screening of diseases

1. SCREENING FOR DISEASES
Dr Suman B
II MDS

2. Contents
• Introduction.
• Concept of screening
• Screening test vs. diagnostic test
• Concept of lead time
• Uses of screening.
• Types of screening.
• Criteria for screening.
• Evaluation of screening programmes
• Conclusion.

3. Introduction
• Biggest Challenge in Preventive Medicine is to
distinguish between people who have the disease and
those who do not.
• Epidemiologist and others who study disease find that
the pattern of disease in hospitals is quite different
from that in a community.

4. Iceberg phenomenon of disease
• It is a concept closely related to the spectrum of disease.
• This concept gives a better idea of the progress of a disease
from its sub clinical stages to overt or apparent disease than
the familiar spectrum of disease.
• According to this concept, disease in a community may be
compared with an “ICEBERG”.

6. Concept of Screening
• The active search for disease among apparently healthy people
is a fundamental aspect of prevention.
• It is defined as “ the search for unrecognized disease or defect
by means of rapidly applied test, examinations or other
procedures in apparently healthy individuals”.
• It detects disease in its early asymptomatic phase whereby
early treatment can be given and disease can be cured or its
progression can be delayed.

7. • Screening is considered a preventive care function, and some
consider it a logical extension of health care.
• Screening differs from periodic health examinations in the
following aspects:-
– Capable of wide application
– Relatively inexpensive
– Requires little physician time.

8. SCREENING TEST vs DIAGNOSTIC TEST
Screening test Diagnostic test
1. Done on apparently healthy
individuals
2. Applied to groups
3. Results are arbitrary and final
4. Based on one criteria and cut-off
5. Less accurate and less expensive
6. Not a basis for treatment
7. Initiative comes from investigator
1. Done on sick or ill individuals
2. Applied on single patient
3. Diagnosis is not final
4. Based on evaluation of a no. of
signs/symptoms & lab findings
5. More accurate and More expensive
6. Used as a basis for treatment
7. Initiative comes from a patient

9. Concept of Lead Time
• LEAD TIME: The advantage gained by screening. (The period
between diagnosis by early detection and diagnosis by other
means.)
• In this Fig. A is the usual outcome of the disease and B is the
outcome to be expected when the disease is detected at the
earliest possible moment.

10. Aims and Objectives
• The basic purpose of screening is to sort out from a large
group of apparently healthy persons those likely to have the
disease or at increased risk of the disease under study, to bring
those who are apparently abnormal under medical supervision
and treatment.
• Possible outcomes of screening:-
Apparently healthy
(Screening tests)
Apparently normal
(Periodic re- Screening)
Apparently abnormal
a) Normal periodic re- screening
b) Intermediate surveillance
c)Abnormal treatment

11. Uses of Screening
• Four main uses have been described as
 Case Detection
 Control of disease
 Research purposes
 Educational opportunities

12. CASE DETECTION
• It is also known as “Prescriptive screening”.
• It is defined as “The presumptive identification of
unrecognized disease, which does not arise from a patients
request”.
• Example:- Neonatal screening.
• The people are screened primarily for their own benefit.
• Heel Prick Blood Sample

13. CONTROL OF DISEASE
• It is also known as Prospective screening.
• People are examined for the benefit of others.
– Screening of Immigrants from infectious diseases like
Ebola, Tb & Syphilis to protect the home population.
– Screening for HIV, STD’s
– Covid testing at Airports etc..
• Screening programme may, by leading to early diagnosis
permit more effective treatment and reduce the spread of
infectious disease and mortality.

14. RESEARCH PURPOSES
• To know the history of many chronic diseases like cancer,
Hypertension etc.
• Screening may aid in obtaining more basic knowledge about
the natural history of such diseases.
• Initial screening provides a prevalence estimate and
subsequent screening provides an incidence figure.

15. EDUCATIONAL OPPORTUNITIES
• Screening programmes help in:-
- Acquisition of information of public health relevance.
- Providing opportunities for creating public awareness.
- For educating health professionals.

16. Types of Screening
Three types of screening have been described:-
1. Mass Screening
2. High Risk or Selective Screening
3. Multiphasic Screening

17. MASS SCREENING
• It Simply means the screening of a whole population or a sub
group.
• Application of screening test to large, unselected population.
Everyone in the group is screened regardless of the probability
of having the disease or condition.
a) Visual defects in all school children
b) Colonoscopy for occult blood.
• Indiscriminate mass screening is not a useful preventive
measure unless it is backed up by suitable treatment that will
reduce the duration of illness or alter its final outcome.

18. HIGH RISK / SELECTIVE / TARGETED
SCREENING
• The screening will be most productive if applied selectively to high
risk groups.
a) Screening fetus for Down’s syndrome in a mother who already has a baby
with Down’s syndrome.
b) Screening for familial cancers, HTN and DM.
c) Screening for CA Cervix in lower social group women.
• Epidemiologists have extended the concept of screening for disease
to screening for risk factors, as these factors apparently antedate the
development of actual disease.
• Besides effectiveness, economical use of resources will also occur
if the screening tests are selectively applied to individuals in high
risk group.

19. MULTIPHASIC SCREENING
• It is defined as the application of 2 or more screening tests in
combination to a large number of people at one time than to
carry out separate screening tests for single diseases.
• In this screening various diagnostic procedures such as
questionnaire, clinical examination and a range of
measurements and investigations are employed during the
same screening program.
• On the other hand, it has increased the cost of health services
without any observable benefit and most of the tests have not
been validated.

20. CRITERIA FOR SCREENING
• Before initiating a Screening Programme, a decision must be
made whether it abides to all the ethical, scientific and
financial justification.
• The principles that should govern the introduction of screening
programmes were first enunciated by Wilson and Junger
(1968).
• The Criteria for Screening is based on two considerations:
– DISEASE.
– SCREENING TEST.

21. DISEASE
• The Disease should be important Health problem (High
Prevalence).
• Disease should have Long & Detectable Preclinical stage.
• The Natural history of disease should be adequately
understood.
• Appropriate test must be available for early detection of
disease (before signs and symptoms appear).

22. • Facilities must be available for Confirmation of disease.
• Early detection of disease and treatment should be able to
reduce Mortality & Morbidity.
• The disease should be treatable, and there should be a
recognized treatment for lesions identified following
screening.
• Expected benefits must exceed risks and costs.
• A Policy should be agreed on, concerning whom to treat as
patients.

23. SCREENING TEST
 The test must satisfy the criteria of
 Acceptability
 Repeatability(Reliability)
 Validity
 Besides others such as
a) Simplicity (Inexpensive & Easy to Apply)
b) Safety
c) Rapidity
d) Yielding

24. Acceptability
• Since a high rate of cooperation is necessary, it is important
that the test should be acceptable to the people at whom it is
aimed.
• In general tests that are painful, discomforting or embarrassing
are not likely to be acceptable to the population in mass
campaigns.
E.g., Screening for prostrate cancer might not be
acceptable to a large proportion of the community.

25. Repeatability(Reliability)
• An attribute of an ideal screening test or any measurement is
its repeatability.
• It is also called as Reliability, Precision or Reproducibility.
• It means the test must give consistent results when repeated
more than once.
• It depends upon 3 major factors:-
– Observer variation.
– Biological (or Subject) variation.
– Errors relating to technical methods.

26. Observer variation
• All observations are subjected to variation(or Error).
• These may be of 2 types:-
– INTRA-OBSERVER VARIATION
– INTER-OBSERVER VARIATION

27. • INTRA-OBSERVER VARIATION( Within-observer)
This is the Variation in the results of the test due to the same
observer examining the result at different times.
E.g: Two readings of Blood pressure by the Same observer.
• INTER-OBSERVER VARIATION( Between-observer)
This is the Variation in the results of the test due to the multiple
observers examining the result.
E.g: Chest X ray read by two different Radiologists

28. Biological (or Subject) variation
• There is a biological variability associated with many
physiological variables such as blood pressure, blood sugar,
serum cholestrol, etc
• The fluctution in the variate measured in the same individual
may be due to:
a) Changes in the parameters observed.
b) Variations in the way patients perceive their symptoms and
answer.
c) Regression to the mean.

29. Errors relating to technical methods.
• Lastly, repeatability may be affected by variations inherent in
the method.
– E.g,: Defective instruments, erroneous calibration, faulty
reagents or the test itself might be unreliable.
• Where these errors are large, repeatability will be reduced and
a single test result may be unreliable.

30. Validity
• Validity determines the Accuracy of the Test.
• Accuracy refers to the closeness with which measured values
agree with true values.
• Validity expresses the ability of a test to separate or distinguish
those who have the disease from those who do not.
• COMPONENTS OF VALIDITY
Sensitivity Specificity

31. WHAT IS VALID AND RELIABLE?
• VALIDITY :- The accuracy of a test.
• RELIABILITY :- The precision of a test.
• ACCURACY: “how close is result of a test to its true value?”
• PRECISION: “how close are the results of a test on
repetition?”

32. SENSITIVITY
• Introduced by Yerushalmy (1940) as a statistical index of
diagnostic accuracy.
• The ability of a test to correctly identify those who have the
disease (True Positives).
• “Proportion of Truly ill Population”.
Expressed as percentage
Sensitivity= TP / TP+FN
OR
Sensitivity= a/a+c

33. SPECIFICITY
• The ability of a test to correctly identify those who do not have
the disease. (True Negatives).
• Proportion of Truly Healthy Population.
Expressed as percentage
Specificity= TN / TN+FP
OR
Specificity = d / d+b

34. Example
An Ideal Screening test should have 100% sensitivity and 100%
specificity. ( Practically not possible )

35. PREDICITVE ACCUARCY
• In addition to sensitivity and specificity, the performance of a
screening test is measured by its “ Predictive value” which
refeclts the diagnostic power of the test.
• Positive Predictive Value: The Proportion of the people who is
screened Positive that actually have the disease. (Are the
people with disease correctly identified
• Negative Predictive Value: The Proportion of the people who
is screened Negative that are actually FREE of the disease.
(Are the people without disease correctly identified?)
• These Values are not fixated for a particular test.

37. • Predictive accuracy depends on
– Prevalence of the Disease.
– Sensitivity and Specificity of the Test.
• Increase in Sensitivity causes a modest increase in PPV, but
increase in Specificity raises PPV markedly.
• More prevalent diseases has high PPV, that’s why screening is
more efficient & productive, If done in High risk population.

38. Yield
• Yield is the amount of previously unrecognized disease that is
detected and brought to treatment as a result of Screening.
• It depends on prevalence of the disease and sensitivity and
Specificity of the screening test, participation in the
programme.
• Hence, yield of a screening test is high in high-risk screening.
– E.g, By limiting a diabetes screening programme to persons
over 40 years, we can increase the yield of the screening
test.

39. • Whereas the epidemiologist thinks in terms of sensitivity and
specificity, the clinician thinks in terms of false negatives and false
positives.
• FALSE NEGATIVES: If a Person with disease is labelled Negative.
– False reassurance
– Ignores any disease signs and symptoms
– Postponement of treatment.
– Detrimental to overall health
• FALSE POSITIVES: If a Person without disease is labeled Positive.
- Further testing with long, expensive tests.
- Discomfort, inconvenience, anxiety
- Burden on health facilities
- Emotional trauma
- Difficulty in “de-labeling”

40. BIMODAL DISTRIBUTION &
UNIMODAL DISTRIBUTION
•If the disease is bimodal, as may be
expected in certain genetically
transmitted characteristics, the border
line group will comprise a mixture of
persons with disease and persons
without the disease.
•The point at which the distributions
intersect is frequently used as the cut off
point.
•In unimodal distribution their values are
continuously distributed around the
mean, confirming to normal or skewed
distribution.

41. CONCEPT OF CUT-OFF POINT -
• Unlike in Bimodal Distribution(Dichotomous), Some diseases
comes in Continuous Variables (Ex: Diabetes, HTN).
• In these Cases, It is difficult to calculate Sensitivity &
Specificity. -So, A Cut Off Point must be set to distinguish
between Positive and Negative Result.
 Consider 20 diabetics and 20 Non-diabetics
screened using a blood sugar test – Vertical axis
From Low to High.

42. Low Cut-Off Point
• False Positives originate (More Non-diabetics are diagnosed
positively)
Sensitivity= 85%
Specificity= 20%

43. High Cut-Off Point
• False Negatives originate (More diabetics are not diagnosed
positively)
Sensitivity= 25%
Specificity= 90%

44. • SO,
• Different Cut-off points yield different sensitivities and
specificities.
• The cut off point that identifies more true negatives will also
identify more false negatives.
• The cut off point that identifies more true positives will also
identify more false positives.
 The choice of a high or low cut off level for screening
therefore depends on the importance we attach to FPs or FNs.
 In case of Lethal diseases (Early Intervention possible) Cut off
point must be set at Low level , as Greater sensitivity is
required. (False Positives can be tolerated)

45. EVALUATION OF SCREENING
PROGRAMMES
• 1. Experimental :-
• Conduct an RCT of the screening test to compare the disease
specific cumulative mortality rate between the intervention and
control group.
• Problems include- Long follow up, Costs and record keeping.
• Allows study of distribution of lead time, effects of early
treatment and identification of prognostic factors.

46. • 2. Non – experimental :-
• Cohort study (comparison of advanced disease or death rates
in those who choose to screen and those who do not)
• Case - control study (comparison of screening history in those
who have advanced disease and those who are healthy)
• Ecological study (correlation of screening pattern and disease
experience of several populations)

47. Conclusion
• Screening, despite its flaws, is a major public Health
determinant, measured by its effect on Mortality, Morbidity &
Disability.
• Establishing appropriate criteria requires considerable
knowledge of the Natural history of disease, adequate facilities
for follow up & Rx.
• It is necessary to ensure that the program is continuously
monitored to confirm that effectiveness is maintained.
• Newer fields such as genetic screening are on the rise which
would help the cause.

48. References
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Lilienfeld, et al
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Studies No. 30
5. Cochrane, A L and Holland, W.w(1971). Br.Med. Bull,
6. Roberts, C J (1977) Epidemiology for clinicians, Pitman Medical.
7. WHO (1981) World Health Forum
8. Garfield, S. R(1970)N,Eng. J Med.
9. http://www.med.uottawa.ca/sim/data/Screening_e.htm
10. https://www.slideshare.net/VishnuYenganti/screening-for-diseases