Screening for Disease.pptx

Screening for
Disease
- Dr. Ritu Randad
- II-year MDS
- 13/01/2021
1

Content list
• Introduction
• Definitions: . Health, Disease,
screening
• Concept of screening.
• Concept of lead time.
• Iceberg phenomenon of Disease.
• Aim and objectives
• Uses of screening
2

• Types of screening
• Test for screening
• Variations in screening
• Bias in screening
• Specificity and sensitivity
• Borderline problems
• Conclusion
• References
3

Introduction
• Globally morbidity patterns are changing rapidly
and are closely linked with lifestyle, lack of
adequate physical activity, excessive use of tobacco
and increased utilization of alcohol.
• With addition of socioenvironmental
determinants, oral diseases are largely related to
the risk factors of morbidity and mortality.
4
K. Park, Park’s textbook of Preventive and Social Medicine, 23rd edition, 2013.

Introduction
• Healthy lifestyles in turn acts as protective
factors.
• Most of the diseases are preventable to a larger
extend if detected earlier.
• Oral diseases are major public health problems
owing to their high prevalence and incidence in all
the regions of the world, mainly in under privileged
and socially marginalized populations.
5

Introduction
• The severe impact of pain and suffering,
impairment of function effects the quality
of life.
• Traditional treatment of oral disease is
extremely costly in several industrialized
countries and not feasible in most low- income
and middle-income countries.
• Hence, periodic screening is recommended
at periodic time interval for diseases.
6

What is Health?..
• The World Health Organization
claims that health is “a state of
complete physical, mental and
social well-being, not merely the
absence of disease or infirmity, the
ability to lead a socially and
productive life.”
WHO (1946) Preamble to the Constitution of the World Health Organization. WHO, New York, USA
7

What is Disease?..
• “Disease” in literal terms mean without
ease. Disease (uneasiness) happens when
something is wrong with the body
function.
• “Illness” refers to not only presence of a
specific disease, but also to the
individual’s perceptions and behavior in
response, as well as the impact of that
disease on the physiological environment.
SS. Hiremath, Concepts of Health and Disease and Prevention, Textbook of Public Health
Dentistry, Elsevier, 3-16, 3rd edition
8

Screening
• Morrison defines screening as
follows: “Screening for disease is the
examination of asymptomatic people
in order to classify them as likely or
unlikely to have the disease.
• People who appear likely to have a
disease are investigated further to
arrive at a final diagnosis. Those
people who are found to have the
disease are then treated.”
Morrison AS. Screening in chronic disease. Oxford: University Press. 1992
9

Concept of screening
• Active search for disease among apparently healthy people is a
fundamental aspect of prevention.
• Traditionally, the health examinations were meant for early
detection of “hidden” disease.
• When this had to be brought to large population with minimal
expenditures of time and money along with alternative
approaches have come to it use.
10
SS. Hiremath, Concepts of Health and Disease and Prevention, Textbook of Public Health Dentistry, Elsevier, 3-16, 3rd edition

• Based on conserving physicians time for diagnosis and treatment
and having technicians to administer simple, inexpensive
laboratory tests and operating other devices.
• Initially the screening test were for individual diseases but over
years it has grown to be considered as
1. preventive care function and
2. extension for health care.
11

Screening differs from Periodic Health
Examination
1. Capable of wide application
2. Relatively inexpensive
3. Less physician time.
12
K. Park, Park’s textbook of Preventive and Social Medicine, 25th edition, Madhya Pradesh.

Concept of “Lead Time”
13
A
Disease
onset
detection
First
possible
point
Final
critical
diagnosis
Usual
time of
diagnosis
Screening time
Lead time
OUTCOME
FIG 1: model for early detection programs.

• “Lead time”- advantage gained by screening.
• It is period between diagnosis by early detection and diagnosis by
other means.
• A stands for usual outcome of disease, and B for outcome to be
expected when disease is detected at the earliest moment.
• The benefit of program is seen in terms of its outcome.
14

• Detection programs should be restricted to those conditions in which
there is considerable TIME LAG between onset of disease and usual
time of diagnosis.( severity of disease and success of treatment)
• Detection programs should concentrate on the conditions where time
lag between disease’s onset and its final critical point is sufficiently
long enough for population screening process.
15
K. Park, Park’s textbook of Preventive and Social Medicine, 25th edition, 23rd edition, 2013.

ICEBERG
PHENOMENON
OF DISEASE
16
Physician sees-
clinical cases
Hidden mass of
disease: latent, Pre-
-symptomatic or
undiagnosed cases
Submerg
ed
portion of
iceberg
Demarcation btw
apparent and
inapparent disease

• Submerged portion represents the unknown morbidity of
undiagnosed reservoir of infection/disease in the community, and
its detection and control is a challenge preventive medicine.
• The major deterrents of disease of unknown etiology is absence of
methods to detect the subclinical state (bottom of iceberg).
17

Aims and objectives of Screening
Apparently healthy (Screening tests)
Apparently abnormal
Normal –
periodic re-
screening
Intermediate-
surveillance
Abnormal
treatment
Apparently normal (Periodic re-screening)
18

• To sort out from a large group of apparently healthy people those likely to
have disease or at risk of disease under study.
• To bring out “apparently abnormal” under medical supervision and
treatment.
19

Uses of screening
Uses
of
screening
Case detection
Control of
disease
Research
purpose
Educational
opportunities
20

a. Case Detection
Prescriptive study.
Presumptive identification of
unrecognized disease, not arising from
patient’s request.
Neonatal screening, breast cancer,
deafness in children, pulmonary
tuberculosis
Since disease detection is carried out by
medical and public health personnel,
under special obligation the suitable
treatment should be started at the
earliest.
21
K. Park, Park’s textbook of Preventive and Social Medicine23rd edition, 2013.

b. Control of disease
Prospective screening
Benefit for others; screening for
immigrants from infectious disease like
tuberculosis, AIDS for protection to
home population.
Screening may lead to early diagnosis
permitting more effective treatment and
reducing the [spread of the
condition/mortality of the disease
affecting].
22

c. Research Purpose
• Screening may aid in obtaining more basic knowledge about the
natural history of chronic diseases, providing prevalence and
incidence of a disease.
• When screening is done for a research purpose, the investigator
should inform the participants that no follow-up therapy will be
provided.
23

d. Educational Opportunities
• Apart for benefits to individual and acquisition of information of
public health relevance, screenings provide opportunities for
creating public awareness and educating health professionals.
24

Types of screening
25
Mass
Screening
High Risk/
Selective
Screening
Multiphasic
Screening

Mass screening
Screening of whole
population/subgroup
Risk of contracting
the disease in
individual
Had support in past
Not useful unless
preventive
measures with
suitable treatment to
alter the outcome. 26

High-risk / selective screening
Selectively applied to
high risk groups
One subgroup where
certain disease
progresses, screening
other members can
detect additional cases
“risk factors” –
antedate for
development of actual
disease
Preventive measures
can be applied before
disease occurs
Economical use of
resources would occur
if screening applied
effectively
27

Multiphasic screening
Combination of two or
more tests
Large population of
people at the one time
than to carry separate
tests for single
diseases.
Can include
questionnaire, clinical
examination and range
of measurements and
investigations
No benefit with
mortality and morbidity
reduction
Increased cost of
health services without
observable benefit.
28

Criteria for screening
1. The condition sought should be an important health problem (prevalence
should be high).
2. Recognizable latent or early asymptomatic stage.
3. Natural history of condition, including development from latent to
declared disease, should be adequately understood (to know at what stage
is it reversible)
4. To detect the disease prior to onset of signs and symptoms.
29
DISEASE
TEST
ETHICAL,
SCIENTFIC and
FINANCIAL
JUSTIFICATION

6. Facilities available for conformation of the diagnosis.
6. Effective treatment
7. Should have agreed-on policy concerning whom to treat as patients.
8.Good evidence that early detection and treatment reduces mortality and
morbidity.
9.Expected benefits of early detection exceeds the risks and costs.
30
When the above criteria are satisfied, then only, it
is appropriate to consider a suitable screening
test.

Bias in screening
Bias in screening
Selection bias
Referral/ volunteer
bias
Length-biased
sampling/
prognostic selection
Lead time bias
Over diagnosis bias
31

1a. Referral bias/ volunteer bias
Bias with who has been
screened and who has
been not.
Assumption that people
been screened will all
have same
characteristics as those
who are not
Unknown fact that in
which direction the
selection bias might
operate and how it
affects the study.
Interpretation of findings
can be addressed by
carrying out RCT in
which two groups have
comparable prognostic
profiles. 32

1b. Selection biased
Depends upon type of
disease that is detected.
Screening tends to
selectively identify those
cases that have loner
preclinical phases of
illness.(longer clinical,
better prognosis).
Survival should be
examined for all
members of each group
Survival should be
calculated for those in
whom disease is
detected by screening
also in disease
detected during
screening examinations
INTERVAL CASES.
33

Lead time bias
Illusion of better survival
because of earlier detection
is lead time bias
Even if there is no benefit
with early detection of
disease, there will be benefit
associated with screening.
Lead time associated with
detection indicates
appearance of benefit in the
form of enhanced survival.
It is result of diagnosis being
made at earliest point in
natural history of the
disease.
Early detection is associated
with improved survival,
survival in screened group
should always be greater
than survival in control group
plus the lead time.
34

Over diagnosis bias
People initiated the
study may lack
enthusiasm for the
program
Due to overrating of the
sample, at times
abnormal people are
grouped with people
who are free of disease.
False impression of
increased rate of
detection and diagnosis
of early–stage disease
as a result of screening.
The essential part is
that the diagnostic
process be rigorously
standardized.
35

Screening test criteria
Acceptability
Repeatability
Ease of
administration
Safety
Validity
(accuracy)
Yield
Simplicity
Cost
36

Acceptability
• Acceptable to people at whom it is aimed.
• Test which are painful, discomforting or embarrassing not
acceptable to the population is mass campaigns.
37

Repeatability/ precision/ reproducibility
• Attribute of an ideal screening test or any measurement is its
repeatability.
• Test gives consistent result when repeated more than once on
the same individual/material, under same conditions.
1. Observer variation
2. Biological/ subject variation
3. Errors relating to technical methods.
38

Intra- observer/ within- observer
variation
• Single observer taking 2
measurements in same
subject, at same time and each
time , he gets a different result.
• Can be minimized by taking
the average of several replicate
measurements at the same
time.
Inter-observer / between-observer
variation
• Variations between different
observers on the same
material/ subject.
• Minimized by :
• 1) standardization of procedures
for obtaining measurements and
classifications
• 2) intensive training of all
observers
• 3)making use of two/ more
observers for independent
assessment.
39

Biological/ subject variation
• A) changes in the parameters observed.
• B) variations in the way patients perceive their symptoms and answer.
• C) regression to the mean.
There is a tendency for values at the extremes of a distribution,
either very high/low, to regress towards the mean or average on
repeat measurements.
Biological variation is tested by repeated measurements over
time.
40

Validity (accuracy)
• Refers to what extend the test accurately measures which it is
suppose to measures.
• Ability of a test to separate or distinguished those who have the
disease from those who do not.
• Accuracy refers to the closeness with which measured values
agree with “true” values.
41
SPECIFICITY
SENSITIVITY

Sensitivity
Yerushalmy in 1940’s
Statistical index of
diagnostic accuracy
Ability of a test to identify
correctly all those who
have disease, is “true
positive”
90% of diseased people
screened means that 90
per cent of diseased
people will give “true
positive” result
Rest, 10 per cent people
will give “false negative ”
result.
42

“False negative”
• Means that patients who actually have the disease are told that they do not have
the disease.
• Amounts to giving them “false reassurance”.
• Can lead to postponing of the treatment as the patient might ignore the signs
and symptoms, can lead to detrimental if disease is serious.
• The lower the sensitivity, the larger will be the number of false negatives.
43

• Take a pregnancy test and it comes back as negative (not
pregnant). However, you are in fact, pregnant.
• A test for cancer might come back negative, when you have the
disease.
• Quality control in manufacturing; a false negative in this area
means that a defective item passes through the cracks.
• In software testing, a false negative would mean that a test
designed to catch something (i.e. a virus) has failed.
44

Specificity
• Ability of a test to identify correctly those who do not have the
disease, “true negatives”.
• 90 per cent specificity means that 90 per cent of the non-diseased
persons will give “true-negative” result, 10 per cent of non-
diseased people screened by the test will be wrongly classified as
“diseased” when they are not.
45

In dealing with diagnostic tests that yield a quantitative result
the situation is different.
There will be overlapping of the distribution of the attribute for
diseased and non-diseased persons, false positives and false
negatives comprise the area of the overlap.
When this overlap happens, it is not possible to assign
individuals correctly with the values to either normal or the
diseased group on basis of screening test alone.
46

“False- positive”
• Refers to that patients who do not have the disease are told that
they have the disease.
• A screening test with a high specificity will have few false
positives, but they also bring discredit to screening programes.
• In fact, no screening test is prefect, 100 per cent sensitive or 100 per
cent specific.
47

• A pregnancy test is positive, when in fact you aren’t pregnant.
• A cancer screening test comes back positive, but you don’t
have the disease.
• A prenatal test comes back positive for Down’s Syndrome,
when your fetus does not have the disorder.
• Virus software on your computer incorrectly identifies a
harmless program as a malicious one.
48

Predictive accuracy
• Performance of a screening test is also measured by its
“predictive value” which reflects the diagnostic power of the
test.
49
specificity
sensitivity
Disease prevalence

• The “predictive value of a positive test” indicates the probability that a
patient with a positive test result has, in fact disease in question.
• The more prevalent a disease in given population, the more accurate will be
the predictive value of a positive screening test.
• The predictive value of a positive test falls as disease prevalence
declines.
50

Yield
• previously unrecognized disease that is diagnosed as
a result of screening efforts.
51
specificity
sensitivity
Prevalence of disease
Participation of the individual in the
detection program
High risk
population
are usually
selected for
screening,
thus
increasing
the yield.

Border line problems..
52
Overlap
The point at which
distribution interest
is frequently used as
the cut-off point
between the
“normal” and
“diseased” persons,
because it will
generally minimize
the false negative
and false positives.
No sharp dividing
line between
“normal” and
“diseased”.

• In screening for disease, a prior decision is made the cut-off
point, based on which individual are classified as “normal” and
“diseased”.
• Disease prevalence
The disease
53

• Points which must be taken into account in screening are :
1. People who participate in the program may not be those who
have most to gain from it.
2. Tests with greater accuracy may be more likely to more
expensive and time-consuming, and the choice therefore often
be based on compromise.
54

• Screening should not be developed in isolation; it should be
integrated into the existing health care services.
• The risks as well as the expected benefits must be explained to the
people who are to be scanned. These results include any possible
complications of examination procedures, and the possibility of
false positives and false negatives test results.
55

Conclusion
• “Health should mean a lot more
than escape from death or, for
that matter, escape from
disease”.
• The screening concept, filled
with potential has been
overburdened with problems,
many of which remain unsolved.
The construction of accurate
tests that are both sensitive and
specific is a key obstacle to the
wide application of screening. 56

Name of the article Level of evidence Results Conclusion
Young GP,
Recommendations for a
step-wise comparative
approach to the evaluation
of new screening tests for
colorectal cancer.
3a A 4‐phase evaluation is
recommended. An initial
retrospective evaluation in
cancer cases and controls
(Phase 1) is followed by
performance across the
continuum of neoplastic
lesions (Phase 2). Phase 3
follows the demonstration
of adequate accuracy in
these 2 prescreening
phases and addresses
programmatic outcomes at
1 screening round on an
intention‐to‐screen basis.
Phase 4 involves more
comprehensive evaluation
over multiple rounds.
New screening tests can
be evaluated efficiently by
this stepwise comparative
approach.
57

References
• WHO (1946) Preamble to the Constitution of the World Health
Organization. WHO, New York, USA.
• SS. Hiremath, Concepts of Health and Disease and Prevention,
Textbook of Public Health Dentistry, Elsevier, 3-16, 3rd edition
• Morrison AS. Screening in chronic disease. Oxford: University
Press. 1992
• K. Park, Park’s textbook of Preventive and Social Medicine, 25th
edition, Madhya Pradesh, 2017.
• K. Park, Park’s textbook of Preventive and Social Medicine, 23rd
edition, 2013.
58

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