Dr. Mohan Lal
Department of Community Medicine
Govt. Medical College ,Amritsar
Iceberg Phenomenon of Disease
Major submerged portion consists of :-
(apparently healthy individuals)
Constitute a mass of unrecognized disease in the
Responsible for the constant prevalence of disease .
Detection and a control is challenge .
1 Diseased, diagnosed &
2 Diagnosed, uncontrolled
3 Undiagnosed or wrongly
4 Risk factors for disease
5 Free of risk factors
wrongly diagnosed disease
Defined as the search for unrecognized disease
or defect by means of rapidly applied test,
examination or other procedures in apparently
Detection programmes should be restricted to those
conditions in which there is considerable time lag
between disease onset and usual time of diagnosis
Lead time is advantage gained by screening i.e. period
between diagnosis by early detection and diagnosis by
To sort out those having the disease and
those not having the disease from a group
of apparently healthy individuals.
To provide treatment to those
detected persons, so that the disease is
controlled in community
Considerations of Screening
Screening must be applied to those people
most likely to benefit.
Selection must be based on the person’s age,
sex, medical history, occupation, family
Test with greater accuracy may be more
expensive and time consuming, so choice of
test is based on compromise.
Screening should be integrated in to
existing health services
The risks as well as expected benefits must
be explained to people to be screened
Regular patient follow up is also important
Criteria for Screening
Public health importance
Recognizable early stage
Disease can be diagnosed before the onset of signs
Facilities available to confirm diagnosis
Must be effective treatment
Disease with treatment ,
Important Public health problem-
High Prevalence & Serious outcome
Recognized in early/ latent phase
Natural history well understood- Early detection and
treatment can affect the course of disease
Safe, Accurate, Cost effective, Suitable &
Agreed policy & Treatment
Cost of screening< Cost of medical care
Continuing process and not a "once for all"
Problem of borderline
In screening for disease, a prior decision is made
about the cut off point
The following factors are take into consideration:
Disease prevalence: when the prevalence is high in the
community, its screening level is set at lower level, which
will increase sensitivity
The disease: if the disease is very lethal and early
detection markedly improves prognosis, a greater degree
of sensitivity, even at the expense of specificity, is desired
Uses of Screening
Case detection (Prescriptive screening)
Control of the disease(Prospective screening)
It is also known as prescriptive screening.
Defined as presumptive identification of unrecognized
disease which does not arise from patients request. e.g.
Diseases sought include breast cancer, cervical cancer,
diabetes, tuberculosis, HDN etc.
Initiated by medical and public health personnel, they
are under special obligation to make sure that
appropriate treatment is started early.
Control of the disease
Also known as prospective screening.
People are examined for benefit of others. e.g.
screening of immigrants from infectious diseases such
as tuberculosis, syphilis, streptococcal infection etc.
The screening programme may, by leading to early
diagnosis permit more effective treatment and reduce
the spread of infectious disease and/or mortality from
Screening programs provide opportunities
for creating public awareness and for
educating health professionals. eg screening
Screening helps in obtaining basic knowledge about
natural history of chronic diseases.
Initial screening provides prevalence estimate and
subsequent screening, an incidence figure.
Where screening is done for research purposes, the
investigator should inform the study participant that
no follow up therapy will be available.
The disease to be screened should fulfill the following criteria
before its considered suitable for screening
1) The condition sought should be an important health
2) There should be a recognizable latent or early
3) The natural history of the condition, including
development from latent to declared disease, should
be adequately understood.
Probably have disease
Probably do not have disease
Have disease Do not have disease
Given treatment Under surveillance Periodic screening
Types of Screening
High –risk screening
Means screening of the whole population or a sub
It is offered to all irrespective of particular risk
individual may run of contracting disease in question.
Indiscriminate mass screening is not a useful
preventive measure unless backed up by suitable
treatment that will reduce duration of illness or alter
High risk or selective screening
Screening is most productive if applied selectively to
high risk groups, groups defined on epidemiological
Diseases tend to aggregate in family so screening other
members of family can detect additional cases.
Epidemiologists have extended concept of screening to
Multi phasic screening
Defined as application of two or more screening test in
combination to large number of people at one time
than to carry out separate screening test for single
Procedure includes health questionnaire, clinical
examination and range of measurements and
investigations. E.g. chemical and hematological tests
on blood and urine, lung function assessment,
Recently multi phasic screening has not
shown any benefit to population in terms of
mortality and morbidity reduction.
Rather it has increased the cost of health
services without any observable benefit.
Thus screening test divides the
apparently healthy population in to
two groups –
Those probably having the disease
Those probably not having disease
The first group is then further subjected to
history taking ,clinical examination and
Which divides this group into two sub-
Those who have the disease ,
Those not having the disease ,requiring
Surveillance and periodic screening
(1) Examination of urine sugar
Persons with negative result Persons with positive result
(2) Examination of random blood for sugar level
Persons with blood glucose
level <120 mgm %
Persons with blood
glucose >120 mgm %
(3) Oral glucose tolerance test
Blood sugar <120 mgm%
2 hrs after glucose
Blood sugar between 120 to
180 mgm% 2 hrs after glucose
Blood sugar > 180 mgm% 2
hrs after glucose
Impaired glucose tolerance diabetic
Screening differs from periodic
o Capable of wide application
o Relatively inexpensive and
o Requires little physician- time
Screening test Diagnostic test
Done on apparently healthy people Done on sick people
Done on groups Done on individual cases
Done by the epidemiologist Done by the physician
Test results are final Diagnosis is not final but based on other
criteria such as history and clinical findings
The purpose is to do community diagnosis
,to launch a control programme
The purpose is to make a diagnosis in the
patient to give treatment
Less expensive More expensive
Initiative is from the epidemiologist Initiative is from patient
Criteria for screening test
Simple, Safe cheap and rapidly applied
Acceptable by the people
Reliable (repeatable or reproducible)
Some Screening Tests
• Rh status
• Cardiovascular disease
• Neural tube defects
• Down syndrome
• Congenital heart disease
• Spina bifida
• visual defects
• Sickle cell anemia
Middle-aged men and women Elderly
• Diabetes mellitus
• Serum cholesterol
• Nutritional disorders
The test must satisfy the criteria of:
besides other such as yield, simplicity, safety, rapidity,
ease of administration and cost also.
Since a high rate of cooperation is necessary, its important
that the test should be acceptable to the people at whom it
The test must give consistent results when repeated more
than once. It depends on 3 major factors:
-- Observer variation
-- Biological (Subject) variation
-- Errors relating to technical methods
These are of 2 types:
a) Intra-observer variations – If a single observer takes
two measurements (e.g. BP & Chest expansion) in the
same subject, at the same time and each time he obtains
a different result, this is termed as intra-observer or
b) Inter-Observer variation – this is variation b/w diff.
observers on the same subject or material, also known
as Between-observer variation.
There is a biological availability associated with many
physiological variables such as BP, Blood sugar, Serum
cholesterol etc. The fluctuation in the variate
measured in the same individual may be due to:
a) Changes in the parameters observed. E.g. Cervical
smears taken from the same woman may be normal
one day, and abnormal on other day.
b) Variations in the way patients perceive
their symptoms and answers.
c) Regression to mean : There is tendency for
values at the extreme of distribution, to
regress toward the mean or average on
E.g. elevated serum cholesterol is associated
with high risk of developing coronary heart
In this way preventive measures can be
applied before the disease occurs.
ERRORS RELATING TO TECHNICAL
Repeatability may be affected by variations inherent in
-- Defective instruments
-- Erroneous calibration
-- Faulty reagents
-- Test inappropriate or unreliable
The term Validity refers to what extent the test accurately
measures which it purports to measure.
In other words, validity expresses the ability of a test to
separate or distinguish those who have the disease from
those who do not.
Diagnosis (screening test results)
Diseased Not diseased Total
Positive (True positive )
Negative (False negative)
Total (Total disease)
Validity has two components:
When assessing the accuracy of a diagnostic test, one
must consider both these components. Both
measurements are expressed in percentages.
Sensitivity & Specificity are usually determined by
applying the test, to one group of persons having
disease, and to a reference group not having the
To evaluate screening test:
Predictive value of positive test
Predictive value of negative test
Percentage of false negatives
Percentage of false positives
Introduced by Yerushalmy as statistical
index of diagnostic accuracy.
Defined as the ability of test to identify
correctly all those who have the disease, that
is “true positive.”
Sensitivity = x100
Percentage of diseased persons ,showing the test
Ability of a test to correctly identify those having disease (true
Ability of the test to correctly identify those not having the
Percentage of non-diseased persons showing the negative
Combination of tests
Two or more tests can be used in combination to
enhance specificity or sensitivity of screening.
For example syphilis screening (RPR test) has high
sensitivity, yet will yield false positives.
However, all those positive to RPR are then
submitted to FTA-ABS which is more specific test
and the resultant positives now truly have syphilis.
Performance of screening test is measured by its
“predictive value” which reflects diagnostic power
Predictive accuracy depends upon sensitivity,
specificity and disease prevalence.
More prevalent a disease in a given population,
more accurate will be the predictive value of a
positive screening test.
Predictive value ( + test)= x100
Percentage of positive probably having the
Predictive value (-test)= x100
Percentage of negative probably not having the
Means patients who do not have the disease are told
that they have.
It causes inconvenience, discomfort, anxiety and
expense to normal healthy people.
Screening test with high specificity will have few false
False positive = x100
Percentage of non-diseased persons wrongly
identified as having a disease
Means patients who actually have the disease are
told that they do not have the disease.
So patient might ignore the development of signs
and symptoms and may postpone the treatment.
A screening test which is very sensitive has few
Lower the sensitivity, larger the number of false
False negative = x100
Percentage of diseased persons wrongly identified
as not having the disease
That is, a proportion of false – positives is
tolerable but not false – negatives.
On the other hand, in a prevalent disease
like diabetes for which treatment does not
markedly alter outcome, specificity must be
high and false positives should be limited.
It is the amount of previously unrecognized
disease that is diagnosed as a result of screening
It depend on sensitivity and specificity of the test,
prevalence of the disease and participation of
individuals in the detection program.
Gratitude for all authors and
sources whose material has been
used for this Presentation