2. Learning Objectives
• Understand different stages of disease
development.
• Understand levels of prevention and stages
of disease development to phases of
prevention.
• Modes of intervention: Describe advantages
and disadvantages of population and high-
risk prevention strategies.
5. Disease Process
• Diseases and other phenomena of interest in
epidemiology are processes, not events.
• Every disease has a natural course of progression.
6. Disease Process
• Therefore, defining, observing and measuring
health and disease require understanding of
concept of “natural history”:-
“the evolution of a pathophysiologic
process”
9. Primordial Prevention
“Prevention of the emergence of living patterns
that contribute to increased risk of disease
(e.g. the maintenance of low-fat diets in
traditional societies)”
10. Primordial Prevention
• Understanding of CVD epidemiology.
• Dietary patterns
• Socioeconomic development -> more
widespread risk factors
• Have we missed the boat?
11. • The goals of medicine are to promote health,
to preserve health, to restore health when it is
impaired, and to minimize suffering and
distress
• These goals are embodied in the word
"prevention"
12. Prevention; Definition and Concept
• Actions aimed at eradicating, eliminating or
minimizing the impact of disease and disability, or if
none of these are feasible, retarding the progress of
the disease and disability
• The concept of prevention is best defined in the
context of levels, traditionally called primary,
secondary and tertiary prevention. A fourth level,
called primordial prevention, was later added
13. Determinants of Prevention
• Successful prevention depends upon:
– a knowledge of causation,
– dynamics of transmission,
– identification of risk factors and risk groups,
– availability of prophylactic or early detection and
treatment measures,
– an organization for applying these measures to
appropriate persons or groups, and
– continuous evaluation of and development of procedures
applied
14. Preventable Causes of Disease
BEINGS
Biological factors and Behavioral Factors
Environmental factors
Immunologic factors
Nutritional factors
Genetic factors
Services, Social factors, and Spiritual factors
[JF Jekel, Epidemiology, Biostatistics, and Preventive Medicine, 1996]
15. Leavell’s Levels of Prevention
Stage of disease Level of prevention Type of response
Pre-disease Primary Prevention Health promotion and
Specific protection
Latent Disease Secondary prevention Pre-symptomatic
Diagnosis and
treatment
Symptomatic Disease Tertiary prevention •Disability limitation for
early symptomatic disease
•Rehabilitation for late
Symptomatic disease
16.
17. Primordial prevention
• Primordial prevention consists of actions and
measures that inhibit the emergence of risk
factors in the form of environmental,
economic, social, and behavioral conditions
and cultural patterns of living etc.
18. Primordial prevention (cont.)
• It is the prevention of the emergence or
development of risk factors in countries or
population groups in which they have not yet
appeared
• For example, many adult health problems (e.g.,
obesity, hypertension) have their early origins in
childhood, because this is the time when
lifestyles are formed (for example, smoking,
eating patterns, physical exercise).
19. Primordial prevention (cont.)
• In primordial prevention, efforts are directed
towards discouraging children from adopting
harmful lifestyles
• The main intervention in primordial
prevention is through individual and mass
education
20. Primary prevention
• Primary prevention can be defined as the action
taken prior to the onset of disease, which removes
the possibility that the disease will ever occur.
• It signifies intervention in the pre-pathogenesis
phase of a disease or health problem.
• Primary prevention may be accomplished by
measures of “Health promotion” and “specific
protection”
21. Primary prevention (cont.)
• It includes the concept of "positive health", a
concept that encourages achievement and
maintenance of "an acceptable level of health that
will enable every individual to lead a socially and
economically productive life"
• Primary prevention may be accomplished by
measures designed to promote general health and
well-being, and quality of life of people or by
specific protective measures
22. Primary prevention
Specific protection
Health promotion
Achieved by
Health education
Environmental modifications
Nutritional interventions
Life style and behavioral changes
Immunization and seroprophylaxis
chemoprophylaxis
Use of specific nutrients or supplementations
Protection against occupational hazards
Safety of drugs and foods
Control of environmental hazards,
e.g. air pollution
23. Health promotion
• Health promotion is “ the process of enabling
people to increase control over the
determinants of health and thereby improve
their health”.
24. Approaches for Primary Prevention
• The WHO has recommended the following
approaches for the primary prevention of
chronic diseases where the risk factors are
established:
–a. Population (mass) strategy
–b. High -risk strategy
25. Population (mass) strategy
• “Population strategy" is directed at the whole population
irrespective of individual risk levels.
• For example, studies have shown that even a small
reduction in the average blood pressure or serum
cholesterol of a population would produce a large reduction
in the incidence of cardiovascular disease
• The population approach is directed towards socio-
economic, behavioral and lifestyle changes
26. High -risk strategy
• The high -risk strategy aims to bring
preventive care to individuals at special risk.
• This requires detection of individuals at high
risk by the optimum use of clinical methods.
27. Secondary prevention
• It is defined as “ action which halts the progress of a disease at its
incipient stage and prevents complications.”
• The specific interventions are: early diagnosis (e.g. screening tests, and
case finding programs….) and adequate treatment.
• Secondary prevention attempts to arrest the disease process, restore
health by seeking out unrecognized disease and treating it before
irreversible pathological changes take place, and reverse
communicability of infectious diseases.
• It thus protects others from in the community from acquiring the infection
and thus provide at once secondary prevention for the infected ones and
primary prevention for their potential contacts.
28. Secondary prevention (cont.)
• Secondary prevention attempts to arrest the
disease process, restore health by seeking out
unrecognized disease and treating it before
irreversible pathological changes take place, and
reverse communicability of infectious diseases.
• It thus protects others from in the community from
acquiring the infection and thus provide at once
secondary prevention for the infected ones and
primary prevention for their potential contacts.
29. Early diagnosis and treatment
• WHO Expert Committee in 1973 defined early
detection of health disorders as “ the detection of
disturbances of homoeostatic and compensatory
mechanism while biochemical, morphological and
functional changes are still reversible.”
• The earlier the disease is diagnosed, and treated
the better it is for prognosis of the case and in the
prevention of the occurrence of other secondary
cases.
30. Tertiary prevention
• It is used when the disease process has advanced
beyond its early stages.
• It is defined as “all the measures available to
reduce or limit impairments and disabilities, and to
promote the patients’ adjustment to irremediable
conditions.”
• Intervention that should be accomplished in the
stage of tertiary prevention are disability limitation,
and rehabilitation.
32. Impairment
• Impairment is “any loss or abnormality of
psychological, physiological or anatomical
structure or function.”
33. Disability
• Disability is “any restriction or lack of ability
to perform an activity in the manner or within
the range considered normal for the human
being.”
34. Handicap
• Handicap is termed as “a disadvantage for a
given individual, resulting from an impairment
or disability, that limits or prevents the
fulfillment of a role in the community that is
normal (depending on age, sex, and social
and cultural factors) for that individual.”
35. Rehabilitation
• Rehabilitation is “ the combined and
coordinated use of medical, social,
educational, and vocational measures for
training and retraining the individual to the
highest possible level of functional ability.”
36. Primary Prevention
• The Population Strategy
–Advantages:-
• Radical
• Large potential for population
37. Primary Prevention
• The Population Strategy
–Disadvantages:-
• Small benefits to individuals
• Poor motivation of subject
• Poor motivation of physician
• Benefit-to-risk ratio may be low as
compared to targeting high-risk group
38. Primary Prevention
• The High-risk Strategy
–Advantages:-
• Appropriate to individuals
• Subject motivation
• Physician motivation
• Benefit-to-risk ratio is favourable
39. Primary Prevention
• The High-risk Strategy
–Disadvantages:-
• High screening costs
• Temporary effects
• Limited effect
40. Levels of Prevention
Level of Prevention Phase of Disease Target
Primordial Underlying conditions
leading to causation
Total population and
selected groups
Primary Specific causal factors Total population, selected
groups and healthy ind’s
Secondary Early stage of disease Patients
Tertiary Late stage of disease Patient
41. Iceberg/pyramid of disease
• In most diseases, as with the iceberg, the larger
presence lurks unseen, unmeasured and easily
forgotten.
• Figure illustrates this idea and develops the iceberg
concept in the form of a pyramid of disease by using its
clear structure and shape.
• Blocks 1 and 2 correspond to the iceberg above the
sea-level and 3 to 5 below sea level.
• Epidemiology that forgets the iceberg phenomenon of
disease is weak and potentially misleading.
42. The pyramid and iceberg of disease
1 Diseased, diagnosed & controlled
2 Diagnosed, uncontrolled
3 Undiagnosed or wrongly
diagnosed disease
4 Risk factors for disease
5 Free of risk factors
Diagnosed disease
Undiagnosed or
wrongly diagnosed disease
43. Iceberg/pyramid of disease
• Unidentified cases may be different to identified ones, both in terms
of the natural history or spectrum of disease.
• Where symptoms and disease progression and outcome are related,
the undiagnosed cases are likely to be less severe.
• When symptoms and signs are not evident in the early stages of
disease, as in high blood pressure or chronic glaucoma, undiagnosed
cases may be just as severe as diagnosed ones.
• Epidemiological studies based on selected cases from the tip of the
iceberg may give an erroneous view.
44. Iceberg/pyramid : severity of
disease
• Prostate cancer based on cases diagnosed in hospital would lead to
the view that the disease is usually, if not always, progressive.
• Unselected cases show that prostatic cancer can in some cases be a
static, or slowly progressive, phenomenon.
• Patients who are at the tip of the iceberg are more likely to have
multiple health problems than others.
• People with cardio-respiratory problems and diabetes are more likely
to be admitted to hospital, than people with only one of these two
problems. This is the basis of the bias known as Berkson's bias.
45. Screening
• Screening is the use of tests to help diagnose diseases (or their
precursor conditions) in an earlier phase of their natural history or at the
less severe end of the spectrum than is achieved in routine clinical
practice.
• Screening attempts to uncover the iceberg of disease.
• On the pyramid model, screening is applied to block 3, and less
commonly, to block 4.
• Aim is to reverse, halt or slow the progression of disease.
• Screening is also done to protect society.
• Screening may be done to select out unhealthy people e.g. for a job.
• Screening is sometimes done to help allocate health care resources.
• Screening may be done simply for research, for example, to identify
disease at an early stage to help understand the natural history.
46. 1 Nil, except vigilance
2 Review
3 Opportunistic or population
screening
4 Screening or health
education
5 Protection of current
status
1 Diseased, diagnosed & controlled
2 Diagnosed, uncontrolled
3 Undiagnosed or wrongly
diagnosed disease
4 Risk factors for disease
5 Free of risk factors
47.
48. Screening: ethics and limitations
• The ethical viewpoint, that the natural history of disease must be
influenced favourably, sets limits on the scope of screening
• Screening could be done for every disease for which there is a
diagnostic test or diagnostic signs and symptoms
• These can be crystallised as six questions
49. Criteria for screening
• Is there an effective intervention?
• Does intervention earlier than usual, improve
outcome?
• Is there an effective screening test that recognises
disease earlier than usual?
• Is the test available and acceptable to the target
population?
• Is the disease one that commands priority?
• Do the benefits exceed the costs?
• If the answer to these six questions is yes then the
case for screening is sound
50. Screening: evaluating the case
• Screening programmes need more careful evaluation than clinical care -
The benefits of screening for hypertension far exceed the costs.
• The screening test is measurement of the blood pressure, usually using
a sphygmomanometer.
• The diagnostic test is, effectively, repetition of the same test on several
occasions combined with a clinical history, examination and other tests
to check for other diseases, particularly those that cause specific forms
of hypertension.
• Additional tests of high blood pressure are possible but used
infrequently, including 24-hour readings using equipment that permits
measurement while the person is ambulatory.
51. Screening: hypertension
• The ideal test would pick up all cases of hypertension in the
population tested. This attribute of the test is known as high
sensitivity (or true positive rate).
• The ideal test would also correctly identify all people who do not have
the disease, that is, the test is specific to those who have the disease
i.e. high specificity (or true negative rate).
• When cases go for more detailed clinical examination, the screening
test result is confirmed, so
• A positive test predicts with accuracy the presence of hypertension,
and similarly a negative test predicts its absence.
52. Screening tests: performance
• These four measures, sensitivity, specificity and
predictive power of a positive and negative test, are the
main way to assess the performance of a screening test
• Measures of performance can be calculated from the 2
x 2 table as shown in table
• As the definitive test is never 100% accurate
• Screening test is being evaluated against another
imperfect, albeit better, test
53. The 2x2 table - validating the
screening test
Disease (true/definitive test)
Present Absent
Screening
test
+ve a b a+b
-ve c d c+d
Total a+c b+d a+b+c+d
Sensitivity or true positive rate = a/a+c
Specificity or true negative rate = d/b+d
Predictive power of a +ve test = a/a+b
Predictive power of a -ve test = d/c+d
54. Exercise: Calculating sensitivity and
specificity, and predictive power
• 500 patients known to have a particular disease were screened with a
new test.
• 500 controls without this disease were also screened.
• Of the 500 patients 473 had a positive test.
• Of the healthy group without the disease 7 had a positive test.
• Create a 2 x 2 table based on table 6.3 and reflect on the
interpretation of the data.
• Calculate sensitivity and specificity of the test.
• Is this a good performance?
• What are the implications for those wrongly classified by the test?
55. The 2x2 table - validating the
screening test
Disease (true/definitive test)
Present Absent
Screening
test
+ve
-ve
Total
Sensitivity or true positive rate = a/a+c
Specificity or true negative rate = d/b+d
Predictive power of a +ve test = a/a+b
Predictive power of a -ve test = d/c+d
56. The 2x2 table - validating the
screening test
Disease (true/definitive test)
Present Absent
Screening
test
+ve
-ve
Total 500 (a+c) 500 (b+d) 1000 (a+b+c+d)
Sensitivity or true positive rate = a/a+c
Specificity or true negative rate = d/b+d
Predictive power of a +ve test = a/a+b
Predictive power of a -ve test = d/c+d
57. The 2x2 table - validating the
screening test
Disease (true/definitive test)
Present Absent
Screening
test
+ve 473 (a) 480 (a+ b)
-ve 27 (c)
Total 500 (a+c) 500 (b+d) 1000 (a+b+c+d)
Sensitivity or true positive rate = a/a+c
Specificity or true negative rate = d/b+d
Predictive power of a +ve test = a/a+b
Predictive power of a -ve test = d/c+d
58. The 2x2 table - validating the
screening test
Disease (true/definitive test)
Present Absent
Screening
test
+ve 473 (a) 7 (b) 480 (a+ b)
-ve 27 (c)
Total 500 (a+c) 500 (b+d) 1000 (a+b+c+d)
Sensitivity or true positive rate = a/a+c
Specificity or true negative rate = d/b+d
Predictive power of a +ve test = a/a+b
Predictive power of a -ve test = d/c+d
59. The 2x2 table - validating the
screening test
Disease (true/definitive test)
Present Absent
Screening
test
+ve 473 (a) 7 (b) 480 (a+ b)
-ve 27 (c) 493 (d) 520 (c+d)
Total 500 (a+c) 500 (b+d) 1000 (a+b+c+d)
Sensitivity or true positive rate = a/a+c = 473/500= .946 = 94.6%
Specificity or true negative rate = d/b+d= 493/500= .986 = 98.6%
Predictive power of a +ve test = a/a+b
Predictive power of a -ve test = d/c+d
60. A study in Bellary
• Information for BCG scar was obtained by
cross checking the BCG scar (Gold standard)
present generally on the lateral side of the left
arm.
• Immunization history was sought
• Immunization cards/register were checked
independently
61. The 2x2 table - validating the
screening test
BCG Scar
Present Absent
BCG Card +ve
812 30 842
-ve
9 4 13
Total
821 34 855
Sensitivity or true positive rate = a/a+c
Specificity or true negative rate = d/b+d
Predictive power of a +ve test = a/a+b
Predictive power of a -ve test = d/c+d
62. The 2x2 table - validating the
screening test
BCG Scar
Present Absent
Parenteral
recall
+ve
672 27 699
-ve
5 5 10
Total
677 32 709
Sensitivity or true positive rate = a/a+c
Specificity or true negative rate = d/b+d
Predictive power of a +ve test = a/a+b
Predictive power of a -ve test = d/c+d
63.
64. Table 6.4 Calculation of sensitivity and specificity based
on data in box 6.4
Diseased
+ ve - ve
Screening
test
+ ve 473 (a) 7 (b) 480 (a+ b)
- ve 27 (c) 493 (d) 520 (c+d)
500 (a+c) 500 (b+d) 1000 (a+b+c+d)
Sensitivity = a/a+c = 473/500 = 94.6%
Specificity = d/b+d = 493/500 = 98.6%
65. Sensitivity and specificity
• The sensitivity (94.6%) and specificity (98.6%) of the test are very
high.
• The test will correctly identify most people who have the disease and
correctly identify most people who are disease free.
• About one person in twenty who does have the disease will be
misclassified as disease free.
• Far fewer people without disease will be misclassified as having the
disease.
• Individuals and their doctors who want to know the implications of
their individual results and this is given by predictive power.
66. Applications of the concepts of natural history, spectrum
and screening
• Health policy that has the objective to shift the natural history of disease
to the right and alter the spectrum so disease is less severe.
• Public health and medical action can be seen as the force spearheading
the attack against ill-health and disease.
• Knowledge of the natural history of disease can radically alter the
organisation of health care so care is proactive.
• Knowing the role of early life events in the genesis of heart disease and
diabetes alters fundamentally our approach to these problems.
• The need to influence the policies which foster good education and
health of mothers and their infants is crystal clear.
• The scientific rationale for health care agencies to seek partnership with
other agencies such as education, housing and social services is overt
• Cross-disciplinary working within health care (primary health care,
paediatrics, obstetrics, nutrition and adult medicine) is seen as essential.
67. Epidemiological theory : symbiosis with clinical medicine
and social sciences
• Theory that many diseases are initiated by events
acting years, or decades, before any clinical
manifestation.
• Diseases may manifest themselves in many ways,
including asymptomatic yet damaging forms.
• To understand why some people with symptoms and
signs of disease seek care, and hence are diagnosed,
while others do not, epidemiology crosses to the social
sciences, linking into theories of illness seeking
behaviour.
68. Summary
• Natural history of disease is the uninterrupted progression of disease
from its initiation by exposure to the causal agents to either
spontaneous resolution, containment by the body’s repair
mechanisms, or to a clinically detectable problem.
• The primary purpose of public health and medicine is to influence
favourably the natural history of disease.
• Natural history of disease is related to (and influences) the changing
pattern of disease in populations or the different levels of severity with
which a disease may present (spectrum of disease).
• For most health problems the number of cases identified is exceeded
by those not discovered.
69. Summary
• The iceberg phenomenon thwarts epidemiological
efforts to assess the true burden of disease.
• It is impossible to identify truly unselected and
representative cases for epidemiological studies.
• Screening is the application of tests to diagnose
disease (or its precursors) in an earlier phase of the
natural history of disease (often in well people) or in a
less severe part of the disease spectrum than is
achieved in routine medical practice.
• These concepts are highly interrelated.
