Screening Pt 1

Question 1

What are the three methods of detecting a disease?

Answer 1

Spontaneous presentation

Opportunistic case finding

Screening

Question 2

What is spontaneous presentation in terms of detecting a disease?

Answer 2

Person presents with symptoms
- Self-defined as a ‘patient’

Occurs at

• GP
• A&E
• Other services

A diagnosis is made

Question 3

What is opportunistic case finding in terms of detecting a disease?

Answer 3

Person presents with symptoms related to a disease/problem

GP takes the opportunity to check for other diseases

• BP measurement
• Urine dipstick

Question 4

What is a diagnosis?

Answer 4

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 HAVING a disease or NOT HAVING a disease

The diagnosis is made following tests
Treatment will follow
The ‘patient’ will be prepared to accept the risk (side effects) associated with the treatment in order to get well

Question 5

What is the definition of screening?

Answer 5

A systematic attempt to detect an UNRECOGNISED condition by the application of tests, examinations, and other procedures, which can be applied rapidly (and cheaply) to distinguish between APPARENTLY WELL PERSONS who PROBABLY have a disease (or its precursor) and those who PROBABLY DO NOT

Question 6

What is screening? (not the definition, just more info)

Answer 6

A process that labels people as screen positive or screen negative

Following screening if a person is labelled as ‘screen positive’ - this does not mean that they DEFINITELY have the disease
Further test tests are required before the diagnosis of disease is made
Treatment will only follow once a definitive diagnosis is made

Question 7

What is the purpose of screening?

Answer 7

To give a better outcome compared with finding something in the usual way (having symptoms self-reporting to the health services)

If treatment can wait until there is symptoms, there is no point in screening

Question 8

What are some NHS population screening programs?

Answer 8

Abdominal Aortic Aneurysm (AAA)
Bowel cancer
Breast cancer
Cervical cancer
Diabetic Retinopathy
Down's syndrome
Foetal anomalies
PKU
Sickle Cell and Thalassaemia
Inherited metabolic diseases

Question 9

What are the benefits and risks of AAA screening?

Answer 9

All men are offered a single ultrasound screening scan when they are 64

Benefits
- Reduces risk of death from rupture by about half

Risks
- 238 men needed to be invited to screening to
prevent one ruptured AAA in next ten years
- Approximately 1 in 10,000 will attend for screening,
have a large AAA and then undergo treatment which
is complicated by death
- Quality of life is impaired (at least short term) for
those who have treatment
- Some cannot tolerate anxiety of knowing they have a
small AAA

Question 10

What are the four criteria for deciding on whether to screen?

Answer 10

Disease/Condition
Test
Treatment
Programme

Question 11

Describe the criteria of disease/condition for deciding on whether to screen or not

Answer 11

Must be an important health problem

Epidemiology and natural history must be well known

Must have an early detectable stage

Cost-effective primary prevention interventions must have been considered and where possible implemented

Question 12

Describe the criteria of the test for deciding on whether to screen or not

Answer 12

Simple & safe (screening healthy people)

Precise & valid (‘tells the truth’)

Acceptable to the population

Distribution of test values in the population must be known i.e. the proportion who test positive and negative
An agreed cut-off level must be defined (who do we count as testing positive)
There must be an agreed policy on whom to investigate further (i.e. the test positives)

Question 13

What are the two types of errors any screening programme makes?

Answer 13

It will refer well people for further investigation

• Put them through stress, anxiety and inconvenience
• Direct costs
• Opportunity costs
• These are known as false positives

It will fail to refer people who actually do have an early form of the disease

• Inappropriate reassurance
• Possibly delay presentation with symptoms
• These are known as false negatives

Question 14

What are the features of test validity?

Answer 14

Sensitivity (detection rate)

Specificity

Positive predictive value (PPV)

Negative predictive value (NPV)

Question 15

What are the four different types of result you can get for a person who has been screened?

Answer 15

True positives - Disease present and a positive test
True negatives - Disease absent and negative test
False positives - Disease absent and positive test
False negatives - Disease present and negative test

Question 16

What is the sensitivity of a test?

Answer 16

It is the proportion of the people with the disease who are test positive

The proportion of the people who really have the disease and are detected by the test as having the disease

Sensitivity is the probability a case will test positive

Sensitivity = (True +ves) / ((True +ves) + (False -ves))

If the sensitivity of the test is high then the test is very good at correctly identifying people with the disease

A high sensitivity is ideal.

Question 17

What is the specificity of a test?

Answer 17

It is the proportion of the people without the disease who are negative test

Probability a non-case will test negative

Specificity = (True -ves) / ((False +ves) + (True -ves)

Question 18

What are the features of sensitivity and specificity?

Answer 18

They are a function of the characteristics of the test

When the same test is applied to different population the test will have the same sensitivity and specificity

Question 19

What is the positive predictive value (PPV) of a test

Answer 19

Probability that someone who has tested positive actually has the disease

This value is strongly influenced by the prevalence of the disease

PPV = (True +ves) / ((True +ves) + (False +ves)

A lower prevalence condition will have a lower PPV than a higher prevalence condition.

Question 20

What is negative predictive value (NPV) of a test?

Answer 20

It is the proportion of the people who are test negative who actually do not have the disease.

NPV = (True -ves) / ((False -ves) + (True -ves))

Question 21

What are the implications of false positive results?

Answer 21

The test indicates they might have a disease when in fact they do not

They will be offered (invasive) diagnostic testing with all its attendant anxieties and risks - for a condition they actually do not have.
They will be turned into “patients” when they are not actually ill

May lead to a lower uptake of screening in the future and greater risk of interval cancer

If the PPV is low there will be a lot of people with false positive results who undergo stress and unnecessary procedures

Question 22

Implications of false negative results

Answer 22

The screening test indicates that they do not have the disease when in fact they do

They will not be offered (invasive) diagnostic testing when in fact they may have benefited from it. Their disease, although present will not be diagnosed

They will be falsely reassured - may present late with symptoms as a consequence

Question 23

Describe the criteria of treatment for deciding on whether to screen or not

Answer 23

Effective evidence based treatment must be available

Early treatment must be advantageous - you must not just bring forward the date of diagnosis

Agreed policy on whom to treat

Clinical management of the condition and patient outcomes should be optimised in health care providers before participation in screening programmes

Question 24

Describe the criteria of the programme for deciding on whether to screen or not

Answer 24

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 should be considered e.g. improving treatment
Think about opportunity costs
Decisions about parameters should be scientifically justifiable to the public
Benefit should outweigh physical and psychological harm (test, diagnostic procedures or treatment)