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South Asian Cardiovascular Research Methodology Workshop. Basic Epidemiology. Screening and its Useful Tools. Thomas Songer, PhD. 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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South Asian Cardiovascular

Research Methodology Workshop

Basic Epidemiology

Screening and its Useful Tools

Thomas Songer, PhD


Screening l.jpg
Screening

  • The early detection of

    • disease

    • precursors of disease

    • susceptibility to disease

      in individuals who do not show any signs of disease

Goel


Purpose of screening l.jpg
Purpose of Screening

  • Aims to reduce morbidity and mortality from disease among persons being screened

  • Is the application of a relatively simple, inexpensivetest, 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


Diagnosis screening l.jpg
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


Natural history of disease l.jpg

Point of Exposure

Onset of symptoms

Screening

Natural History of Disease

Detectable subclinical disease

Clinical Disease

Stage of Recovery, Disability, or Death

Subclinical Disease

Susceptible Host

Diagnosis sought


Screening process l.jpg
Screening Process

Population

(or target group)

Screening

Test

Test

Clinical

Exam

Negative

Positive

Unaffected

Affected

Intervene

Re-screen


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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

Key Measures

  • Sensitivity

  • Specificity

  • Positive Predictive Value

  • Negative Predictive Value

Paneth


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Terminology

Validity 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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Disease

Present

Absent

a

b

a + b

Positive

Screening

Test

c

d

c + d

Negative

N

a + c

b + d


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Disease

Present

Absent

False

positives

True

positives

Positive

Screening

Test

False

negatives

True

negatives

Negative


Sensitivity l.jpg
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

Disease

a

yes

no

=

Sensitivity

a

b

a + b

+

Screening

Test

a + c

c

d

c + d

-

a + c

b + d

N


Specificity l.jpg
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

Disease

d

yes

no

=

Specificity

a

b

a + b

+

Screening

Test

b + d

c

d

c + d

-

a + c

b + d

N


Screening principles l.jpg
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


Slide16 l.jpg

Disease

Present

Absent

a

b

a + b

Positive

Screening

Test

c

d

c + d

Negative

N

a + c

b + d


Positive predictive value l.jpg
Positive Predictive Value

  • Proportion of individuals who test positive who actually have the disease

Disease

a

yes

no

=

P.P.V.

a

b

a + b

+

Screening

Test

a + b

c

d

c + d

-

a + c

b + d

N


Negative predictive value l.jpg
Negative Predictive Value

  • Proportion of individuals who test negative who don’t have the disease

Disease

d

yes

no

=

N.P.V.

a

b

a + b

+

Screening

Test

c + d

c

d

c + d

-

a + c

b + d

N


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A test is used in 50 people with disease and

50 people without. These are the results.

Disease

Present

Absent

51

48

3

Positive

Screening

Test

2

47

49

Negative

100

50

50

Paneth


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Disease

Present

Absent

51

48

3

Positive

Screening

Test

2

47

49

Negative

100

50

50

Sensitivity = 48/50

Specificity = 47/50

Positive Predictive Value = 48/51

Negative 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


Considerations in screening l.jpg

Considerations in Screening

Severity

Prevalence

Understand Natural History

Diagnosis & Treatment

Cost

Efficacy

Safety


Criteria for a successful screening program l.jpg
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


Screening strategies l.jpg

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

Screening Strategies

High-Risk Strategy

Population Approach


Nci guidelines for screening mammography l.jpg

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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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


Risks of screening32 l.jpg
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


Lead time bias l.jpg
Lead time bias

  • Lead time: interval between the diagnosis of a disease at screening and the usual time of diagnosis (by symptoms)

Lead Time

Diagnosis

by screening

Diagnosis

via symptoms


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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.

Survival = 14 years

Diagnosis

by screening

in 1994

Death

in 2008


Lead time bias37 l.jpg
Lead time bias

Survival = 14 years

True Survival = 10 years

Lead Time 4 years

Diagnosis

by

screening

in 1994

Usual time of

diagnosis

via symptoms

in 1998

Death

in 2008


Bias in screening l.jpg
Bias in Screening:

Length Bias

  • 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 l.jpg

O

Biological

onset of

disease

P

Disease

detectable

via screening

Y

Symptoms

Begin

D

Death

Length bias

Screening

O

P

Y

D

O

P

Y

D

O

P

Y

D

O

P

Y

D

O

P

Y

D

O

P

Y

D

Time


Volunteer bias l.jpg
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


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