Download Screening - Nottingham

Screening
Puja Myles
[email protected]
Learning outcomes
You should be able to:
• Define screening
• Describe the criteria that should be met
before implementing a screening
programme
• Discuss the properties of a screening test
• Outline the benefits and disadvantages of
screening programmes
Lecture outline
•
•
•
•
•
What is screening?
UK National Screening Programmes
Screening Criteria
Properties of a screening test
Disadvantages of screening
What is screening?
“Screening is the presumptive
identification of unrecognised disease or
defect by the application of tests,
examinations or other procedures that can
be applied rapidly”
US Commission on Chronic Illness, 1957
What is screening?
Screening is a public health service in which
members of a defined population, who do not
necessarily perceive they are at risk of, or are
already affected by a disease or its
complications, are asked a question or offered a
test, to identify those individuals who are more
likely to be helped than harmed by further tests
or treatment to reduce the risk of a disease or its
complications.
– UK National Screening Committee
UK National Screening
Programmes
•
•
•
•
•
Cervical cancer
Breast cancer
Bowel cancer
Diabetic retinopathy
Antenatal screening
– e.g. infectious diseases, foetal anomalies
• Neonatal screening
– e.g. blood spot (hypothyroidism, PKU, sickle cell),
neonatal hearing
Principles for introducing a
screening programme
• Originally proposed by Wilson and Junger
in 1968.
• Subsequently modified by a number of
authors.
• NHS National Screening Committee
criteria:
http://www.nsc.nhs.uk/uk_nsc/uk_nsc_ind.htm
Principles for introducing a
screening programme
•
•
•
•
The condition
The test
The treatment
The programme
Criteria for introducing a screening
programme
The condition
– Should be an important health problem i.e.
substantial burden of morbidity or mortality
– Natural history should be known
– Should be a detectable preclinical phase:
• Asymptomatic disease, e.g. breast cancer
• Markers of risk, e.g. pre-malignant changes in
cervical cancer
– Cost-effective primary prevention should have
been implemented
Criteria for introducing a screening
programme
The test
– Safe
– Acceptable to population
– Validated
– Relatively low cost
Criteria for introducing a screening
programme
The treatment
– Effective treatment available
– Evidence that early treatment gives better
outcomes
Criteria for introducing a screening
programme
The programme
– Evidence from RCTs that the screening
programme reduces mortality or morbidity
– Evidence that programme is clinically, socially
and ethically acceptable
– Economic costs should be considered
Properties of screening tests
1. Let’s start with an imaginary population of 1000
2. 100 people have asymptomatic disease
(prevalence=__%)
3. The screening test correctly identifies 90 of
these 100 people as having disease (sensitivity
of the test= 90%)
Sensitivity is the proportion of people with the
disease who are correctly identified as being
positive (true positive)
Properties of screening tests
What happens to the 10 people with
disease who are missed?
The screening test wrongly identifies them
as negative (false negative)
Screening Tests
Disease +
Test +
Test -
Disease -
Screening Tests
Disease +
Disease -
Test +
True positive
False positive
Test -
False negative
True negative
Specificity
• Ability of test to identify correctly all those who
are free of the disease in the screened
population
• The proportion of people free of the disease in
whom a screening test gives a negative result
• High specificity means low false positives
• Negative consequences for people who test
false positive: anxiety, unnecessary
diagnostic tests which may be invasive
Sensitivity and Specificity
Disease +
Test +
Test Total
Disease -
Total
a
b
a+b
c
d
c+d
a+c (total
b+d (total
a+b+c+d
disease)
healthy)
• Sensitivity = those who screen positive/ total
number with disease
• Specificity = those who screen negative/ total
number of disease-free people
Test yourself!
Test +
Test Total
Disease +
Disease -
Total
90
?
100
90
810
900
180
?
1000
• Sensitivity = ?
• Specificity = ?
PPV and NPV
• Positive predictive value (PPV)
– The proportion of those who test positive who actually
have the disease
• Negative predictive value (NPV)
– The proportion of those who test negative who re free
of the disease
• Influenced by the sensitivity and specificity of the
test AND by prevalence of disease in population
Two problems with cancer
screening…
Length time bias:
Screening is more effective at detecting
slow developing cancers which are less
likely to be the cause of an individual’s
death but by detection can cause
considerable anxiety
Lead time bias
Time
Survival time from diagnosis with screening
Survival time from diagnosis
without screening
Screening
Diagnosis
Symptoms
Death
Disadvantages of screening
• Significant harm to false positives:
communication of anxiety producing
information; follow-up tests which may be
risky (e.g. colonoscopy in the case of
bowel cancer)
Final word…
• Screening is not a diagnostic test. It just
separates individuals into groups who
have either a low probability or a high
probability of disease being present.
• It can be misunderstood by the public that
a positive result is ‘bad news’ and a
negative result is ‘all clear’.
Revision points
You should be able to:
• Define screening
• Describe the criteria that should be met
before implementing a screening
programme
• Discuss the properties of a screening test
• Outline the benefits and disadvantages of
screening programmes