Screening tests are used to detect disease or risk factors for disease in asymptomatic individuals. They differ from diagnostic tests in that they test large groups of people rather than single individuals, are less accurate but less expensive, and are not intended to conclusively diagnose disease. Successful screening programs require the disease to be an important public health problem, screening and early intervention to improve outcomes, reliable and valid screening tests that are safe, acceptable and cost-effective, and availability of diagnostic services and treatment for positive cases. Sensitivity measures the test's ability to correctly identify those with disease while specificity measures its ability to correctly identify those without disease. Both have implications for the predictive values of screening tests.

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 Mass
High Risk
Multiphasic
Types of Screening Test
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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
Fails To
D eal With
Ro o t Cau se
O f D isease.
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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:
Successful
Screening
Criteria for
Disease
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:
• RELIABILTY:
What Is The Definition Of
Reliability ?
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. Validity
Sensitivity
Specificity
Predictive
Value
Yield
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30. DISEASE TOTAL
TEST
Diseased No disease
Test +ve a b a+b
Test -ve c d c+d
TOTAL a+c b+d a+b+c+d
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31. Sensitivity
Ability of the test
to truly identify
those who have
the disease
Sensitivity=
a/(a+c)
True
Positive
DISEASE TOTAL
TEST
Diseased
No
disease
Test
+ve
a b a+b
Test
-ve
c d c+d
TOTAL a+c b+d a+b+c
+d
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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”]]
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Positive test
and have the
disease.
Negative test
and have the
disease.

33. Specificity
The ability of the test
to correctly identify
those who do not
really have the disease
Specificity=
d/(b+d)
True
Negative
DISEASE TOTAL
TEST
Diseased
No
disease
Test
+ve
a b a+b
Test
-ve
c d c+d
TOTAL a+c b+d a+b+c
+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”]]
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Negative test and do
not have the disease.
Negative test and have the
disease.

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
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Sensitivity = 80/ 100 X 100= 80%
Specificity = 800/ 900 X 100 = 88%

37. Practical Example
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Brain Tumor
EEG Results Present Absent
Positive 36 54,000
Negative 4 306,000
Total 40 360,000
Sensitivity = 36/40 X 100 = 90%
Specificity = 306,000/360,000 X 100 = 85%

38. DISEASE TOTAL
TEST
Diseased No disease
Test +ve a (True Positive) b (false Positive) a+b
Test -ve c (false Negative) d (True Negative) c+d
TOTAL a+c b+d a+b+c+d
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39. Predictive value
Positive
Predictive value
Proportion of
Individuals with
positive test really
have the disease
PPV=a/(a+b)
Negative
Predictive value
Proportion of
Individuals with
negative test really
have no disease
NPV= d/(c+d)
DISEASE TOTAL
TEST
Diseased
No
disease
Test
+ve
a b a+b
Test
-ve
c d c+d
TOTAL a+c b+d a+b+c
+d
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40. Practical Example
Screening Test
Results
Diagnosis Total
Diseased Not Diseases
Positive 40 20 60
Negative 100 9840 9940
Total 140 9860 10,000
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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%

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. 44
What is better a test with high sensitivity
or with high specificity…?

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
Anxiety
Fear From
Future Test
Monetary
Expenses
False
Negative
Delayed
Diagnosis
Delayed
Intervention
Complications
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