Screening

Question 1

What is spontaneous presentation?

Answer 1

-Pt presents to A&E/GP –> presents with symptoms, diagnosis made

Question 2

Describe opportunistic case finding

Answer 2

• Person presents with symptoms related to a
disease/problem
• Health professional takes opportunity to check for other potential conditions
– BP measurement – Urine dipstick

Question 3

Definition of diagnosis

Answer 3

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

Question 4

What are 3 ways of detecting disease?

Answer 4

• Spontaneous presentation
• Opportunistic case finding
• Screening

Question 5

What is screening?

Answer 5

A systematic attempt to detect an unrecognised condition by the application of tests, examinations, or 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

Name 3 purposes of screening

Answer 6

To give a better outcome compared with finding
something in the usual way (having symptoms and self-reporting to health services)
• If treatment can wait until there are symptoms, there is no
point in screening
• Finding something earlier is not the primary objective

Question 7

What are the 5 areas of criteria for a screening programme?

Answer 7

1. Condition
2. Test
3. Intervention
4. Screening programme
5. Implementation

Question 8

Describe the condition criteria wrt (with respect to) screening

Answer 8

An important health problem (frequency/severity) with epidemiology, incidence, prevalence and natural history understood
• All the cost-effective primary prevention interventions
should have been implemented as far as practicable
• If the carriers of a mutation are identified as a result of
screening the natural history of people with this status
should be understood, including the psychological
implications

Question 9

Describe the Test criteria wrt screening programmes

Answer 9

Simple, safe, precise and validated screening test
• Distribution of test values in the population must be known and an agreed cut-off level must be defined and agreed
• Acceptable to target population
• Agreed policy on further diagnostic investigation of those who test positive and choices available to them
• If the test is for a particular mutation or set of genetic variants the method for their selection and the means through which these will be kept under review in the programme should be clearly set out

Question 10

What types of error can be made in any screening test?

Answer 10

• It will refer well people for further investigation: puts them through stress/anxiety, direct costs/opportunity costs
• It will fail to refer pple who do actually have an early form of the disease: inappropriate reassurance, possibly delay presentation with symptoms

Question 11

What are the features of test validity?

Answer 11

• Sensitivity (detection rate)
• Specificity
• Positive predictive value
• Negative predictive value

Question 12

Discuss the test validity feature sensitivity

Answer 12

Is the proportion of the people with the disease who are test positive
• Also known as the detection rate
• The proportion of the people who really have the disease
who are identified correctly by the test as having the disease
• Sensitivity is the probability a case will test positive
-If the sensitivity is high, then the test is v good at correctly identifying ppl with the disease you are screening for

Question 13

Discuss specificity wrt testing

Answer 13

Is the proportion of the people without the disease who are test negative
• The proportion of the people who really do not have the
disease who are identified correctly by the test as not
having the disease
• Probability a non-case will test negative
- a high specific is good -> correctly identifies ppl without the disease as not having the disease

Question 14

What is important about sensitivity and specificity wrt inherent characteristics of the test?

Answer 14

• Sensitivity and specificity are a function of the
characteristics of the test
• When the same test is applied in the same way in
different populations the test will have the same
sensitivity and specificity

Question 15

Discuss positive predictive value wrt test validity

Answer 15

• Probability that someone who has tested positive actually has the disease
• This value is strongly influenced by the prevalence of the disease

Question 16

Discuss the relationship between PPV and prevalence

Answer 16

If prevalence of a condition is high in the particular population sampled, then the PPV will also be high

Question 17

Discuss the negative predictive value as a feature of test validity

Answer 17

-Pair of PPV
- is the proportion of the people who are test negative who actually do not have the disease
• The NPV is the answer to the question “If the screening test is negative – what are the chances that I really don’t have the disease?”

Question 18

What are the implications of false positive results? (4 points to discuss)

Answer 18

• False positive – the test indicates patients MAY have the 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 also lead to lower uptake of screening in 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 19

What are the implications of false negative results?

Answer 19

• False positive – the test indicates patients MAY have the 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 also lead to lower uptake of screening in 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 20

Discuss the intervention after screening

Answer 20

-What you do yo people after they have had a positive screening test result
• Effective intervention for patients identified through
screening, with evidence that intervention at a pre-
symptomatic phase leads to better outcomes for the
screened individual compared with usual care.
• There should be agreed evidence based policies covering
which individuals should be offered interventions and the
appropriate intervention to be offered.

Question 21

What are the (4) criteria you need to meet before implementing a screening program?

Answer 21

• Proven effectiveness in reducing mortality or morbidity (high quality RCT data)
• Evidence that the complete screening programme is
clinically, socially and ethically acceptable to health
professionals and public
• Benefit gained by individuals should outweigh any harms
for example from overdiagnosis, overtreatment, false
positives, false reassurance, uncertain findings and
complications
• Opportunity cost of the screening programme should be
economically balanced in relation to expenditure on
medical care as a whole

Question 22

Discuss the (6) important criteria for implementation of a screening programme

Answer 22

• Clinical management and patient outcomes should be optimised in all healthcare providers
• All other options for managing the condition should have been considered
• Management and monitoring programme – quality
assurance
• Adequate staffing and facilities for programme
• Evidence-based information available to potential
participants (informed choice)
• Public pressure should be anticipated - decisions should
be scientifically justifiable to the public

Question 23

What is the lead time bias evaluation difficulty?

Answer 23

• Early diagnosis falsely appears to prolong survival
• Screened patients appear to survive longer, but only
because they were diagnosed earlier
• Patients live the same length of time, but longer knowing
they have the disease

Question 24

What is the length time bias evaluation difficulty?

Answer 24

• Screening programmes better at picking up slow growing, unthreatening cases than aggressive, fast-growing ones
• Diseases that are detectable through screening are more
likely to have a favourable prognosis, and may indeed never have caused a problem
• Could lead to false conclusion that screening is beneficial
in lengthening the lives of those found positive – curing
people that didn’t need curing?

Question 25

What is the selection bias evaluation difficulty?

Answer 25

• Studies of screening often skewed by ‘healthy volunteer’ effect
• Those who have regular screening likely to also do other
things that protect them from disease
• An RCT would help deal with this bias