screening puja myles [email protected]
TRANSCRIPT
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)
Disease + Disease -
Test +
Test -
Screening Tests
Disease + Disease -
Test + True positive False positive
Test - False negative True negative
Screening Tests
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
Disease + Disease - Total
Test + a b a+b
Test - c d c+d
Total a+c (total disease)
b+d (total healthy)
a+b+c+d
• Sensitivity = those who screen positive/ total number with disease
• Specificity = those who screen negative/ total number of disease-free people
Sensitivity and Specificity
Disease + Disease - Total
Test + 90 90 180
Test - ? 810 ?
Total 100 900 1000
• Sensitivity = ?
• Specificity = ?
Test yourself!
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
DeathDiagnosis
Symptoms
Screening
Lead time bias
Time
Survival time from diagnosis without screening
Survival time from diagnosis with screening
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