Lecture 8 & 9. Outbrek invesitgation and screening test.pptx

Outbreak Investigation & Management
Erean Sh. (MPH/E)
1

Learning objectives
After the end of this session, students will
be able to:
o State different level of disease occurrences
o List the rationale to investigate outbreak occurrence
o Discuss steps in the investigation of an outbreak occurrence
o Describe types of outbreak occurrence
o Discuss the outbreak controlling strategies
2

Level of disease occurrences
Endemic
o Presence of a disease condition at more or less stable level
o It can be defined as ‘the constant presence of a disease or
infectious agent within a given geographic area or population
group’
Sporadic
o The occurrence of individual case or outbreak of disease at
occasional, irregular and unpredictable intervals over time
Periodic (cyclic changes)
o The occurrence of disease outbreaks at regular intervals, in
cycles
3

Epidemic
o The occurrence of more cases of disease than expected in a
given area among a specific group of people over a particular
time
Outbreak
o Epidemic of shorter duration covering a more limited area
affecting small proportion of populations
E.g. in a village, town or closed institution
Cluster
o An ‘aggregation of relatively uncommon health outcomes in
space and/or time in amounts that are believed to be greater
than could be expected by chance’
4

Pandemic
o An epidemic of disease involving several countries or
continents affecting a large proportion of populations
o If the epidemic crosses many international boundaries
Secular trends
o Slow and gradual changes of a disease over long period
of time such as decades (long term trends of disease
occurrence like cancer)
5

Enzootic
o An “endemic” disease occurring in animal populations
Epizootic
o An “epidemic” of disease occurring in animal
populations
6

Endemic versus Epidemic
Endemic
Epidemic
Number
of
Cases
of
a
Disease
Time
7

Common conditions for the occurrence of an
outbreak
Agent–host–environment states may change and
precipitate an outbreak occurrences:
o The new appearance or sudden increase of an infectious
agent
o An increase in susceptibles in an environment that has an
endemic pathogen
o The introduction of an effective route of transmission
from source to susceptible host
8

Outbreak occurrence…
o Are there cases in excess of the baseline rate for that
disease and setting?
o The excess frequency should be found out with
epidemic threshold curve
9

The following points are worth noting about
epidemic declaration:
ƒ
The term epidemic can refer to any disease and health related
condition
The minimum number of cases that fulfils the criteria for
epidemic is not specific and the threshold may vary
Knowledge of the expected number is crucial to label an
occurrence of a particular event as an epidemic
The expected level varies for different diseases and different
geographic locations
10

Thresholds
Thresholds are markers that indicate when something
should happen or change
They help surveillance and program managers answer the
question, “When will you take action, and what will that
action be?”
11

Types of thresholds
 An alert threshold suggests to health staff that further
investigation is needed and preparedness activities
should be initiated
 An action threshold triggers a definite response
12

What is outbreak occurrence?
14

What does outbreak investigation & control?
It is the process of identifying:
o the cause of the epidemic
o the source of the cause
o the mode of transmission
o taking of preventive and control measures
15

Ways of Outbreak Detection
o One of the uses of public health surveillance is detecting
an outbreak occurrence
o Outbreak is detected when a routine surveillance data
reveals an increase in reported cases of a disease
o It can also be detected when the outbreak come to
attention of health care providers
o Members of affected group are another important sources
for outbreak information and concerned citizens
o Media like TV, newsletters and radio
16

Deciding whether to investigate a possible
outbreak
Factors related to the problem itself include:
o the severity of the illness/ virulence
o the number of cases
o the source
o mode of transmission/communicability
o availability of preventive and control measures
17

Objectives for outbreak
investigation
1) To initiate control & preventive measures
The most important public health reason for
investigating an outbreak occurrence is to guide disease
prevention and control strategy
Before the investigators do a control strategy, they
should identify where the outbreak is in its natural
course:
ois the outbreak continuing?
ois the outbreak just over?
18

To initiate control & prevention measures …
o If the outbreak is continuing, the major goal should be to
prevent additional cases
o If the outbreak is almost over, the goal should be to
identify risk factors to prevent future episodes of similar
outbreak occurrence
19

Outbreak Detection and Response Without Preparedness
0
10
20
30
40
50
60
70
80
90
Delayed Response
Days
Cases
Opportunity
for control
Late Detection
Index Case
20

Outbreak Detection and Response With Preparedness
0
10
20
30
40
50
60
70
80
90
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
Rapid Response
Days
Cases
Early Detection
Potential
cases prevented
21

o Choosing between control measures versus further
investigation depends on how much is known about:
the cause
the source of the outbreak
the mode of transmission
o If the investigators know only little about the outbreak,
further investigation is needed
o If investigators know well about the outbreak, control
measures should be initiated immediately
22

o Decision regarding how extensively to investigate an
outbreak occurrence is influenced by:
– Severity of the illness
– Knowledge of the source of the agent
– Mode of transmission of the agent
– Availability of effective preventive and control
measures
23

2) Research and training opportunity
o Each outbreak should be viewed as an experiment
waiting to be analyzed
o It presents a unique opportunity to study the natural
history of the disease
o It could be a good opportunity to gain additional
knowledge by assessing:
– The impact of prevention and control measures
– The usefulness of new epidemiology and laboratory techniques
24

Research and training opportunity …
o For a newly recognized disease, field investigation
provides an opportunity to define the natural history of a
disease including:
 causative agent
 mode of transmission
 incubation period
 clinical spectrum of the disease
 Outcome of the disease
o Investigators also attempt to characterize the populations
at greatest risk and to identify specific risk factors for
prevention and control 25

3) Public, political and legal obligations
o Politicians and leaders are usually concerned with control
of the epidemic
o Politicians and leaders may sometimes override scientific
concerns
o The public are more concerned in cluster of disease and
potentials of getting medication
o Such investigations almost never identify a causal link
between exposure and disease of interest
o It’s advantage could be educational for the community
26

4) Program considerations (evaluation)
o Occurrence of an outbreak notifies the presence of a
program weakness
o This could help program directors to change or
strengthen the program’s effort in the future to prevent
potential episodes of similar outbreak occurrence
27

Steps of outbreak investigation and control
1. Prepare to field work
2. Establish the existence of outbreak
3. Verifying the diagnosis
4. Case definition and case findings
5. Perform descriptive epidemiology
6. Formulate hypothesis
7. Evaluate hypothesis
8. Refine hypothesis and conduct additional studies
9. Intervention and follow up strategies
10. Communicate findings and design post-outbreak surveillance
system
28

First spot
o When we got information of an epidemic:
– We should identify where the outbreak is in its natural
course?
– Is it starting ? (Early recognition)
– Is it just about over? (Late recognition)
29

Time that
come
to attention of
Investigators
Time
Early recognition
Major aim:
To prevent additional disease occurrence
(i.e. initiation of preventive and control measures)
30

Time that come
to attention of
Investigators
Time
Late recognition
Major aim:
to prevent future similar outbreak
occurrence
(i.e. further investigation of risk factors) 31

Rapid Response Team (RRT)
o A good field investigator must be a good manager,
collaborator and epidemiologist
o It is not only health professionals but also it may need
involvement of others including:
 An epidemiologist
 A clinician
 A laboratory technician
 Environmental health specialist
 Public health officer
 Microbiologist
 A representative of the local health authority etc…
32

Step 1: Prepare for field work
Before leaving for the field, an investigator must be well
prepared to under take the investigation:
o Investigation (Knowledge in epidemiology and the disease of
concern is important)
o Administrative (Logistics, administrative procedures, travel
arrangements)
o Consultation (Health workers should know their role, and should
participate in the planning phase)
33

Step 2: Confirm outbreak occurrence
o An outbreak is the occurrence of more cases of disease
than expected level
o But be careful, excess cases may not always indicate an
outbreak occurrence rather it may be because:
 Change in population size
 Change in case definition
 Change in reporting procedure
 Improvement in diagnostic procedure
 Increase local awareness or interest etc…
34

Step 3: Verify the diagnosis
o The initial report may be spurious and arise from
misinterpretation of the clinical features
o Review clinical and laboratory findings to establish
diagnosis
o Goals in verifying the diagnosis includes:
 To ensure that the problem has been properly diagnosed
 To rule out laboratory error as a basis for the increase in
diagnosed cases
 To ensure the diagnosed disease is possibly epidemic
35

Step 4: Workable case definition & case findings
o Prepare “case definition” before starting identification
of cases
o It’s aim is to count all cases of the illness
o It includes clinical criteria restricted by time, place and
person
o Use the case definitions objectively
o Do not include an hypothesised exposure to risk
factor(s) in your case definition
36

Step 4: Workable case definition …
o The clinical criteria should be simple having objective
measures
o Whatever the criteria, they must be applied consistently
to all persons under investigation
o Use sensitive or "loose case definition” early in the
investigation using descriptive epidemiology to identify
the extent of the problem
o But, during testing the hypothesis generated from this
process using analytic epidemiology, specific or "tight or
strict case definition” must be used
37

Step 4: Workable case definition …
o Direct the case finding to take place both in health
institutions and outreach sites
o If a localized form of epidemic, case finding should go
to the epidemic area
o Finally, you can ask case patients if they know anyone
else with the same signs and symptoms
38

Two ways of case findings:
Stimulated/enhanced passive surveillance
o Sending a letter describing the situation and asking for reports
o Alerting the public directly through local media to visit health
facility if they have symptoms compatible with the disease in
question
o Asking cases if they know anyone else with the same signs and
symptoms
Active surveillance
o Making telephone call or visit the health facilities to collect
information on cases
o Conducting a survey of the entire population
39

Step 5: Performing Descriptive
Epidemiology
o Once data is collected, it should be analyzed by time,
place and person
o The tools to be used when characterizing the epidemic
are epidemic curve, spot map and attack rate
o The characterization often provides clues about etiology,
source and modes of transmission that can be turned into
testable epidemiologic hypothesis
40

Analysis of epidemic by time
 One can distinguish several types of epidemics
according to the mode of transmission and duration:
o Analysis by time of onset
o The epidemic curve can help to identify the type of
epidemic
o An epidemic curve provides a simple visual display of
the outbreak’s magnitude & time trend
41

Types of epidemic
o Epidemics can be classified according to the mode of
spread or propagation, nature and length of exposure to
the infectious agent and duration
o There are three principal types of epidemic
42

1. Common source epidemic
 It occurs as a result of the exposure of a group of
population to a common source (etiological agent)
o It can result from a single exposure of the population to
the agent
E.g: contaminated water supply, or the food in a
certain restaurant
43

A) Point common source epidemic
o If the exposure is brief and simultaneous
o All exposed hosts will develop the disease within one
incubation period
o The epidemic usually decline after a few generations,
either because the number of susceptible hosts fall below
some critical level, or because intervention measures
become effective
o A rapid rise and gradual fall of an epidemic curve
suggests a point source epidemic occurrence
44

E.g. Food borne outbreak of “AGE” in a wedding
feast
45

Point Source Epidemic: Meningitis in A.A Ethiopia, 2000
46

B) Continuous common source epidemic
o If the duration of exposure is prolonged
o The epidemic is continuous common source epidemic
and the epidemic curve has a plateau (multimodal epi
curve)
47

Continuous common source epidemic
48

Continuous common source
epidemic…
49
Usual rate
Number of cases
Time
Flat top

C) Intermittent common source epidemic
o An intermittent common source epidemic (exposure to
the causative agent is sporadic over time)
o Usually produces an irregularly jagged epidemic curve
reflecting the intermittence and duration of exposure and
the number of persons exposed
E.g. waterborne outbreak
50

Intermittent common source
epidemic
51

Intermittent common source epidemic…
52

2. Propagative epidemic
o It occurs as a result of transmission of an infectious
agent with a multiple sources
o It could be transmission from one person to another
directly or indirectly
o The epidemic curve in a progressive epidemic is usually
presence of successive several peaks, a prolonged
duration, and usually a sharp fall
53

Propagative source epidemic
54

3. Mixed Epidemic
o It shows the features of both types of epidemics
o It begins with a common source of infectious agent with
subsequent propagated spread
o For example a common source outbreak may be followed
by secondary person-to-person spread
E.g. Food borne outbreaks
56

Analysis of epidemic by place
– Using spot map you may ascertain localized epidemic
by place (cluster epidemic)
– Area map if large area is affected
– It is identified by intensity of shading corresponding to
incidence of a disease
– A spot map is a simple and useful technique for
illustrating where cases live, work or may have been
exposed
– It is important to indicate source of outbreak
occurrence
57

Spot Map (Cholera Outbreak in London )
58

Analysis of epidemic by place…
 Spot map does not take in to account underlying geographic
differences in population density.
 Therefore the spot map needs to be supplemented by
calculations of place specific attack rates.
 Arti-factual differences in disease occurrence between
places can occur because of regional differences in
diagnostic capacity, case detection procedures and compliance
with reporting etc.
60

Analysis of epidemic by person
o Characterizing an outbreak occurrence by person is how
we determine what populations are at risk for the disease
o Host characteristics: age, race, sex, or medical status and
exposures-occupation, leisure activities, use of
medications, tobacco and drug use etc…
o These influence susceptibility to disease and
opportunities for exposure to risk factors
o We use attack rates to identify high risk groups
61

Example: Attack Rate by age group per 100,000
populations
Age
Groups
(years)
Cases Populations Attack Rates
<1 2 522 383
1 – 5 36 16,014 225
6 – 14 22 30,385 72
15 – 64 29 157,989 18
> 65 9 41,948 22
Total 98 246,858 40
62

Step 6: Formulating epidemiological
hypothesis
Depending on the outbreak, the hypothesis may address:
o the source of the agent
o the mode of transmission
o the exposures that caused the disease
 The hypothesis should be workable & testable!
63

Step 7: Testing of the hypothesis
o After a hypothesis has been developed, the next step
is to evaluate the plausibility of the hypothesis
o Association between the postulated exposure to risk
factor(s) and the disease will be tested using analytic
studies:
 Retrospective cohort study
 Case-control study
64

Step 8: Refining hypothesis & conduct
additional studies
o Search for additional cases: Locate unrecognised or
unreported cases
o Environmental studies are equally important in some
settings
65

Step 9: Intervention and follow up
strategies
o Simultaneous to data collection and hypothesis formation, steps
should be taken to control the epidemic occurrence
o These measures depend upon knowledge of etiologic agent, mode
of transmission and other contributing factors
o In most outbreak investigations, the primary goal is control of the
outbreak and prevention of additional cases
o Implementing control measures should be done as soon as possible
if the source and/or mode of transmission is known
o It should go in parallel to investigating the outbreak or even before
investigation
66

 Control measures are aimed at the weak link in the
chain of infection:
o It may be aimed at the specific agent, source or
reservoir
o Interrupting the transmission or exposure to risk factors
o Instructing (educating) people to reduce their risk of
contacting possible exposure to the risk factors
o Reducing host susceptibility
67

Management of outbreak occurrences
o Measures directing against the reservoir
o Measures interrupting the mode of transmission
o Measures reducing host susceptibility
68

Measures directed against the reservoir
Domestic animals as reservoir
oImmunization
oTesting of herds
oDestruction of infected animals
Wild animals as reservoir
oPost-exposure prophylaxis
Humans as reservoir
oRemoval of the focus of infection
oIsolation of infected persons
oTreatment to make them non-infectious
oDisinfections of contaminated objects 69

Measures interrupting the mode of
transmission
Actions to prevent transmission of disease by ingestion:
o Purification of water
o Pasteurisation of milk
o Inspection procedures designed to ensure safe food supply
o Improve housing conditions
Attempts to reduce transmission of respiratory infections
Action to interrupt transmission of diseases whose cycles
involve an intermediate host
E.g. Clearing irrigation farms from snails to control
schistosomiasis
70

Measures to reduce host susceptibility
Control measures also include strengthening the host’s
immunity to resist disease through the following
activities:
o Active immunization: E.g. EPI preventable diseases
o Passive immunization: E.g. TAT, Rabies
o Chemoprophylaxis: E.g. Tuberculosis, Malaria , HIV (PEP)
71

Step 10: Communicating findings of
investigation
 The final responsibility is to prepare a written
report to document the investigations, findings
and the recommendations
 An oral briefing for local authorities and
implementers of control and preventive
activities:
o What was done
o What was found
o What should be done
72

A written report
At the end, prepare a comprehensive report and submit to
the appropriate responsible bodies
The report should follow the scientific reporting format
which includes:
o introduction
o objectives
o methods
o results
o discussion
o conclusion, and
o recommendations
o references
73

The report should be discussed in detail:
o Factors leading to the epidemic occurrence
o Measures taken for the controlling of the epidemic
o Recommendations for the prevention of similar episodes
of outbreaks in the future
74

Post-Epidemic Surveillance
o It is epidemic prevention strategy
o The efficacy of control measures should be assessed day
by day during the outbreak, a final assessment being made
after it has ended
o This will provide a logical basis for post-epidemic
surveillance, and preventive measures aimed at avoiding
similar outbreaks in the future
o Develop long term early warning system
o Monitor environmental risk factors for the disease
occurrence
75

SCREENING IN DISEASE CONTROL
Definition:-
• Screening is a public health intervention intended to
improve the health of a precisely defined target
population.
• The presumptive identification of unrecognized disease
by application of rapid tests or examinations.
• The early detection of disease
– Precursors of disease
– Susceptibility to disease in individuals who do not
show any signs of disease

Definition…
• Detecting disease or risk factors in asymptomatic
individuals
• The examination of asymptomatic people in order to
classify them as likely or unlikely to have the diseases
of interest.
– Diagnosis is not equal to Screening
• Screening sort out apparently well persons who
probably have a disease from those probably do not.

Diagnostic test
o Diagnostic tests are used to confirm the presence or
absence of illness, provide prognostic information, and
predict a response to treatment
o Identify and confirm a disease condition in individuals
– Diagnostic test is performed in persons with symptoms
or a signs of an illness
– Tests performed in patients
– The objective is case finding within a population that
is probably “diseased”
79

Screening and diagnostic tests
o May be based on:
–Standardized interviews
–Physical examinations
–Laboratory tests
–More sophisticated measurements
radiography
electro-cardiograph etc…
80

Clinical Aim of Screening
• To reverse, halt, or slow the progression of disease
more effectively than would probably normally
happen.
• To alter the natural course of disease for a better
outcome for individuals affected.
• Reduce morbidity and mortality through early
detection and treatment

Public health aim of screening
program
o To protect society from communicable disease
o To reduce mortality
o For rational allocation of resources…
Other Use:
o Research: study on natural history of disease
o Selection of healthy individuals usually for employment
E. g: Military and driving license …
83

Screening…..
• Screening is only useful for disease which can be
detected in their early, pre-clinical stage, and for which
early treatment significantly improves the outcome
• Screening can also be of useful for the communicable
disease control, such as trachoma, Schistosomiasis, or
TB, for which early treatment helps to prevent
transmission

Types of screening
programme
1. Mass/population screening
– It is offered to all, irrespective of the particular risk factors
2. Multiple/multi-phase screening
– The purpose of two or more screening tests in combination to a
large number of people at one time
3. Case finding/opportunistic screening
– It is restricted to patients who consult a health professional for
some other purposes
4. Targeted screening
– High risk or selective screening
– It will be most productive if applied selectively to high risk
groups
85

Types of screening
Selective Vs Mass.
– Selective – screening of people with selective exposure
– Mass – screening of people without reference to specific
exposure
• Single Vs Multiple
• Multiple-Parallel Vs Series
– Parallel testing – applying two screening tests and a positive
result on either test is sufficient to be labeled as positive E.g. –
Breast ca screening
– Series testing – applying two screening tests and both must
be positive in order to prompt action
E.g. – HIV testing, Syphilis

Criteria for establishing screening program
1. The condition should be an important health problem (Life-
threatening diseases)
2. There should be a treatment for the condition.
3. Facilities for diagnosis and treatment should be available.
4. There should be a latent stage of the disease.
5. A suitable screening test must be available

Modified Wilson criteria for introducing a screening program
1. The disease condition
– Important health condition (prevalent, serious)
• Has early stage
– High prevalence at asymptomatic stage
– Adequately understood natural history
2. The test
– Simple, precise, validated
– Acceptable to population
– Agreed diagnostic test(s)
– Available, adequate, effective and acceptable
– Sensitive, specific, reliable, simple, cheap, safe

Modified Wilson criteria, cont..
3. The treatment
– Effective treatment Available, adequate, and
acceptable
– Evidence based policies on whom to treat
4. The program
– Acceptable to public and professionals
– Benefits outweigh risks
– Available facilities and staff
– Overall costs assessed

WHO criteria for screening
programme
Is it a health problem?
Is there treatment?
Are there facilities in place?
Is it detectable pre-clinically?
Is there a suitable screening test?
Is the screening test acceptable to people?
Is the natural history of disease understood?
Are the costs acceptable?
90

Evaluation of a screening
program
Reliability
Feasibility
Validity
Performance
Yields
Effectiveness

Characteristics to be evaluated
• These characteristics are:
1. Validity
2. Predictive value
3. Reliability
4. Yield
1. Validity is analogous to accuracy
– The validity of a screening test is how well the given
screening test reflects another test of known greater accuracy
– Validity assumes that there is a gold standard to which a test
can be compared
• Validity- does it truly measure what it sets out to measure?
• Variability/reliability -does it gives the same measurement each time

Accuracy of screening tests…
o A test should be unbiased, precise, and be able to
determine the disease status of an individual without
error
o Accuracy is the ability of the test to correctly classify
individuals according to their disease status
Examples:
o Classify exposed versus non-exposed individuals
o Classify infected versus non-infected individuals
o Classify abnormal versus normal individuals etc…
93

Characteristics of a screening test
Validity of a categorical test
Screening
Test result
Definitive Diagnosis(Gold Standard)
Diseased Non diseased Total
Positive True Positive
(TP)
False Positive
(FP)
TP + FP
Negative False
negative
(FN)
True
Negative
(TN)
TN + FN
Total TP + FN FP + TN TP +FP +
TN + FN

Two-by-two contingency table
Screening
Test
Diagnostic tests
Total
Diseased Non-diseased
Positive
True positive [TP]
(a)
False positive[FP]
(b) a + b
Negative
False negative [FN]
(c)
True negative[TN]
(d) c + d
Total a + c b + d n
95

Two-by-two contingency table…
True positive
o The number of individuals for whom the screening test is
positive and the individual actually has the disease
False positive
o The number for whom the screening test is positive but
the individual does not have the disease
True negative
o The number for whom the screening test is negative and
the individual does not have the disease
False negative
o The number for whom the screening test is negative but
the individual does have the disease 96

Measures of accuracy of screening
tests
o Validity is the ability of a test to come up
with a result which is closer to the actual
value
o Reliability is the ability of a test to come up
with similar values upon repeated
measurements in similar occasions
97

Measures of accuracy…
Validity:
o The application of a screening test is to identify correctly
individuals who do and do not have preclinical disease
o Those who have preclinical disease should test positive,
and those who do not have it should test negative
o The ability of a screening test to successfully separate
these two groups is a measure of its validity which is
expressed by its sensitivity and specificity
98

Reliability versus validity of a test
99

A. Sensitivity
– The ability of the test to identify correctly those who have the
condition, Sensitivity + Fn = 100%
– A test with high sensitivity will have few false negatives
%
100



Fn
Tp
Tp
y
Sensitivit
Sensitivity (detection rate) =a/a+c
=(Tp/Tp+Fn)X100
Proportion of true positive with diseases

B. Specificity
The ability of the test to identify correctly those who
don’t have the disease
–Specificity + false positive = 100%
–test that has high specificity will have few
false positives
%
100
y
Specificit 


Fp
Tn
Tn
Specificity=d/d+b=(Tn/Tn+Fp)X100
Proportion of true negatives identified

Validity of Screening Tests
5/15/2023 102
132
63650
45
983
Breast Cancer
+ -
Physical Exam
and Mammo-
graphy
+
-
Sensitivity: a / (a + c)
Sensitivity = 132 / (132 + 45) = 74.6%
Specificity: d / (b + d)
Specificity = 63650 / (983 + 63650) = 98.5%

Predictive values of screening
test
o It is the probability that a person tested positive or
negative will or will not have a disease respectively
o Predictive values of screening test depends on:
– Prevalence of preclinical disease
– Sensitivity
– Specificity
103

Positive predictive value
o The probability of a person having the preclinical disease
when the test is positive
o It is defined as the proportion of people with the condition
among all those who received a positive test result
o It is the probability of the presence of the disease in a
person with a positive (abnormal) test
o It is calculated as a percentage; the number of individuals
with preclinical disease who test positive is in the
numerator, and the total number of individuals who test
positive is in the denominator
104

Negative predictive value
o The probability of a person not having the preclinical
disease when the test is negative
o It is the proportion of people without the condition
among all those who received a negative test result
o It is the probability of not having the disease when the
test result is negative
o It is calculated as a percentage; the number of
individuals without the disease who test negative is in
the numerator, and the total number of individuals who
test negative is in the denominator
105

Validity and predictive values of
screening test
106

Screening test trade-offs
o Tests with dichotomous results – tests that give either
positive or negative results
E.g: HIV positive or negative
o Tests with continuous variables – tests that do not
yield obvious “positive” or “negative” results, but
require a cutoff level to be established as criteria for
distinguishing between “positive” and “negative” groups
E.g: Blood pressure measurement
107

Screening test trade-offs…
 There is always a trade-off between sensitivity
and specificity; as one increases, the other
tends to decreases
o The higher the cutoff used to say a test is positive, the
more specific the test becomes, but this higher specificity
comes at a price
o As the cutoff is increased, the sensitivity decreases and
the test is more likely to miss affected individuals (life
lost) i.e. false negative increases
108

Screening test trade-offs…
o A lower cutoff might be used to create a very
sensitive test, so as not to miss anyone with the
disorder in question =false +ve
o In other situations, when using a test to confirm a
diagnosis, a higher cutoff, making the test highly
specific, is more desirable, so as not to incorrectly
label anyone with the disorder =false –ve
109

Choosing appropriate test
– Choose a test with a high sensitivity and high
negative predictive value to rule out a diagnosis
in the early stage of the investigation
– When a False Negative is DANGEROUS or
SERIOUS
110

Choosing appropriate test…
o Choose a test with a high specificity and
high positive predictive value to confirm a
diagnosis
o When a False Positive is DANGEROUS or
SERIOUS
111

Choosing appropriate test…
High sensitivity High
specificity
112
Sensitivity and specificity are affected by the ‘cut-off’ value of
measure at
which a test is defined as positive.

Yield of screening test
o The proportion of truly diseased individuals identified by the
screening test among screened population.
o The yield of a test result is a function of:
 Specificity of the test
 Prevalence of the preclinical disease
Strategies to increase yield of a test
o Select disease with high prevalence of preclinical
stage
o Target high risk group for screening program
o Select a test with high specificity test
113

Yield…
• The number of cases of the condition detected by screening
test in relation to the total number of persons screened
• Affected by sensitivity, prevalence of unrecognized cases,
number of tests employed, frequency of screening and
participation of people in screening and follow-up
%
100





Fp
Fn
Tp
Tn
Tp
Yeild
%
100
screened
Total
test
by the
detected
dx
with the
persons


Yeild

,
Yields of screening test
Reference test
Screening test
positive negative total
+ve 1555 988 2543
-ve 514 1394 1908
T 2069 2382 4451

Example…,
Sensitivity =1555/2069 =75.2%
Specificity=1394/2382 =58.5%
PVPT=1555/2543 = 61.1%
PVNT=1394/1908 =_____
Yield=1555/4451=______

RELIABILITY
• The ability of a test to give consistent results when it is
performed more than once on the same individual under the
similar conditions
• The reliability of a test is its capacity to give the same result -
positive or negative, whether correct or incorrect - on repeated
application in a person with a given level of disease
• Affected by variation in the method, observer and the
characteristic to be measured

Reliability…
o Intra-observer reliability – agreement among
measurements made by the same observer
o Inter-observer reliability – agreement among
measurements made by different observers
o Test-retest reliability – agreement among
measurements on the same subject at different times
o Biological (or subject) variation
o Technical (instrumental) problems
o Reagent problems
118

RELIABILITY…
• The reliability of a dichotomous test can be described
using a 2×2 table in which the first and second test
results are presented on the left-hand side and top of
the table, respectively.

RELIABILITY…
Second test
First test
+ve tests agree tests disagree
-ve tests disagree tests agree
T

RELIABILITY…
• A simple way to summarize these findings is to
calculate the proportion of tests that agree:
• Percent agreement = Number of tests that agree
Total number of tests
• Percent agreement = a+d *100%
n
• However, test agreement due to chance alone is
possible, even with an imprecise test.
• The Kappa statistic is used to describe test agreement
beyond that expected from chance alone.
• Calculation of the Kappa statistic is presented below.

RELIABILITY…
• The Kappa statistic ranges from +1 (perfect agreement), to 0 (no
agreement beyond that expected from chance), to −1 (perfect
disagreement).
• In general, a Kappa statistic <0.2 is considered poor agreement,
0.2 – 0.6 is considered fair agreement, and > 0.6 is considered
good agreement.
• A common application of the Kappa statistic is to measure
agreement between two different testers who evaluate a
subjective test characteristic

Example
Second test
First test
+ve 5 3
8
-ve 2 30
32
T 7 33 40

Potential Source of Bias in
Screening
• Self-selection (volunteer) bias
• Lead time bias (early diagnosis)
• Length Bias (chronicity and progression)

Self-selection (volunteer) bias
• Implies that those accepting screening are different from those
declining it.
• Comparing populations accepting and declining for all
relevant characteristics.

Lead time bias (early diagnosis)
• Bias caused by picking screened cases at an early
stage of the disease, i.e., before they develop signs
and symptoms of the disease

Length Bias (chronicity and
progression):
• Related to the variation in the speed of progression of the disease.
• Cases picked up by a screening may be less severe, and slow
progressive compared with others.
• It is very important to be aware of such a possibility in interpreting
survival gains from a screening program.

Success Factors for Screening
Programs
• The health problem should be important enough to be worth
detecting.
• Availability of an acceptable intervention which is effective.
• Presence of a recognizable latent or early "asymptomatic" stage.
• Presence of a suitable and acceptable test.

Success Factors…
• The natural history of the condition should be adequately
understood.
• Presence of policy regarding when the intervention is appropriate.
• The cost of detecting the problem and its remedy should be
reasonable.
• The screening program should be ongoing, and not a "one-time"
effort.

Study Design for Evaluation of
Screening
• Ideally experimental
• Most commonly used is cohort

Summary
 A screening program is defined as a set of procedures for
early detection and treatment of a disease
─ It includes both diagnostic and therapeutic components, and
includes the screening test and the follow-up evaluation for people
with positive tests
─ The therapeutic component consists of treatment of confirmed cases
of the disease
─ Thus, a screening program is much more than just a screening test
132

Lecture 8 & 9. Outbrek invesitgation and screening test.pptx

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