This Lecture not only Usefull for Undergraduate but also for MPhil, FCPS, and PhD students

1. SCREENING
.
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2. What is Screening
• Screening is the testing of apparently healthy
populations to identify previously undiagnosed diseases
or people at high risk of developing a disease.
• Screening aims to detect early disease before it
becomes symptomatic.
• Screening is an important aspect of prevention, but not
all diseases are suitable for screening.
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3. Definitions
1. Screening program -- comprehensive disease control
activity based on the identification and treatment of
persons with either unrecognized disease or
unrecognized risk factors for disease.
2. Screening test -- specific technology (survey
questionnaire, physical observation or measurement,
laboratory test, radiological procedure, etc.) used to help
identify persons with unrecognized disease or
unrecognized risk factors for disease.
Definitions
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4. Definitions
3. Primary prevention -- disease control approach based on the
elimination or reduction of risk factors for disease. Primary
prevention aims to prevent the occurrence of disease. Primary
prevention may use screening tests to identify persons with risk
factors.
4. Secondary prevention -- disease control approach based on the
active identification and treatment of persons with unrecognized
disease. Secondary prevention aims to prevent the occurrence of
adverse outcomes from disease (such as fatal outcomes), without
necessarily reducing the occurrence of disease. Secondary
prevention must screen to identify persons with unrecognized
disease
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5. Generalities
1. Screening often implies a public health related
activity involving asymptomatic or healthy
subjects coming from the general population.
2. Case-finding refers to special clinical efforts to
recognize disease among persons who
consult a health professional.
Generalities
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6. Screening, Case financing and Diagnostic
test
Terminology
for testing
Target Persons
Screening Apparently healthy individuals who
are not seeking health care
Case-finding To detect disease in individuals
seeking health care for other
reasons
Diagnostic
tests
To confirm or disprove the existence
of disease in patients presenting
with complaints (Symptoms & signs
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7. The Principles of Screening
• The choice of disease for which
to screen;
• There should be longer latent or
early a symptomatic stage
• Facilities for confirmation of
diagnosis must be available
• The availability of a treatment
for those found to have the
disease;
• The relative costs of the
screening.
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8. • The disease must be an important health problem.
• There should be a recognizable latent or early symptomatic
stage.
• The natural history of the disease, including latent to
declared disease, should be adequately understood.
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9. When to screen?
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10. • There should be a suitable test or examination.
• The test should be acceptable to the population.
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11. Examples of screening
• Screening the healthy people for hypertension
• Screening healthy adults for diabetes
• Screening of high-risk population for HIV/AIDS and Hepatitis
• Screening of pregnant ladies for anemia/ Cervical cancers
etc
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12. Screening and diagnostic tests
Screening tests Diagnostic tests
Conducted on apparently
health population
Conducted on sick or with
some indications
Applied to groups or
communities
Applied to the patients under
consideration
The initiative comes from the
investigator or some agency
Initiative based on patient
complaints
The objectives are
predominantly preventive
The objective is to modify the
treatment on basis of tests
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13. Screening and diagnostic tests
Screening tests Diagnostic tests
Based on one criterion or cut-
off point
Based on clinical evaluation of
signs and symptoms
Less expensive More expensive
Less accurate More accurate
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14. True Disease Status
Screening
Test
Positive Negative Total
Positive True Positives
(TP)
False Positives
(FP)
TP+FP
Negative False Negatives
(FN)
True Negatives
(TN)
FN+TN
Total TP+FN FP+TN TP+FP+FN+TN
Outcomes of a Screening Test
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15. • There should be an acceptable treatment for the patients
with recognized disease.
• There should be facilities for diagnosis
and treatment should be available.
• There should be an agreed policy on whom to treat as
patients.
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16. • The cost of case finding (including diagnosis and treatment of
patients diagnosed) should be economically balanced in relation to
possible expenditure on medical care as a whole.
• Case finding should be a continuing process and not a "once for all"
project.
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17. Uses of Screening
Case detection Objectively done to identify the
unrecognized diseases e.g.
neonatal screening
Control of disease Objectively done to identify the
diseases to prevent transmission
in the community
Epidemiology /
Research
Initial screening to identify the
prevalence subsequent for
research purpose
Educational
Opportunities
Objectively done for health
education purposes e.g. screening
of diabetics
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18. Screening Strategies
Mass
Screening
Screening of whole population or
subgroups of population e.g. Screening
of all adults for tuberculosis
High risk or
Selective
Screening is applied to selectively to
high-risk for a particular health problem
or disease
Multiphase
Screening
The people are subjected to more than
one screening test. First screening for
identification of suspect and second for
confirmation of diseases
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19. Latent or Incubation period
Time period lapse between the start of the
disease process up to the appearance of
sign and symptoms of disease.
Disease
onset
Possible
detectio
n
Final
critical
point
Usual time
of
diagnosis
Latent/ incubation
period
outcom
e
A B C D
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20. • Time between possible detection and the usual time of
diagnosis by signs and symptoms is the “Lead Time”
• Time between first possible detection and the finial critical
detection is the “Screening Time”
Screening time and lead time
Disease
onset
Possible
detectio
n
Final
critical
point
Usual time
of
diagnosis
outcom
e
Screening
time
Lead time
A B C D
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21. Concept of Latent period, Screening
time and Lead time
Disease
onset
Possible
detection
Final critical
point
Usual time
of
diagnosis
Latent/ incubation
period
outcome
Disease
onset
Possible
detection
Final
critical
point
Usual time of
diagnosis
outcome
Screening
time
Lead time
A B C D
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22. Summary
• Screening is the testing of apparently healthy populations
to identify previously undiagnosed diseases or people at
high risk of developing a disease.
• Principles of Screening: disease, test, treatment and cost.
What is the next step?
Define the validity of the screening test and
put screening to use in the population.
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23. Terms Related to
Screening Tests
• Validity - relates to accuracy (correctness)
• Reliability - repeatability
• Yield - the # of tests that can be done in a time period
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24. Terms Related to Screening Tests (cont’d)
• Sensitivity - ability of a test to identify those who
have disease
• Specificity - ability of a test to exclude those who
don’t have disease
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25. Terms Related to Screening Tests
(cont’d)
• Tests with dichotomous results – tests that give either
positive or negative results
• Tests of continuous variables – tests that do not yield
obvious “positive” or “negative” results, but require a
cutoff level to be established as criteria for
distinguishing between “positive” and “negative”
groups
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26. How will you test the accuracy of
screening test?
• Identify the screening test to be evaluated
• Identify the confirmatory test for counter testing also
known as “Gold Standard Test”
• Screened the population of interest by screening test
• Apply counter test or Gold Standard Test to all the
positive and negative identify by screening test
• Determine the accuracy by 2x2 Table analysis
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27. Examples of Screening and Gold
Standard
Disease Screening test Gold Standard or
Counter test
Diabetes Blood Glucose Glucose tolerance
test
Brain tumor EEG CT Scan
Breast
cancer
Mammography FNA
(histopathology)
Tuberculosi
s
Tuberculin test Sputum for AFB
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28. Sensitivity
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
Sensitivity =
a
a + c
True positive
True positive + False
Negative
X 100
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29. Specificity
X 100
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
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30. Percentage of false Positive
Percentage false
positive =
b
b + d
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
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31. Percentage of false negative
X 100
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
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32. Predictive value of positive test (PPV)
a
a + b
True Positive X 100
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
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33. Predictive value of Negative test (NPV)
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
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34. Apparent or false prevalence
False/apparent
prevalence =
a +
G. total
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
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35. True Prevalence
a +c
G. total Total patient Screened
X
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
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36. Accuracy of the test
Dis. Yes Dis. No Total
Dis.
yes
a
(True
positive)
b
(False
Positive)
a + b
Dis. No c
(False
Negative)
d
(True
Negative)
c + d
Total a + c b + d Grand total
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37. Sensitivity and Specificity
• Sensitivity and specificity has reciprocal
relationship with each other
• If we increase the sensitivity of a test
specificity will be decreased
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38. Sensitivity and specificity
At cut-off 120 mg all above those will be declared as disease
Which are included as disease by the test
Which are normal but declared as disease (b/ False Positive)
Which are disease but excluded by the test (d/ False Negative)
Comment on Sensitivity and specificity
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39. Conclusion at cut-off value 120 mg / 100 ml
• Nearly 99% of those having diabetes will be picked up by the
test that means test become highly sensitive
• The test is falsely including a large number of normal persons
as the diseased increasing the false positive
• The increasing false positive means that the ability of the test to
exclude those not having the disease is decreasing (decrease in
specificity)
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40. Sensitivity and specificity
a
c
d b
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41. Conclusion at cut-off value 160 mg / 100 ml
• Nearly99% of those not having the diabetes will be
excluded by the test mean test become highly specific
(increasing specificity)
• The test will include many of the diseased persons as the
normal increasing the false negative cases
• Increasing number of false negative mean the ability of test
to pick up the diseased people is decreasing (decreasing the
sensitivity)
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42. Sensitivity and specificity
a
b
d
c
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43. Conclusion at cut-off value 140 mg/100
ml
• The ability of the test to include or exclude the diseased
person is nearly equal or critical (Balance sensitivity and
specificity)
• The number of false positive and false negative are also in
balance
• Therefore the point B is the suitable cut-off value for diabetic
screening with sensitivity and specificity nearly above 90%
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44. Reliability of the Screening tests
What are the factors that determine the
reliability of screening tests?
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45. Three type of factors effect the
reliability of test
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46. Observational Variation
• Intra-observer Variations (variation in
observation when a single observer repeat
the same observation)
• Inter-observer Variation (Different observers
when the same observation is repeated by
different observers
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47. •Use of Multiple Screening Tests
Sequential (Two-stage) Testing
Simultaneous Testing
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48. Hypothetical Two-Stage Screening
Only Pos. Test 1 are
given Test 2
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49. Hypothetical Two-Stage Screening (cont.)
TEST 2 (Glucose Tolerance Test)
Sensitivity = 90%
Specificity = 90%
DIABETES
+ -
TEST
RESULTS +
315 190 505
- 35 1710 1745
350 1900 2250
Net Sensitivity = 315/500 = 63%
Net Specificity = 7600 + 1710 = 98%
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50. Predictive Value
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51. Prevalence & Predictive Value
Note: Test has 95%
sensitivity and 95%
specificity
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52. Specificity &
Predictive Value
As specificity increases,
positive predictive value
increases.
As sensitivity increases, positive
predictive value also increases, but
to a much lesser extent.
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55. Study designs for screening
1. Correlation Studies
Use:
Description of population
Strength:
Suggest possibility of benefit
Limitation:
Can’t test hypothesis 55
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56. Study designs for screening
2. Analytical Studies
Types:
Case-control
Cohorts
Use:
Comparison
of rates
Advantage:
 Test
hypothesis
Limitation:
Selection
Lead time
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57. Study designs for screening
3. Randomized Trials
Use:
Comparison of rates
Strength:
Most valid test of hypothesis
Limitation:
Cost, ethics & feasibility 57
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58. Review Questions (Developed by the
Supercourse team)
• What is screening and what types of screening can you name?
• What are the objectives of screening?
• For what type of diseases would it be appropriate to set up
screening programs? List characteristics.
• How is screening program evaluated?
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60. Thank You
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