Screening as a tool for secondary prevention in public health

1. SCREENING
Prepared By Dr. Anees AlSaadi
CMT-P R4
December 2013
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2. SCREENING
• Outline:
– Definition of Screening.
– Differentiation between Screening and Diagnostic test?
– Uses/Purpose of Screening.
– Types and Examples of Screening test.
– Criteria for successful screening program.
– Relation ship between sensitivity and specify.
– Risk of Screening.
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3. How We Can Define Screening ….?
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4. Definition
The early detection of:
– Disease
– Precursors of Disease
– Susceptibility to Disease
In individuals who do not show any signs of
disease.
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5. Definition
The presumptive identification of those who
probably have disease from those who do not
have
by
means of rapidly applied tests in apparently
healthy individuals.
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6. Definition
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7. Definition
Screening
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8. How we can Differentiate Between
Screening and Diagnostic Tests ….?
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9. Differentiate Between Screening and
Diagnostic Tests
Screening
Diagnostic
Asymptomatic
Suggestive clinical picture
Large group
Single subject
Less accurate
Accurate
Not conclusive
Conclusive
Less expensive
Expensive
Not basic for treatment
Basic to treatment
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10. What are the Uses of Screening ….?
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11. Uses of Screening
Uses of Screening
Case Detection
Perspective Screening
Case / Disease Control
Prospective Screening
Research
Natural History of Disease
Health Education
Public Awareness
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12. Purpose of Screening
• Reducing disease burden.
• Classifying people to likelihood of having a
particular disease.
• Mean of identifying high risk groups who
warrant further evaluation.
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13. Screening Process
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14. Types of Screening Test
What are the Type of Screening Tests …?
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15. Types Of Screening
Test
Types of Screening Test
Mass
High Risk
Multiphasic
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16. Types of Screening Test
Population Approach
Not Cost Effective
Potential To Alter The Root Cause Of Disease
Large Chance To Reduce Disease Incidence
Small Benefit To The Individual
Poor Subject Motivation
Large Chance Of Reducing Disease Incidence
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17. Types of Screening Test
High Risk Strategy
F a i l s To
D eal With
Root Cause
Of Disease.
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18. Can You Give Some Examples For
Screening Tests … ?
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19. Examples for Screening Tests
Infancy
Pregnancy
Elderly
Adults
Growth Charts
Weight
Cancers
Lipid profile
Metabolic
Screening
CBC
Depression
Blood pressure
Hearing Test
Blood sugar
Vitamin
deficiencies
BMI
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20. Criteria For Successful Screening Test:
Criteria for
Disease
Successful
Screening
Criteria for Test
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21. Criteria For Successful Screening Test:
• Criteria for Disease:
– Present in population screened.
– High burden &of high public health concern.
– Screening +Intervention must improve
outcome.
– Known natural history of the disease.
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22. Criteria For Successful Screening Test:
• Criteria for Test:
– Reliable.
– Valid.
– Simple and inexpensive.
– Very safe.
– Acceptable to subjects and providers.
– Cost-effective.
– Exit strategy.
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23. Criteria For Successful Screening Test:
• Exit strategy:
• Facilities for diagnosis and appropriate
treatments should be available for positive
subjects.
• Ethically not acceptable to offer screening
without available management.
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24. Criteria For Successful Screening Test:
What Is The Definition Of
Reliability ?
• RELIABILTY:
What Are The Causes Of
Unreliability ?
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25. Criteria For Successful Screening Test:
Definition of Reliability:
– Repeatability, Reducibility, Precision.
– Getting the same results, when the test
repeated in same target individuals in the
same settings.
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26. Criteria For Successful Screening Test:
Causes of unreliability:
– Observer variation.
– Subject variation – Biological.
– Technical method error variation.
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27. Criteria For Successful Screening Test:
• ACCEPTABILITY:
– The test should not be:
• Painful.
• Unsafe.
• Discomforting /Embarrassing.
• Socially/ believes not accepted.
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28. Criteria For Successful Screening Test:
VALIDITY:
Ability of the test to distinguish between who
has the diseases and who does not.
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29. Sensitivity
Yield
Validity
Specificity
Predictive
Value
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30. DISEASE
TOTAL
TEST
a+b
Test -ve c
TOTAL
Diseased No disease
Test +ve a
b
d
c+d
b+d
a+b+c+d
a+c
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31. Sensitivity
DISEASE
TOTAL
Ability of the test
to truly identify
those who have
the disease
TOTAL
True
Positive
Test
+ve
a
b
a+b
Test
-ve
TEST
Diseased
No
disease
c
d
c+d
a+c
b+d
a+b+c
+d
Sensitivity=
a/(a+c)
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32. Sensitivity
[[A 90% Sensitivity means that 90% of the diseased
people screened by the test will give a “true
positive” and the remaining 10% a “false negative
results”]]
Positive test
and have the
disease.
Negative test
and have the
disease.
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33. Specificity
DISEASE
Diseased
TOTAL
True
Negative
No
disease
Test
+ve
a
b
a+b
Test
-ve
TEST
The ability of the test
to correctly identify
those who do not
really have the disease
TOTAL
c
d
c+d
a+c
b+d
a+b+c
+d
Specificity=
d/(b+d)
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34. Specificity
[[A 90% Specificity means that 90% of the non
diseased people screened by the test will give a “true
negative” result, and the remaining 10% a “false
negative results”]]
Negative test and do
not have the disease.
Negative test and have the
disease.
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35. • False Positive error rate= (1-specificity)
• False Negative error rate = (1-sensitivity)
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36. Assume a population of 1,000 people 100 have a disease 900
do not have the disease A screening test is used to identify the
100 people with the disease
Sensitivity = 80/ 100 X 100= 80%
Specificity = 800/ 900 X 100 = 88%
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37. Practical Example
EEG Results
Brain Tumor
Present
Absent
Positive
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54,000
Negative
4
306,000
Total
40
360,000
Sensitivity = 36/40 X 100 = 90%
Specificity = 306,000/360,000 X 100 = 85%
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38. DISEASE
TEST
Diseased
TOTAL
TOTAL
No disease
Test +ve a (True Positive)
b (false Positive)
a+b
Test -ve c (false Negative)
d (True Negative)
c+d
b+d
a+b+c+d
a+c
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39. Predictive value
DISEASE
TOTAL
Diseased
Negative
Predictive value
Proportion of
Individuals with
positive test really
have the disease
TOTAL
PPV=a/(a+b)
Test
+ve
a
b
a+b
Test
-ve
Proportion of
Individuals with
negative test really
have no disease
TEST
Positive
Predictive value
No
disease
c
d
c+d
a+c
b+d
a+b+c
+d
NPV= d/(c+d)
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40. Practical Example
Screening Test
Results
Diagnosis
Total
Positive
Negative
Diseased
40
100
Not Diseases
20
9840
60
9940
Total
140
9860
10,000
Sensitivity = 40/140 X100 = 28.57%
Specificity = 9840/9860 X100 =99.79%
Positive predictive value = 40/60X100 = 66.66%
Negative predictive value = 9840/9940X100 = 98.9%
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41. Effects on Predictive Value
Prevalence Increases
PPV Increases; NPV Decreases
Prevalence Decreases
PPV Decreases; NPV Increases
Specificity Increases
PPV Increases
Sensitivity Increases
NPV Increases
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42. Yield –the amount of previously unrecognized
disease that is diagnosed and brought to treatment as
a result of the screening program.
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43. Practical Exercise
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44. What is better a test with high sensitivity
or with high specificity…?
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45. Relation Between Sensitivity &
Specificity
Sensitivity
Specificity
False
Positive
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46. Relation Between Sensitivity &
Specificity
Specificity
Sensitivity
False
Negative
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47. Relation Between Sensitivity &
Specificity
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48. What
about
Risk of
Screening …?
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49. Risk of Screening
True
Positive
Labelling
Effect
False
Positive
False
Negative
Anxiety
Delayed
Diagnosis
Fear From
Future Test
Delayed
Intervention
Monetary
Expenses
Complications
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