screening of the diseases

1. Screening for
Disease
Dr Meenakshi Khapre

3. “LEAD TIME” It is the period between diagnosis by early detection
and diagnosis by other means.
Detection programmes should be restricted to those conditions in
which there is considerable time lag between disease onset and the
usual time of diagnosis.

4. “Screening is the search for unrecognized disease or defect
by means of rapidly applied tests, examinations or other
procedures in apparently healthy individuals.”
“Screening includes methods; technique; procedure and
examinations for early and rapid detection of unrecognized
disease or defect in apparently healthy persons.”
A screening program tests people for a disease or condition
before they have symptoms of the disease.
Screening for disease

5. Definitions of screening have been changing with time
& they range from simplest to most complicated ones,
one of which defines it as
‘It is a medical investigation which doesn’t arise from
patient’s request for advice for a specific complaints’
while the latest one by National Screening Committee
2nd report(2000) is
‘Screening is a public health service in which members
of a defined population who do not necessarily
perceive that they are at risk of or are already
affected by a disease or it’s complications are asked a
question or offered a test to identify those individuals
who are more likely to be helped than harmed by
further tests or treatment to reduce the risk of
disease or it’s complications’

6. The benefits of screening were 1st introduce by the use of
mass miniature radiography (MMR) for the identification of
individual with TB. With the introduction of effective
treatment for this condition after the second word war.
The use of MMR became widespread in many western
countries particularly with USA and UK.
In the late 1950’s and early 1960’s special campaign of
mass radiography were used to good effect in Scotland 1st
in Glasgow and then throughout the country to try and find
unknown and infection sufferer from TB.

7. Screening is carried out in the hope that earlier diagnosis
and subsequent treatment favorably alters the natural
history of the disease in significant proportion of those who
are identified as “positive”
The goal of this program is to discover conditions in there
earliest and most treatable stages. The program only works
for certain conditions where there is effective treatment, and
effective and sensitive screening tests. In these cases,
minimally invasive treatments can be used, and improved
life expectancy should result.
1 Control of disease 2 Researches
3 Early diagnoses 4 Educational Purposes

8. • Case detection
Also known as ‘Prescriptive screening’ it is defined as
presumptive identification of unrecognized disease which doesn't from
patient’s request.
• Control of disease
Also known as ‘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.

9. Mass Screening
It is screening of whole population or a sub group
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
no. of people at one time than to carry out separate screening tests for
single disease

10. Cont…
• Multipurpose screening
• Opportunistic screening

11. Screening tests Diagnostic tests
Done on apparently healthy.
Applied on groups.
Test results are arbitrary
and final.
Based on one criteria or
cut-off point .
Less accurate.
Less expensive.
Not basis for treatment.
Done on those with
indication or sick.
Applied on single patient,
all diseases are consider.
Diagnosis is not final but
modified in light of new
evidence, diagnosis is sum
of all evidence.
Based on evaluation of a
number of symptoms, sign
and lab findings.
More accurate.
More expensive.
Used as a basis for
treatment.

12. CRITERIA for screening for diseases
Criteria acceptable, and safe screening tests are
essential; the disease screened must be common
and should have a detectable preclinical stage, for
which effective treatment should be available; a
large proportion of people at risk should participate
in screening, investigations and treatment; the local
health services infrastructure should be
sufficiently developed to provide the diagnostic,
treatment and follow-up services.

13. Consideration for criteria
 Diseases to be screened
The tests to be applied

14. Diseases to be screened should
fulfill the following criteria
1. The condition sought
2. Latent or early asymptomatic stage
3. The natural history of condition
4. Facilites available to confirm diagnosis
5. Effective treatment
6. Policy to treat
Early detection & treatment reduces morbidity
& mortality.

15. Screening tests to be applied
The test must satisfy the criteria of
acceptability,
repeatability,
validity besides others like
yield,simplicity, safety,rapidity,ease of
adsministration & of cost.

16. Screening test
1.Acceptability
The test should be acceptible to all the
peoples at whom it is aimed.
2.Reapeatability
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.

17. Screening test
 3 factors on which repeatability
depends upon are
1.Observer variation
(a)Intra observer variation
(b)Inter observer variation
2.Biological variation
3.Errors related to technical methods

18. Repeatability
1.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.

19. Repeatability
 2.Biological variation
It is assotiated with physiologic variables such as
blood pressure,blood sugar etc.
Fluctuation is due to changes in the parameters
obs,variations in the way the patient percive their
sympt & ans & regression to the mean.
 3.errors related to technical methods
Repeatability may be affected by variations inherent
in the methods.

20. Screening test
 3.validity
It refers to what extent the test accurately
measures which it purports to measure.
Eg: glycosuria is a useful test for screening for
diabetes,but a more valid test is the glucose
tolerence test.
It has two components sensitivity &
specificity.

22. Evaluationof screeningtests
Following measures are used to evaluate a
screening test :
Sensitivity
Specificity
Predictive value of a positive test
Predictive value of a negative test
Percentage of false negative
Percentage of false positive

23. 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”.
Predictive accuracy
It reflects diagnostic power of the test.
Predictive value of a positive test reflects the probability that a patient with a positive
test result has in fact the disease in question. The more prevalent a disease is in a given
population, the more accurate will be the predictive value of a positive screening test
.

24. False negatives & False positives
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.
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

25. Summary of criteria for evaluation
of screening
• Factors
• Simplicity
• Acceptibility
• Accuracy
• Cost
• Repeatability
• Sensitivity
• Specificity
• Criteria
• Simple to perform easy to interupt &
paramedical person should be able to
perform.
• It must be acceptable to all who are
undergoing it
• It should give true measurements of
condition or symptom
• Expense of test must be considered in
relation of early detection of disease
• The test should give consistent result
in repeated trials
• The test should be capable of giving a
positive finding when individual being
is screened has condition being sought
• Test should be capable of giving
negative results when individual being
screened doesn’t condition being
sought

26. Yield
It is the amount of previously unrecognized disease that is diagnosed as a
result of the screening efforts.
It depends upon many factors – Sensitivity & specificity of the test,
prevalence of the disease, the participation of individuals in the detection
programme.
Hence to enhance the yield high risk population are usually selected.
Higher the Yield, higher is the efficiency of screening programme.
Combination of tests
Two or more tests can used in combination in order to enhance the
efficiency of screening

27. The Problem Of Borderline
• The question arises which of the two qualities
(sensitivity or specificity) is more important in
screening?
• (a) Bimodal distribution.The two curves
overlap.The shaded area will comprise a
mixture of persons with diseases & persons
without diseases.
• (b) Unimodal distribution.No line dividing
between the “normal” & “diseased”.

28. Decision Making
Prior decision is made about the cut-off point.
The following factors are taken into consideration.
(a) DISEASE PREVALENCE: When the prevalence is
high in the community,the screening level is set at a
lower level.
(b) THE DISEASE: IF the disease is very
lethal (e.g;cervical cancer,breast cancer)
and early detection markedly improves