Session 6 - Screening

Question 1

What is the definition of diagnosis?

Answer 1

The definitive identification of a suspected disease or defect by application of tests, examination or other procedures (which can be extensive) to definitely label people as either having a disease or not having a disease.

Question 2

What is screening?

Answer 2

A systematic attempt to detect an unrecognised condition by the application of tests, examinations, or other procedures which can be applies rapidly (and cheaply) to distinguish between apparently well persons who probably have a disease (or its precursor) and those who probably do not.

Question 3

What is the purpose of screening?

Answer 3

To give a better outcome compared with finding something in the usual way - if treatment can wait until symptoms show then it is not worth screening

Question 4

What are some NHS population screening programmes?

Answer 4

Breast Cancer
Bowel Cancer
Cervical Cancer
AAA
Diabetic Retinopathy 
Down's Syndrome/PKU/Sickle Cell/Thalassaemia/Inherited Metabolic disease.

Question 5

What are the four criteria that must be fulfilled for a screening programme to go ahead?

Answer 5

Disease/Condition
Test
Treatment
Programme

Question 6

What criteria must be fulfilled for the disease/condition factor of screening?

Answer 6

Must be an important health problem
Epidemiology and natural history must be well understood.
Must have an early detectable stage
Cost effective primary prevention interventions must have been consider where possible implemented.

Question 7

What criteria must be fulfilled for the test factor of screening?

Answer 7

Simple and safe - testing healthy people
Precise and Valid
Acceptable to population - e.g. bowel cancer test

Question 8

What must be known about a test before it can be used?

Answer 8

The distribution of test values in the population e.g the proportion of people who will test positive/negative.
An agreed cut off level must be defined.
Agreed policy on who to investigate further.

Question 9

What are the consequences of referring well people for further investigation (false positives)?

Answer 9

Put them through stress, anxiety and inconvenience
Direct Costs
Opportunity Costs (could have used resources to test people who actually needed it)
May be lower uptake of screening in the future

Question 10

What are the consequences of falling to refer people who actually have the disease (false negatives)?

Answer 10

Inappropriate reassurance given

Possible delay in presentation with symptoms.

Question 11

What features can be calculated to see whether a test is valid?

Answer 11

Sensitivity (detection rate)
Specificity
Positive prediction value
Negative prediction value

Question 12

What is sensitivity and how can it be calculated?

Answer 12

Proportion of the people with the disease who are test positive or detection rate. High sensitivity is ideal.
a/a+c
or
Disease present and positive test / all those that have the actual disease
or
True positives / True positive + False negatives

Question 13

What is specificity and how can it be calculated?

Answer 13

Proportion of people without the disease who test negative. Proportion of the people who really do not have the disease who are identified correctly as not having the disease.
d/b+d
or
Those who test negative and don’t have disease / all those who don’t have the disease
or
True negatives / True negatives + False positives

Question 14

How would you draw a table to help you calculate the validity values?

Answer 14

PICTURE HERE

Question 15

What will happen to the specificity and sensitivity values when the same test is applied to a different population?

Answer 15

They will stay the same.

Question 16

What is positive predictive value and how is it calculated?

Answer 16

Probability that if you test positive that you actually have the disease. This is strongly influenced by the prevalence of a disease. The lower the prevalence the less accurate the test generally is and patients have to undergo unnecessary investigations.
a/a+b
or
True positive / true positives + false positives

Question 17

What is negative predictive value and how is it calculated?

Answer 17

The proportion of people who test negative who actually do not have the disease.
d/ c+d

True negative / true negatives + false negatives

Question 18

What criteria must be fulfilled for the Treatment factor of screening?

Answer 18

Effective evidence based treatment must be available
Early treatment must be advantageous not just bring forward date of diagnosis.
Agreed policy on who to treat.
Management of condition should be optimised before implementation of programme.

Question 19

What criteria must be fulfilled for the Programme factor of screening?

Answer 19

Proven effectiveness - preferably with RCT data
Quality assurance for the whole programme not just the test
Facilities for counselling
Facilities for diagnosis and treatment
Other options first considered: could money be better spent improving treatment/prevention
Parameters should be scientifically justifiable to public.
Benefit should outweigh harm

Question 20

What are some of the issues raised by screening?

Answer 20

Alteration of usual doctor-patient contract
Complexity of screening programmes
Evaluation of screening programmes
Limitations of screening 
Sociological critiques

Question 21

What effect does screening have on the usual doctor-pateint relationship?

Answer 21

Usually people self-present and define themselves as patients.
Screening targets health people who have not sought help and are being given help about something they may not have even thought about. People are turned into patients.

Question 22

What are some of the problems with the cervical cancer screening programme?

Answer 22

Natural history of disease well understood?
How many abnormalities would regress spontaneously?
Are the right women being tested?
Debate over whether screening has caused reduction in mortality
Over-treatment?
Psychological impact?

Question 23

What is lead time bias?

Answer 23

When early diagnosis appears to falsely prolong survival.
Screened patent appear to survive longer but only because they were diagnosed earlier. Patients live for the same amount of time but know they have the condition for longer.

Question 24

What is length time bias?

Answer 24

Screening programmes are better at detecting slow growth, unthreatening cases rather than fast aggressive ones. Therefore the disease that are picked up via screening are more likely to have a favourable prognosis and so it will seem that screening has improved survival. Curing people who didn’t need curing?

Question 25

What is selection bias?

Answer 25

Healthy volunteer effect

The people who are likely to participate are more likely to look after their health or be aware of the risks.

Question 26

What are some of the structural critiques of screening?

Answer 26

Victim blaming - Individuals are encourage to take responsibility for own health - but are we all equally able to do this?
Individualising pathology - what about addressing underlying material causes of disease

Question 27

What are some of the surveillance critiques of screening?

Answer 27

Individuals and populations are increasingly subject to surveillance. Prevention part of a wider apparatus of social control.

Question 28

What are some of the social constructionist critiques of screening?

Answer 28

Health and illness practices can be seen as moral - given meaning through particular social relationships.