Screening

Question 1

Define screening

Answer 1

The systematic attempt to detect an unrecognised condition through tests, examinations or other procedures which can be applied rapidly and cheaply to distinguish between apparently well people who probably have a disease and those who do not

Question 2

What is the screening process?

Answer 2

Screen with a rapid cheap test
Screen positive - high risk, have diagnostic tests, find out if there is disease or no disease
Screen negative - low risk of disease

Question 3

If the person has a positive result, what are they labelled as?

Answer 3

Screen positive

Question 4

Purpose of screening?

Answer 4

Give a better outcome compared with finding something in the usual way (symptoms then self-report to health service)

Question 5

Criteria for a screening programme concerning the disease?

Answer 5

Must be an important health problem
Epidemiology and natural history well-understood
Early detectable stage
Cost-effective primary interventions must have been considered and where possible, implemented

Question 6

Criteria for a screening programme concerning the test?

Answer 6

Simple and safe
Precise and valid (tells truth)
Acceptable to population
Proportion who test positive and negative in a population must be known
Agreed cut off point for who tests positive must be known
Must be an agreed policy on who to investigate further

Question 7

What are the two types of error that screening programmes make?

Answer 7

False positive

False negative

Question 8

What is sensitivity?

Answer 8

Proportion of people with the disease who test positive

ie the probability a case will test positive

Question 9

What is specificity?

Answer 9

Proportion of people without the disease who test negative

Probability a non-case will test negative

Question 10

Do you want specificity to be high or low?

Answer 10

High - test is good at identifying people who don’t have the disease as not having the disease

Question 11

What is the positive predicted value?

Answer 11

Probability that somebody who has tested positive actually has it

Question 12

What is the positive predicted value (PPV) strongly influenced by?

Answer 12

Prevalence of the disease

Question 13

If the PPV for breast cancer is 8%, what does this mean?

Answer 13

That for every case found, 11 women who have been screened will test positive and actually don’t have it and will have had unnecessary tests

Question 14

How is PPV worked out?

Answer 14

True positives/

True +ves + true -ves

Question 15

How is PPV worked out?

Answer 15

True positives/

True +ves + true -ves

Question 16

How is sensitivity worked out?

Answer 16

True positives/

True positives + false negatives

Question 17

How is specificity worked out?

Answer 17

True negatives/

False positives + true negatives

Question 18

What is the negative predictive value?

Answer 18

Proportion of people who test negative who actually don’t have the disease

Question 19

How is NPV worked out?

Answer 19

True negatives/

True negatives + false negatives

Question 20

Implications of false positive results?

Answer 20

Have invasive diagnostic testing
Unnecessary anxiety and risks
Could lead to lower uptake of screening, greater risk of interval cancer
Low PPV -> many undergo stress and unnecessary procedures.

Question 21

Implications of false negative results?

Answer 21

Not offered diagnostic testing when they would have benefitted
Disease not diagnosed
Falsely reassured and present late with symptoms

Question 22

What criteria are there about the treatment of the disease that may be screened for?

Answer 22

Must be effective evidence based treatment
Early treatment must be advantageous
Clinical management of the condition and patient outcomes should be optimised in healthcare providers before participation in a screening programme

Question 23

Criteria about the screening programme before it can be introduced?

Answer 23

Proven effectiveness from RCT data
Quality assurance for the whole programme, not just the test
Facilities for counselling, diagnosis and treatment
Other options should be considered eg improving treatment
Decisions about parameters should be scientifically justifiable to the public
Benefit should outweigh physical and psychological harm

Question 24

Explain the issue of how screening alters the patient-doctor contract

Answer 24

Sick people normally present to the doctor
Screening targets healthy people for something people may never have thought about
Doctor -> healthy person

Question 25

Give some complexities of a screening programme for eg cervical cancer

Answer 25

Is the natural history understood
How many abnormalities would regress spontaneously 
Are the right women being screened
Has screening decreased mortality 
Is there over-treatment
Is there a psychological impact

Question 26

What is lead time bias?

Answer 26

Early diagnosis falsely appears to prolong survival

Question 27

What is length time bias?

Answer 27

Screening is better at picking up slow-growing, unthreatening cases
Disease detectable by screening more likely to have a favourable prognosis and may never even have caused a problem without screening
Leads to false conclusion that screening lengthens lives of those found positive - curing people who don’t need curing

Question 28

How does selection bias arise in screening?

Answer 28

Studies of screening are often skewed by the healthy volunteer effect
Those who have screening are more likely to do other things that protect them from disease

Question 29

Explain some sociological critiques of screening

Answer 29

Victim blaming - individuals are encourage to take responsibility of the own health, are all equally able to do this?
Individualising pathology - addressing underlying material causes of disease?
Individuals are increasingly subject to surveillance
Health and illness practices can be seen as moral
Is screening targeted more at women than men?