Screening involves using tests on apparently healthy people to identify disease. The document discusses screening concepts like lead time and sensitivity/specificity. Screening must meet criteria like being for important diseases with recognizable early stages. It should use acceptable, repeatable, valid tests. Evaluation measures include sensitivity, specificity, and predictive values. Screening programs must be continuously monitored to ensure benefits outweigh costs.

1. SCREENING

2. Learning Objectives
To understand the
concept of screening
and lead time
To describe the uses,
types and criteria for
screening
To understand about
the evaluation of
screening
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3. Contents
 Introduction
 Concept of screening
 Screening and diagnostic test
 Uses and types of screening
 Criteria of screening
 Screening tests
 Evaluation of screening tests (sensitivity and specificity)
 Conclusion
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4. Introduction
 Screening is the process of using tests on a
large scale to identify the presence of disease
in apparently healthy people.
 A screening test is never 100% accurate; it
does not provide certainty but only a
probability that a person is at risk (or risk-free)
from the condition of interest.
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5. “Screening is the search for unrecognized disease or defect by means of rapidly applied tests,
examinations or other procedures in apparently healthy individuals.”
Screening differs from periodic health examinations in the following respects :
Concept of Screening
Capable of wide application
Relatively inexpensive
Requires little physician time
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6. Disease
onset
detection
First
possible
point
Final critical
diagnosis
Usual time
of
diagnosis
A
B
• A: usual outcome of the disease
• B: expected outcome
• B-A: Benefits of the programme
Screening time
Lead time
• “LEAD TIME” is the period between diagnosis by early detection and diagnosis by other means.
• Detection programs should be restricted to those conditions in which there is considerable time lag between disease onset and
the usual time of diagnosis.
Concept of Lead Time
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Screening and Diagnostic Test
Screening Test Diagnostic Test
Done on apparently healthy. Done on those with indication or sick.
Applied on groups.
Applied on single patient, all diseases are
considered.
Test results are arbitrary and final.
Diagnosis is not final but modified in light of
new evidence, diagnosis is sum of all
evidence.
Based on one criteria or cut-off point
Based on evaluation of a number of
symptoms, sign and lab findings.
Less accurate. More accurate.
Less expensive. More expensive.
Not basis for treatment Used as a basis for treatment.

8. Case detection/ ‘prescriptive screening’ is defined as presumptive identification of
unrecognized disease which doesn’t arise from patient’s request.
Control of disease/ ‘prospective screening’ ie people are examined for benefit of others.
Research purposes : Screening may aid in obtaining basic knowledge about natural
history of diseases whose etiology/pathogenesis is obscure.
Educational opportunities: Helpful in creating public awareness & educating health
professionals.
Uses of Screening 8

9. Types of Screening
Mass screening: it is screening of whole population or a subgroup. It has gone out of use
nowadays.
High risk / selective screening: it is most productive because it is applied to selectively to the
high-risk group on the basis of epidemiological research
Multi phasic screening: it is application of two or more screening tests in combination to large
number of people at one time than to carry out separate screening tests for single disease
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10. Diseases to be screened should fulfill the following criteria
1. the condition should be an important health problem
2. there should be a recognizable latent or early symptomatic stage
3. the natural history of the condition, including development from latent to declared
disease, should be adequately understood
4. there is a test that can detect the disease prior to the onset of signs and symptoms,
Criteria for Screening 10

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5. facilities should be available for confirmation of the diagnosis
6. there is an effective treatment
7. there should be an agreed-on policy on whom to treat as patients
8. there is good evidence that early detection and treatment reduces morbidity and
mortality
9. The expected benefits of early detection exceed the risks and costs.

12. Screening Tests
The test must satisfy the criteria of
• acceptability,
• repeatability,
• validity
besides others like
• yield,
• simplicity,
• safety,
• rapidity,
• ease of administration &
• cost.
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13. 1. Acceptability
The test should be acceptable to all the peoples at whom it is aimed.
2. Repeatability
The test must give consistent results when repeated more than once on same
individual or material under same condition. It depends upon 3 major factors.
3 factors on which repeatability depends upon are
A. Observer variation
(a) Intra observer variation
(b) Inter observer variation
B. Biological variation
C.Errors related to technical methods
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14. A. Observer Variation
• All observations are subjected to variation
• If a single observer takes 2 measurements in the same subject, at the
same time & each time, he obtained a different result it is termed as
intra-observer variation.
• The variation between different observers on the same subject or
material is called inter-observer variation.
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15. B. Biological variation
• It is associated with physiologic variables
such as blood pressure, blood sugar etc.
• Fluctuation is due to changes in the
parameters observed; variations in the way
the patient perceive their symptom and
answer; and regression to the mean.
C. Errors related to technical methods
• Repeatability may be affected by variations
inherent in the methods.
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3. Validity
• It refers to what extent the test accurately
measures which it suppose to measure.
• Eg: glycosuria is a useful test for screening for
diabetes, but a more valid test is the glucose
tolerance test.
• It has two components sensitivity & specificity.

17. Evaluation of
Screening Tests
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Following measures are used to evaluate a screening
test :
1. Sensitivity
2. Specificity
3. Predictive value of a positive test
4. Predictive value of a negative test
5. Percentage of false negative
6. Percentage of false positive

18. Screening test result by diagnosis:
 Sensitivity = a/a+c x100
 Specificity = d/ b+d x 100
Diseased Not Diseased Total
Positive
( true positive)
a
(false positive)
b
(total positive)
a+b
Negative
(false negative)
c
(true negative)
d
(total negative)
c+d
Total
(total diseased)
a+c
(total non-
diseased)
b+d
(grand total)
a+b+c+d
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Diagnosis
Results
from
screening
tests
Example:
Point to remember: Sensitivity and specificity are calculated among columns

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Sensitivity
• The term was introduced by
Yerushalmy in 1940’s.
• It is define as the ability of a test
to identify correctly all those who
have the disease. i.e. true
positive.
Specificity
• It is define as the ability of a test
to identify correctly those who do
not have disease i.e. true
negative.

20. False negatives
 The term false negatives means that the patient who actually have the disease are told
that they do not have the disease.
 It amounts to giving them false reassurance.
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False positives
 It means that patients who do not have the disease they are told they have.
 In this case normal people are subjected to further investigations at some inconvenience,
discomfort, anxiety & expense.
 They not only are burden to diagnostic facilities, but they also bring discredit to screening
programme

21. • Screening, is a major public health determinant, measured by its effect on mortality,
morbidity & disability.
• It is necessary to ensure that the program is continuously monitored to confirm that
effectiveness is maintained. (benefits>costs)
• The construction of accurate tests that are both sensitive and specific is a key obstacle
to the wide application of screening.
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Conclusion

22. Suggested Reading
Park, Park’s Textbook of Preventive &Social Medicine, 25th Edition, Jabalpur:
Banarsidas Bhanot,2019.
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23. Expected Questions
• Differences between screening and
diagnostic tests.
• Criteria of screening
• Screening tests
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