screening and its useful tools thomas songer, phd basic epidemiology south asian cardiovascular...
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Screening and its Useful Tools
Thomas Songer, PhD
Basic Epidemiology
South Asian CardiovascularResearch Methodology Workshop
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Screening
• The early detection of– disease
– precursors of disease
– susceptibility to disease
in individuals who do not show any signs of disease
Goel
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Purpose of Screening• Aims to reduce morbidity and mortality from
disease among persons being screened
• Is the application of a relatively simple, inexpensive test, examinations or other procedures to people who are asymptomatic, for the purpose of classifying them with respect to their likelihood of having a particular disease
• a means of identifying persons at increased risk for the presence of disease, who warrant further evaluation
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Diagnosis = Screening
• Screening tests can also often be used as diagnostic tests
• Diagnosis involves confirmation of presence or absence of disease in someone suspected of or at risk for disease
• Screening is generally in done among individuals who are not suspected of having disease
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Susceptible Host
Subclinical Disease
Clinical Disease
Stage of Recovery, Disability, or Death
Point of Exposure
Screening
Onset of symptoms
Diagnosis sought
Natural History of DiseaseDetectable subclinical disease
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Screening Process
TestNegative
Re-screen
Unaffected
Intervene
Affected
TestPositive
Population(or target group)
Screening
ClinicalExam
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Examples of Screening Tests
• Questions
• Clinical Examinations
• Laboratory Tests
• Genetic Tests
• X-rays
Goel
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Validity of Screening Tests
• Sensitivity
• Specificity
• Positive Predictive Value
• Negative Predictive Value
Paneth
Key Measures
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TerminologyValidity is analogous to accuracy
The validity of a screening test is how well the given screening test reflects another test of known greater accuracy
Validity assumes that there is a gold standard to which a test can be compared
Paneth
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Present Absent
Positive a b
Negative c d
a + b
c + d
a + c b + d
DiseaseS
cree
nin
gT
est
N
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DiseaseS
cree
nin
gT
est
Present Absent
PositiveTrue
positives
Negative
Falsepositives
Falsenegatives
Truenegatives
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Sensitivity
• Proportion of individuals who have the disease who test positive (a.k.a. true positive rate)
• tells us how well a “+” test picks up disease
a
a + c=Sensitivityyes no
+ a b
- c d
a + b
c + d
a + c b + d
Disease
Scr
eeni
ngT
est
N
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Specificity• Proportion of individuals who don’t have
the disease who test negative (a.k.a. true negative rate)
• tell us how well a “-” test detects no disease
d
b + d=Specificityyes no
+ a b
- c d
a + b
c + d
a + c b + d
Disease
Scr
eeni
ngT
est
N
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Screening Principles• Sensitivity
– the ability of a test to correctly identify those who have a disease• a test with high sensitivity will have few false negatives
• Specificity– the ability of a test to correctly identify those who
do not have the disease• a test that has high specificity will have few false
positives
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Predictive Value• Measures whether or not an individual
actually has the disease, given the results of a screening test
• Affected by – specificity
– prevalence of preclinical disease
– Sensitivity
• Prevalence = a + c
a + b + c + d
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Present Absent
Positive a b
Negative c d
a + b
c + d
a + c b + d
DiseaseS
cree
nin
gT
est
N
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Positive Predictive Value
• Proportion of individuals who test positive who actually have the disease
a
a + b=P.P.V.yes no
+ a b
- c d
a + b
c + d
a + c b + d
Disease
Scr
een
ing
Tes
t
N
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Negative Predictive Value
• Proportion of individuals who test negative who don’t have the disease
d
c + d=N.P.V.yes no
+ a b
- c d
a + b
c + d
a + c b + d
Disease
Scr
een
ing
Tes
t
N
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Present Absent
Positive 48 3
Negative 2 47
51
49
50 50
Disease
Scr
een
ing
Tes
t
100
A test is used in 50 people with disease and50 people without. These are the results.
Paneth
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Present Absent
Positive 48 3
Negative 2 47
51
49
50 50
Disease
Scr
een
ing
Tes
t
100Sensitivity = 48/50Specificity = 47/50Positive Predictive Value = 48/51Negative Predictive Value = 47/49 Paneth
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So… you understand the accuracy of a screening test …
What is the next step?
Put screening to use in the population
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Considerations in Screening
Severity
Prevalence
Understand Natural History
Diagnosis & Treatment
Cost
Efficacy
Safety
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Criteria for a Successful Screening Program
• Disease– present in population screened
– high morbidity or mortality; must be an important public health problem
– early detection and intervention must improve outcome
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Criteria for a Successful Screening Program
• Disease– The natural history of the disease
should be understood, such that the detectable sub-clinical disease stage is known and identifiable
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Criteria for a Successful Screening Program
• Screening Test– should be relatively sensitive and
specific
– should be simple and inexpensive
– should be very safe
–must be acceptable to subjects and providers
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Criteria for a Successful Screening Program
• Have an Exit Strategy– Facilities for diagnosis and appropriate
treatments should be available for individuals who screen positive
– It is unethical to offer screening when no services are available for subsequent treatment
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Screening Strategies
• Cost-effective• Intervention appropriate
to the individual• Fails to deal with the
root causes of disease• Subjects motivated• Small chance of reducing
disease incidence
• Potential to alter the root causes of disease
• Large chance of reducing disease incidence
• Small benefit to the individual
• Poor subject motivation• Problematic risk-benefit
ratio
High-Risk Strategy Population Approach
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NCI Guidelines for Screening Mammography
“There is a general consensus among experts that routine screening every 1-2 years with
mammography and clinical breast exam can reduce breast cancer mortality by about one-
third for women ages 50 and over.”
“Experts do not agree on the role of routine screening mammography for women ages 40 to
49. To date, RCTs have not shown a statistically significant reduction in mortality in
this age.”
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Screening is not always free of risk
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In population screening….
False positives tend to swamp true positives in populations, because most diseases we test for are rare
Paneth
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Risks of Screening
• True Positives– “labeling effect” (classified as diseased
from the time of the test forward)
• False Positives– anxiety
– fear of future tests
– monetary expense
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Risks of Screening
• False Negatives– delayed intervention
– disregard of early signs or symptoms which may lead to delayed diagnosis
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Sources of Bias in the Evaluation of Screening Programs
• Lead time bias
• Length bias
• Volunteer bias
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Lead time bias
• Lead time: interval between the diagnosis of a disease at screening and the usual time of diagnosis (by symptoms)
Diagnosis by screening
Diagnosis via symptoms
Lead Time
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Bias in Screening:
Lead-Time Bias
•Consider a condition where the natural history allows for an earlier diagnosis, however, survival does not improve despite identifying it earlier •A screening program here will…
– over-represent earlier diagnosed cases– survival will appear to increase
• but in reality, it is increased by exactly the amount of time their diagnosis was advanced by the screening program
– Thus there is no benefit to screening from a survival standpoint.
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Lead time bias
• Assumes survival is time between screen and death
• Does not take into account lead time between diagnosis at screening and usual diagnosis.
Diagnosis by screening
in 1994
Deathin 2008
Survival = 14 years
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Lead time bias
Diagnosis by
screeningin 1994
Usual time of diagnosis
via symptomsin 1998
Lead Time 4 years
Deathin 2008
True Survival = 10 years
Survival = 14 years
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Bias in Screening:
• Most chronic diseases, especially cancers, do not progress at the same rate in everyone.
• Any group of diseased people will include some in whom the disease developed slowly and some in whom it developed rapidly.
• Screening will preferentially pick up slowly developing disease (longer opportunity to be screened) which usually has a better prognosis
Paneth
Length Bias
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Len
gth
bias
OBiological onset of disease
Screening
YSymptoms
Begin
DDeath
PDisease
detectable via screening
O DP Y
O DP Y
O DP Y
O DP Y
O DP Y
O P Y D
Time
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Volunteer bias
• Type of bias where those who choose to participate are likely to be different from those who don’t
• Volunteers tend to have:
– Better health
– Lower mortality
– Likely to adhere to prescribed medical regimens