Screening is a public health intervention aimed at early disease detection in asymptomatic individuals to improve health outcomes. Key points about screening include: - Screening tests should be valid, reliable, and have acceptable sensitivity and specificity to accurately classify individuals. - Screening is only useful if early detection leads to effective treatment and the condition has a latent, pre-clinical stage. - Important factors for establishing successful screening programs include the health burden of the condition, availability of treatment, understanding of disease progression, and use of an acceptable and accurate screening test.

1. SCREENING IN PUBLIC
HEALTH
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2. SCREENING IN DISEASE CONTROL
Definition:-
• Screening is a public health intervention intended to
improve the health of a precisely defined target
population.
• The presumptive identification of unrecognized disease
by application of rapid tests or examinations.
• The early detection of disease
– Precursors of disease
– Susceptibility to disease in individuals who do not
show any signs of disease

3. Definition…
• Detecting disease or risk factors in asymptomatic
individuals
• The examination of asymptomatic people in order to
classify them as likely or unlikely to have the diseases
of interest.
– Diagnosis is not equal to Screening
• Screening sort out apparently well persons who
probably have a disease from those probably do not.

4. Diagnostic test
o Diagnostic tests are used to confirm the presence or
absence of illness, provide prognostic information, and
predict a response to treatment
o Identify and confirm a disease condition in individuals
– Diagnostic test is performed in persons with symptoms
or a signs of an illness
– Tests performed in patients
– The objective is case finding within a population that
is probably “diseased”
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5. Screening and diagnostic tests
o May be based on:
–Standardized interviews
–Physical examinations
–Laboratory tests
–More sophisticated measurements
radiography
electro-cardiograph etc…
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6. Natural history of diseases

7. Clinical Aim of Screening
• To reverse, halt, or slow the progression of disease
more effectively than would probably normally
happen.
• To alter the natural course of disease for a better
outcome for individuals affected.
• Reduce morbidity and mortality through early
detection and treatment

8. Public health aim of screening
program
o To protect society from communicable disease
o To reduce mortality
o For rational allocation of resources…
Other Use:
o Research: study on natural history of disease
o Selection of healthy individuals usually for employment
E. g: Military and driving license …
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9. Screening…..
• Screening is only useful for disease which can be
detected in their early, pre-clinical stage, and for which
early treatment significantly improves the outcome
• Screening can also be of useful for the communicable
disease control, such as trachoma, Schistosomiasis, or
TB, for which early treatment helps to prevent
transmission

10. Types of screening
programme
1. Mass/population screening
– It is offered to all, irrespective of the particular risk factors
2. Multiple/multi-phase screening
– The purpose of two or more screening tests in combination to a
large number of people at one time
3. Case finding/opportunistic screening
– It is restricted to patients who consult a health professional for
some other purposes
4. Targeted screening
– High risk or selective screening
– It will be most productive if applied selectively to high risk
groups
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11. Types of screening
Selective Vs Mass.
– Selective – screening of people with selective exposure
– Mass – screening of people without reference to specific
exposure
• Single Vs Multiple
• Multiple-Parallel Vs Series
– Parallel testing – applying two screening tests and a positive
result on either test is sufficient to be labeled as positive E.g. –
Breast ca screening
– Series testing – applying two screening tests and both must
be positive in order to prompt action
E.g. – HIV testing, Syphilis

12. Criteria for establishing screening program
1. The condition should be an important health problem (Life-
threatening diseases)
2. There should be a treatment for the condition.
3. Facilities for diagnosis and treatment should be available.
4. There should be a latent stage of the disease.
5. A suitable screening test must be available

13. Modified Wilson criteria for introducing a screening program
1. The disease condition
– Important health condition (prevalent, serious)
• Has early stage
– High prevalence at asymptomatic stage
– Adequately understood natural history
2. The test
– Simple, precise, validated
– Acceptable to population
– Agreed diagnostic test(s)
– Available, adequate, effective and acceptable
– Sensitive, specific, reliable, simple, cheap, safe

14. Modified Wilson criteria, cont..
3. The treatment
– Effective treatment Available, adequate, and
acceptable
– Evidence based policies on whom to treat
4. The program
– Acceptable to public and professionals
– Benefits outweigh risks
– Available facilities and staff
– Overall costs assessed

15. WHO criteria for screening
programme
Is it a health problem?
Is there treatment?
Are there facilities in place?
Is it detectable pre-clinically?
Is there a suitable screening test?
Is the screening test acceptable to people?
Is the natural history of disease understood?
Are the costs acceptable?
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16. Evaluation of a screening
program
Reliability
Feasibility
Validity
Performance
Yields
Effectiveness

17. Characteristics to be evaluated
• These characteristics are:
1. Validity
2. Predictive value
3. Reliability
4. Yield
1. Validity is analogous to accuracy
– The validity of a screening test is how well the given
screening test reflects another test of known greater accuracy
– Validity assumes that there is a gold standard to which a test
can be compared
• Validity- does it truly measure what it sets out to measure?
• Variability/reliability -does it gives the same measurement each time

18. Accuracy of screening tests…
o A test should be unbiased, precise, and be able to
determine the disease status of an individual without
error
o Accuracy is the ability of the test to correctly classify
individuals according to their disease status
Examples:
o Classify exposed versus non-exposed individuals
o Classify infected versus non-infected individuals
o Classify abnormal versus normal individuals etc…
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19. Characteristics of a screening test
Validity of a categorical test
Screening
Test result
Definitive Diagnosis(Gold Standard)
Diseased Non diseased Total
Positive True Positive
(TP)
False Positive
(FP)
TP + FP
Negative False
negative
(FN)
True
Negative
(TN)
TN + FN
Total TP + FN FP + TN TP +FP +
TN + FN

20. Two-by-two contingency table
Screening
Test
Diagnostic tests
Total
Diseased Non-diseased
Positive
True positive [TP]
(a)
False positive[FP]
(b) a + b
Negative
False negative [FN]
(c)
True negative[TN]
(d) c + d
Total a + c b + d n
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21. Two-by-two contingency table…
True positive
o The number of individuals for whom the screening test is
positive and the individual actually has the disease
False positive
o The number for whom the screening test is positive but
the individual does not have the disease
True negative
o The number for whom the screening test is negative and
the individual does not have the disease
False negative
o The number for whom the screening test is negative but
the individual does have the disease 21

22. Measures of accuracy of screening
tests
o Validity is the ability of a test to come up
with a result which is closer to the actual
value
o Reliability is the ability of a test to come up
with similar values upon repeated
measurements in similar occasions
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23. Measures of accuracy…
Validity:
o The application of a screening test is to identify correctly
individuals who do and do not have preclinical disease
o Those who have preclinical disease should test positive,
and those who do not have it should test negative
o The ability of a screening test to successfully separate
these two groups is a measure of its validity which is
expressed by its sensitivity and specificity
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24. Reliability versus validity of a test
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25. A. Sensitivity
– The ability of the test to identify correctly those who have the
condition, Sensitivity + Fn = 100%
– A test with high sensitivity will have few false negatives
%
100



Fn
Tp
Tp
y
Sensitivit
Sensitivity (detection rate) =a/a+c
=(Tp/Tp+Fn)X100
Proportion of true positive with diseases

26. B. Specificity
The ability of the test to identify correctly those who
don’t have the disease
–Specificity + false positive = 100%
–test that has high specificity will have few
false positives
%
100
y
Specificit 


Fp
Tn
Tn
Specificity=d/d+b=(Tn/Tn+Fp)X100
Proportion of true negatives identified

27. Validity of Screening Tests
11/4/2023 27
132
63650
45
983
Breast Cancer
+ -
Physical Exam
and Mammo-
graphy
+
-
Sensitivity: a / (a + c)
Sensitivity = 132 / (132 + 45) = 74.6%
Specificity: d / (b + d)
Specificity = 63650 / (983 + 63650) = 98.5%

28. Predictive values of screening
test
o It is the probability that a person tested positive or
negative will or will not have a disease respectively
o Predictive values of screening test depends on:
– Prevalence of preclinical disease
– Sensitivity
– Specificity
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29. Positive predictive value
o The probability of a person having the preclinical disease
when the test is positive
o It is defined as the proportion of people with the condition
among all those who received a positive test result
o It is the probability of the presence of the disease in a
person with a positive (abnormal) test
o It is calculated as a percentage; the number of individuals
with preclinical disease who test positive is in the
numerator, and the total number of individuals who test
positive is in the denominator
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30. Negative predictive value
o The probability of a person not having the preclinical
disease when the test is negative
o It is the proportion of people without the condition
among all those who received a negative test result
o It is the probability of not having the disease when the
test result is negative
o It is calculated as a percentage; the number of
individuals without the disease who test negative is in
the numerator, and the total number of individuals who
test negative is in the denominator
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31. Validity and predictive values of
screening test
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32. Screening test trade-offs
o Tests with dichotomous results – tests that give either
positive or negative results
E.g: HIV positive or negative
o Tests with 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
E.g: Blood pressure measurement
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33. Screening test trade-offs…
 There is always a trade-off between sensitivity
and specificity; as one increases, the other
tends to decreases
o The higher the cutoff used to say a test is positive, the
more specific the test becomes, but this higher specificity
comes at a price
o As the cutoff is increased, the sensitivity decreases and
the test is more likely to miss affected individuals (life
lost) i.e. false negative increases
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34. Screening test trade-offs…
o A lower cutoff might be used to create a very
sensitive test, so as not to miss anyone with the
disorder in question =false +ve
o In other situations, when using a test to confirm a
diagnosis, a higher cutoff, making the test highly
specific, is more desirable, so as not to incorrectly
label anyone with the disorder =false –ve
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35. Choosing appropriate test
– Choose a test with a high sensitivity and high
negative predictive value to rule out a diagnosis
in the early stage of the investigation
– When a False Negative is DANGEROUS or
SERIOUS
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36. Choosing appropriate test…
o Choose a test with a high specificity and
high positive predictive value to confirm a
diagnosis
o When a False Positive is DANGEROUS or
SERIOUS
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37. Choosing appropriate test…
High sensitivity High
specificity
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Sensitivity and specificity are affected by the ‘cut-off’ value of
measure at
which a test is defined as positive.

38. Yield of screening test
o The proportion of truly diseased individuals identified by the
screening test among screened population.
o The yield of a test result is a function of:
 Specificity of the test
 Prevalence of the preclinical disease
Strategies to increase yield of a test
o Select disease with high prevalence of preclinical
stage
o Target high risk group for screening program
o Select a test with high specificity test
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39. Yield…
• The number of cases of the condition detected by screening
test in relation to the total number of persons screened
• Affected by sensitivity, prevalence of unrecognized cases,
number of tests employed, frequency of screening and
participation of people in screening and follow-up
%
100





Fp
Fn
Tp
Tn
Tp
Yeild
%
100
screened
Total
test
by the
detected
dx
with the
persons


Yeild

40. ,
Yields of screening test
Reference test
Screening test
positive negative total
+ve 1555 988 2543
-ve 514 1394 1908
T 2069 2382 4451

41. Example…,
Sensitivity =1555/2069 =75.2%
Specificity=1394/2382 =58.5%
PVPT=1555/2543 = 61.1%
PVNT=1394/1908 =_____
Yield=1555/4451=______

42. RELIABILITY
• The ability of a test to give consistent results when it is
performed more than once on the same individual under the
similar conditions
• The reliability of a test is its capacity to give the same result -
positive or negative, whether correct or incorrect - on repeated
application in a person with a given level of disease
• Affected by variation in the method, observer and the
characteristic to be measured

43. Reliability…
o Intra-observer reliability – agreement among
measurements made by the same observer
o Inter-observer reliability – agreement among
measurements made by different observers
o Test-retest reliability – agreement among
measurements on the same subject at different times
o Biological (or subject) variation
o Technical (instrumental) problems
o Reagent problems
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44. RELIABILITY…
• The reliability of a dichotomous test can be described
using a 2×2 table in which the first and second test
results are presented on the left-hand side and top of
the table, respectively.

45. RELIABILITY…
Second test
First test
positive negative total
+ve tests agree tests disagree
-ve tests disagree tests agree
T

46. RELIABILITY…
• A simple way to summarize these findings is to
calculate the proportion of tests that agree:
• Percent agreement = Number of tests that agree
Total number of tests
• Percent agreement = a+d *100%
n
• However, test agreement due to chance alone is
possible, even with an imprecise test.
• The Kappa statistic is used to describe test agreement
beyond that expected from chance alone.
• Calculation of the Kappa statistic is presented below.

47. RELIABILITY…

48. RELIABILITY…
• The Kappa statistic ranges from +1 (perfect agreement), to 0 (no
agreement beyond that expected from chance), to −1 (perfect
disagreement).
• In general, a Kappa statistic <0.2 is considered poor agreement,
0.2 – 0.6 is considered fair agreement, and > 0.6 is considered
good agreement.
• A common application of the Kappa statistic is to measure
agreement between two different testers who evaluate a
subjective test characteristic

49. Example
Second test
First test
positive negative total
+ve 5 3
8
-ve 2 30
32
T 7 33 40

50. Potential Source of Bias in
Screening
• Self-selection (volunteer) bias
• Lead time bias (early diagnosis)
• Length Bias (chronicity and progression)

51. Self-selection (volunteer) bias
• Implies that those accepting screening are different from those
declining it.
• Comparing populations accepting and declining for all
relevant characteristics.

52. Lead time bias (early diagnosis)
• Bias caused by picking screened cases at an early
stage of the disease, i.e., before they develop signs
and symptoms of the disease

53. Length Bias (chronicity and
progression):
• Related to the variation in the speed of progression of the disease.
• Cases picked up by a screening may be less severe, and slow
progressive compared with others.
• It is very important to be aware of such a possibility in interpreting
survival gains from a screening program.

54. Success Factors for Screening
Programs
• The health problem should be important enough to be worth
detecting.
• Availability of an acceptable intervention which is effective.
• Presence of a recognizable latent or early "asymptomatic" stage.
• Presence of a suitable and acceptable test.

55. Success Factors…
• The natural history of the condition should be adequately
understood.
• Presence of policy regarding when the intervention is appropriate.
• The cost of detecting the problem and its remedy should be
reasonable.
• The screening program should be ongoing, and not a "one-time"
effort.

56. Study Design for Evaluation of
Screening
• Ideally experimental
• Most commonly used is cohort

57. Summary
 A screening program is defined as a set of procedures for
early detection and treatment of a disease
─ It includes both diagnostic and therapeutic components, and
includes the screening test and the follow-up evaluation for people
with positive tests
─ The therapeutic component consists of treatment of confirmed cases
of the disease
─ Thus, a screening program is much more than just a screening test
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58. END
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