24

How do we evaluate test accuracy?

Answer 24

• the sensitivity is high if the proportion of true positives are high
• the specificity is high if the number of true negative are high

Question 25

What are the fixed characteristics of a new test?

Answer 25

Sensitivity and specificity

Question 26

What are the two predictive values?

Answer 26

Positive predictive value (PPV) Negative predictive value (NPV)

Question 27

Why are sensitivity and specificity different between populations?

Answer 27

Because the prevalence of disease is different in different populations

Question 28

So what is a predictive value?

Answer 28

How well a new screening test works in a specific population

Question 29

What does the positive predictive value measure?

Answer 29

Out of everyone who tests positive (false and true), how many truly have the disease? Or the probability of having disease if the test is positive

Question 30

Calculation for the positive predictive value?

Answer 30

True positives/all who test positive x100 Or A/(a+b) x100

Question 31

Why is a low predictive value bad?

Answer 31

Because of the emotional and monetary cost of getting a false positive

Question 32

What is negative predictive value?

Answer 32

The proportion of people who truly don't have the disease out of everyone who test negative Or the probability of not having the disease if you test negative

Question 33

Calculation of negative predictive value?

Answer 33

Number of people truly with out disease/everyone who tests negative x100 Or D/(c+d) x100

Question 34

Are PPV and NPV fixed test characteristics?

Answer 34

No

Question 35

What do positive predictive values and negative predictive values reflect?

Answer 35

Test accuracy and the prevalence of the disease

Question 36

Why is suitable treatment a necessary aspect of screening?

Answer 36

No point if no treatment to improve chances

Question 37

What is suitable treatment criteria?

Answer 37

- evidence of early treatment leading to better outcomes - effective, accessible and acceptable treatment - evidence-based policies covering who should be offered treatment and the appropriate treatment to be offered (don't create disparities)

Question 38

What is a suitable screening programme?

Answer 38

- when benefits must outweigh harm - when randomised control trials have provided evidence that the screening programme will result in: - reduced mortality - increased survival time

Question 39

What are two types of bias sometimes present in screening programme?

Answer 39

- lead bias | - length time bias

Question 40

What is lead time bias?

Answer 40

When there's an "apparent increase in life expectancy or lead time" by screening, but just because you identify it early, doesn't mean you're increasing length of survival (ie still could be a fixed death point)

Question 41

Do what aspect can give a screening programme a false impression of success?

Answer 41

If the screening programme is evaluated in terms of survival time

Question 42

What is length time bias?

Answer 42

Could be two firms of a disease (eg slow and rapid), and screening might say they have an increased/longer average survival rate if they're using data from people with slow disease (and miss those with rapid, due to death etc)

Question 43

Criteria of a suitable screening programme?

Answer 43

- benefits outweigh harm - RCT evidence that screening programme will result in increased survival/reduced mortality - adequate resourcing and agreed policy for test/diagnosis/treatment/programme management - cost effective - healthcare system must support all elements of screening pathway - needs to reach all those likely to benefit from it (specific initiatives for particular population groups)

Question 44

How is breast cancer a suitable disease for screening?

Answer 44

- NZ high risk - common among women - Maori have high rates - also pacific women have high rates - incidence rates increase at age 50+

Question 45

How is breast cancer screening a suitable test ?

Answer 45

Screening mammogram: Detects lumps in breast Sensitivity=75-90% Specificity= 90-95%

Question 46

How is breast cancer screening suitable treatment?

Answer 46

There's surgical treatment and a 5 year survival rate of 95-100%

Question 47

How is the breast screen programme suitable in NZ?

Answer 47

"Breast screen Aotearoa": Women who test positive get the diagnostic test Reduces cancer mortality by 20% (under 50), 30% (50-65), 45% (65-69) Specific goal for Maori/Polynesian 10 year survival rates