Screening for diseases

1. Dr. Madhu S M
1

2. SCREENING
2
• 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.

3. Definition
The search for unrecognized disease or
defect by
means of rapidly applied tests in apparently
healthy individuals.
3

4. Concept of screening
•The active search for disease among apparently
healthy individuals.
•It is the fundamental aspect of prevention.
•Screening differs from periodic health
examinations by following respects:
1. Capable of wide application.
2. Relatively inexpensive.
3. Requires little physicians time. It fact the
physician is not required to administer the
test, but only to interpret it.

5. Definition
5

6. Definition
Screening
6

7. How we can Differentiate Between
Screening and Diagnostic Tests ….?
7

8. 8
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

9. What are the Uses of Screening ….?
9

10. 10
Uses of Screening
Uses of Screening
Case Detection Perspective Screening
Case / Disease Control Prospective Screening
Research Natural History of Disease
Health Education Public Awareness

11. 11
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.

12. Screening Process
12

13. Types of Screening Test
What are the Type of Screening Tests …?
13

14. T
est
Types
Of
Screening
Mass
High Risk
Multiphasic
Types of Screening Test
14

15. 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
15

16. Types of Screening Test
High Risk Strategy
Fails To
D eal With
Root Cau se
O f D isease.
16

17. Can You Give Some Examples For
Screening Tests … ?
17

18. 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
18

19. Criteria For Successful Screening Test:
Successful
Screening
Criteria for
Disease
Criteria for Test
19

20. 20
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.

21. 21
Criteria For Successful Screening Test:
• Criteria for Test:
– Reliable.
– Valid.
– Simple and inexpensive.
– Very safe.
– Acceptable to subjects and providers.
– Cost-effective.
– Exit strategy.

22. 22
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.

23. Criteria For Successful Screening Test:
• RELIABILTY:
What Is The Definition Of
Reliability ?
23
What Are The Causes Of
Unreliability ?

24. 24
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.

25. 25
Criteria For Successful Screening Test:
Causes of unreliability:
– Observer variation.
– Subject variation – Biological.
– Technical method error variation.

26. 26
Criteria For Successful Screening Test:
• ACCEPTABILITY:
–The test should not be:
• Painful.
• Unsafe.
• Discomforting /Embarrassing.
• Socially/ believes not accepted.

27. 27
Criteria For Successful Screening Test:
VALIDITY:
Ability of the test to distinguish between who
has the diseases and who does not.

28. Validity
Sensitivity
Specificity
Predictive
Value
Yield
28

29. 29
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

30. 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
30

31. Sensitivity
[[A90% 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.
31

32. 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
32

33. 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.
33
Negative test and have the
disease.

34. 34
• False Positive error rate= (1-specificity)
• False Negative error rate = (1-sensitivity)

35. 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%
35

36. 36
Practical Example
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%

37. 37
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

38. 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)
38
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

39. 39
Practical Example
Screening Test
Results
Diagnosis Total
Diseased Not Diseases
Positive 40 20 60
Negative 100 9840 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%

40. Effects on Predictive Value
Prevalence Increases PPV Increases; NPV Decreases
Prevalence Decreases PPV Decreases; NPV Increases
Specificity Increases PPV Increases
Sensitivity Increases NPV Increases
41

41. 41
Yield –the amount of previously unrecognized
disease that is diagnosed and brought to treatment as
a result of the screening program.

42. Practical Exercise
42

43. What is better a test with high sensitivity
or with high specificity…?
43

44. Relation Between Sensitivity &
Specificity
Sensitivity Specificity
False
Positive
44

45. Relation Between Sensitivity &
Specificity
Specificity Sensitivity
False
Negative
45

46. Relation Between Sensitivity &
Specificity
46

47. What
about
Risk of
Screening …?
47

48. Risk of Screening
True
Positive
Labelling
Effect
False
Positive
Anxiety
Fear From
Future Test
Monetary
Expenses
False
Negative
Delayed
Diagnosis
Delayed
Intervention
Complications
48

49. 49