Download Screening

Screening in Public
Health Practice
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Screening

Definition: Presumptive identification of an
unrecognized disease or defect by the
application of tests, examinations, or other
procedures. Classifies asymptomatic people
as likely or unlikely to have a disease or
defect. Usually not diagnostic.
 Purpose: Delay onset of symptomatic or
clinical disease. Improve survival.
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Screening

Seems simple but is complex. There are
hidden costs and risks. Screening can create
morbidity and anxiety. Must be aware of
biases.

For screening to be successful you need a:



Suitable disease
Suitable test
Suitable screening program
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What type of screening program is
this?
Drs. Poke and Jab (1993) conducted
research at all shopping malls in
California to detect high blood pressure
and to warn people of the potential for
hypertension. Their subjects were
chosen from volunteers passing by the
mall. Which type of program is this?
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Suitable Disease





Has serious consequences
Is progressive
Disease treatment must be effective at an
earlier stage
Prevalence of the detectable pre-clinical phase
must be high
Examples of suitable diseases: breast cancer,
cervical cancer, hypertension
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Natural History of Disease
20
30
A
Biological
Onset
40
50
B
60
70
C
D
Years
Disease Symptoms Death
Detectable
Develop
By Screening
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Natural History of Disease

Total pre-clinical phase = A to C (Age 30 to
Age 60) = 30 years

Detectable pre-clinical phase (DPCP) = B to C
(Age 45 to Age 60) = 15 years

DPCP varies with the test, the disease, and
the individual

Lead Time: Duration of time by which the
diagnosis is advanced as a result of screening.
B to C (Age 45 to Age 60) = 15 years
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Suitable Test

Ideally, it's inexpensive, easy to administer,
has minimal discomfort has high level of
validity and reliability

Valid Test: Does what it's supposed to do, that
is, correctly classify people with pre-clinical
disease as positive and people without preclinical disease as negative

Reliable Test: Gives you same results on
repetition

Validity is more important than reliability
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Suitable Test
Disease Status (Truth)
Screening
Test Result
Yes
No
Total
Positive
a
b
a+b
Negative
c
d
c+d
Total
a+c
b+d
a + b + c+ d
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Suitable Test
Measures of test validity
Sensitivity - enables you to pick up the cases of disease
Sensitivity =
disease
a/a+c
= those that test positive / all with
Specificity - enables you to pick out the no diseased people
Specificity- enables you to pick out the non diseased people
Specificity =
disease
d/b+d
= those that test positive / all with
Valid test has high sensitivity and specificity
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Suitable Test
Breast Cancer Screening Program of Heath Insurance Plan
(HIP)
Women assigned to screening or usual care. Screening consisted
of yearly
Breast
Cancer
mammogram and physical exam.
Five
years of follow-up produced
these results:
Screening
Test
Total
Confirmed
Not
Confirmed
Positive
132
983
1,115
Negative
45
63,650
63,695
Total
177
64,633
64,810
Result
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Suitable Test

Sensitivity = 132/177 = 74.6%

Specificity = 63,650/64,633 = 98.5%

Interpretation: The screening was very good at picking
out the women who did not have cancer (see
specificity) but it missed 25% of the women who did
have cancer (see sensitivity).

To measure sensitivity and specificity you can wait for
disease to develop (as above) or you can measure the
results of the screening test against the outcome of
another screening or diagnostic test (the Gold
Standard).
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Suitable Test

Criterion of Positivity - test value at which the
screening test outcome is considered positive
Test Result
Clearly Negative
Grey Zone
Clearly Positive
-------------------------??????????????????--------------------A
B
C
Criterion of positivity affects sensitivity and specificity.
Must trade off between the two.
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Suitable Test

What are the sensitivity and specificity if A (or B or C)
is used as the cutoff for a positive result?

If criterion is low (Point A) then sensitivity is good but
specificity suffers. If criterion is high (Point C) then
specificity is good but sensitivity suffers.

Decisions about criterion of positivity involves
weighing the cost of false positives against the cost of
false negatives.
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Suitable Screening Program

Definition of a screening program: Application
of a specific test in a specific population for a
specific disease

You want to determine if screening program is
successful. Does it reduce morbidity and
mortality? How to evaluate?


Feasibility Measures
Effectiveness Measures
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Evaluation of Screening Program
1. Feasibility Measures
Acceptability, cost, predictive value of a positive test
Status
(PV+), predictive valueDisease
of a negative
test (PV-)
Screening
Test
Result
Positive
Yes
a
No
b
Total
a+b
Negative
c
d
c+d
Total
a+c
b+d
a+b+c+d
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Evaluation of Screening Program
Predictive value of a
positive test (PV+)
Predictive value of a
negative test (PV-)
=
A/aa
a/a+b
Number who test positive
= with disease / Number with
positive result
=
d/c+d
= Number who test negative
without disease / Number
with negative result
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Evaluation of Screening Program

Breast Cancer Screening Program of HIP
Breast Cancer
Cancer
Cancer Not
Confirmed
Confirmed
Screening Test
Total
Positive
132
983
1,115
Negative
45
63,650
63,695
Total
177
64,633
64,810
PV+ = 132/1115 = 11.8%
PV- = 63,650/63,695 = 99.9%
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Evaluation of Screening Program

PV will increase when sensitivity, specificity,
and disease prevalence increases.
 For example, PV+ will increase if you
perform breast cancer screening on higher
risk population (i.e. women with a family
history of breast cancer)
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Evaluation of Screening
Program
Demonstration of how prevalence effects PV

Use screening test with 99.9% sensitivity and 99.9% specificity
in two populations:

Population A)
Two positive results. One will be true positive. One will be a test
error. PV+ is 50%

: 1,000 people with low prevalence of disease (1/1,000
 Population B: 1,000 people with high prevalence of disease
(10/1,000)

Eleven positive test results. 10 will be true positives. One will
be a test error. PV+ is 10/11 or 90.9%
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Summary of Screening

Screening is the presumptive identification of
unrecognized disease by the application of
tests, exams, etc.
 Suitable disease must be serious with
important consequences and progressive
 Suitable test must have low cost, be
acceptable, and have a high degree of validity
 Validity is measured by sensitivity and
specificity
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A new screening test for Lyme disease is
developed for use in the general population.
The sensitivity and specificity of the new test
are 60% and 70%, respectively. Three
hundred people are screened at a clinic during
the first year the new test is implemented.
(Assume the true prevalence of Lyme disease
among clinic attendees is 10%.) What are the
number of false positives?
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What are the number of false
positives?
Screening
Test
Result
Disease
Status
Positive
Yes
a
No
b
Total
a+b
Negative
c
d
c+d
Total
a+c
b+d
a+b+c+d
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What are the number of false
positives?
Screening
Test
Result
Disease
Status
Positive
Yes
18
No
81
Total
99
Negative
12
189
201
Total
30
270
300
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Summary of Screening
Screening programs administer
screening tests in particular populations
 Programs are evaluated mainly by
examining predictive value and outcome
measures such as stage distribution and
cause-specific mortality
 Evaluation must consider lead-time bias,
length-biased sampling, and volunteer
bias.

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