Interpretation of laboratory and
epidemiological evidence
Yvan Hutin and Aftab Jasir
Conducting a collaborative epidemiologylaboratory investigation
Formulating the
objectives
Drawing conclusions
Planning
Analysing
Data analysis
Lab analysis
Data
Preparing
Instruments
Specimens
Collecting
Sampling
strategy
Data
Specimens
When faced with the need to interpret evidence,
bear in mind why the investigation was conduced
Dictionary definition of the verb: Interpret
“Explain the meaning of…”
Description and interpretation
in neurophysiology
• Occipital cortex, visual zone:
 I see concentric circles
• Pre-visual zone:
 This is a TARGET!
Description and interpretation in a relationship
Description
• S/he did not call back 
Interpretation
• S/he is angry
• S/he is shy
• S/he is too in love
• There is someone else
• I said something wrong
• S/he lost my number
Probably not enough
data to conclude….
• I was too insistent
• The parents disagree
• Horoscopes do not match
Description and interpretation
in clinical practice
Situation
Description
Interpretation
Chest X-ray
•Alveolar opacity
•Systematized, lobar
opacity
•Consolidation
(Pneumonia?)
Dermatology
•Copper papules
•Soles and palms
•Desquamation
•Secondary syphilis
Language and interpretation
(1) Describe and (2)
interpret: What is this?
Jastrow
Pay attention to the
language you used
• If you used the word
“ear” you make it a
rabbit
• If you used the word
“beak”, you make it a
duck
Interpretation of descriptive epidemiological
data to generate hypotheses
Time
Place
Person
Narrow epidemic
curve
Cluster of cases
@ public tap
Case patients
used the tap
Hypothesis:
The public tap was
contaminated for a
brief duration and
caused the outbreak
Interpretation of analytical epidemiological data
and additional investigations
to test an hypothesis
OR
AFP
Water
analysis
Sanitary
assessment
Strong association
water drinking /
illness
High attributable
fraction
Water positive for
S. Typhi
X-contamination
sewage / water
supply
Epidemiological
evidence supported
the hypothesis of the
tap as the source of
the outbreak
Sewage contaminated
the tap with S. Typhi
and caused the
outbreak
Elements to consider before interpreting
association as causation
?
?
?
?
Chance
Bias
Confounding factor
Causation
?
?
?
?
?
Strength of the association
Dose response
Consistency
Biological plausibility
Exposure/ outcome sequence
Casting the net and pulling it up
Descriptive
epidemiological data
generates hypotheses
Analytical
epidemiological data
tests hypotheses
Can you guess why two different fishermen?
Language used for data description
and data interpretation
Description: Results
Interpretation: Discussion
• Cases started to occur at 5AM,
peaked at 7AM and decreased
with a last case at 10AM
• The shape of the epidemic
curve suggested a point source
outbreak
• Cases clustered around cooling
tower A
• We suspected that tower A
was the source of outbreak
• Malaria rates were high in all
age groups
• Unstable transmission does not
lead to population immunity
• Cases were more likely than
controls to lack health
insurance
• Access to health care may
increase the risk of illness
Interpretation of data in a discussion
section of a paper
+
=
Example of integration of various pieces
of evidence into an interpretation
• Outbreak of cutaneous anthrax
• Beef slaughter in West Bengal, India
• Cohort study
• Contact with meat is a risk factor
• Null hypothesis:
 Eating meat does not cause cutaneous anthrax
Attack rate of anthrax by exposures,
Sarkarpara, Murshidabad, WB, India, 2007
Exposures
Age > 20
AR in exposed (%) AR in unexposed (%)
18
11
Association
RR 95% CI
1.6
0.9-3
Female sex
17
14
1.2
0.7-2
Slaughter
83
9
8.7
6-13
Handling
26
10
2.6
2-4
100
15
6.7
5- 9
17
0
Carry skins
Eating
Undefined
All case-patients who had eaten meat had also other exposures
Other elements of evidence beyond
epidemiological data
• Elements available before the outbreak
– Heat inactivate spores
– Infected meat causes intestinal disease
– Meat involved in intestinal outbreaks
was poorly cooked (e.g., Kebabs)
• Elements from the outbreak investigation
– The beef meat was boiled
Testing the hypothesis that eating meat
could cause cutaneous anthrax
+
=
Always consider other hypotheses
✘ Avoid:
 We found that…
 This could be due to... [this real phenomenon]
 Prefer:
 The results are …
 Two possibilities
• This could be due to this real phenomenon
• This could be an artifact of the study
 Examine both options
 See what the data support and conclude
Dealing with an un-expected finding
• One unexpected exposure is associated with outcome
• Absence of context
 No other studies
 No biological rationale
• Treat as a hypothesis generation:
 This association should be examined in other studies
• Do not force an explanation/ rationalization
✘ “This may be due to…”
Risk factors for post-traumatic stress
disorder (PTSD), Indian Tsunami, 2005
• Unexpected effect modification:
• Single woman more PTSD than married
 This may be due to the fact that they are alone…
• Coding error: It’s the converse that is true!
• Married women have more PTSD
 This may be due to the fact that they have to deal with their
whole family…
✘ Do not force interpretations
 Propose further studies to look into it
Take home message
• Interpretation has a subjective component
– Requires a careful, documented approach
• We raise hypotheses with descriptive epidemiology
and test them with analytical epidemiology
• Findings acquire a meaning in the context of what
was known before
Conducting a collaborative epidemiologylaboratory investigation
Formulating the
objectives
Drawing conclusions
Planning
Analysing
Data analysis
Lab analysis
Data
Preparing
Instruments
Specimens
Collecting
Sampling
strategy
Data
Specimens
When faced with the need to interpret evidence,
bear in mind why the investigation was conduced
Possible objectives of joint laboratory
epidemiology investigations
• Test a hypothesis (Qualitative outcome)
– Test a hypothesis
• About an etiologic agent
(e.g., Is West Nile virus the cause of the outbreak?)
• About the relatedness of isolates
(e.g., Are the cases caused by an identical pathogen?)
• Measure a quantity (Quantitative outcome)
– Estimate a quantity
• Prevalence
• Incidence
Using laboratory evidence to confirm a
diagnosis during an outbreak
• Short list potential etiologic agents (Hypothesis
generating) according to:
– Epidemiological characteristics
– Clinical characteristics
– Setting
• Test for agents short listed (Hypothesis testing)
– Positive test
– Negative test
• Use predictive values positive and negatives
Case scenario 1
Viral Hemorrhagic Fever (VHF)
• Fever
• Bleeding disorders
• Progress to high fever
• Shock
• High case fatality
Virus families causing VHF
sensitive or specific?
Short listing?
Diverse group of animal and human illnesses that may be
caused by five distinct families of RNA viruses
• Arenaviridae, (Lymphocytic choriomeningitis virus, Lassa
virus, Argentine, Bolivian, Brazilian and Venezuelan
hemorrhagic fevers viruses)
• Filoviridae (Ebola virus and Marburg virus)
• Bunyaviridae (Hantaviruses, Crimean-Congo
hemorrhagic fever)
• Flaviviridae (dengue, yellow fever, WNV)
• Rhabdoviridae (Lyssavirus)
Interpreting positive tests results
during an outbreak
• Use the predictive value positive that depends upon:
– The frequency of the disease
– The specificity of the test +++
• Elements that support the hypothesis of a true positive
– The disease is frequent (GAS)
– The test is specific ( emm typing)
• Elements that support the hypothesis of a false positive
– The disease is rare (WNV)
– The test is not sufficiently specific (CFT)
Interpreting negative tests results
during an outbreak
• Use the predictive value negative that depends upon:
– The frequency of the disease
– The sensitivity of the test +++
• Elements that support the hypothesis of a true negative
– The disease is rare (WNV)
– The test is sensitive (IgM ELISA)
• Elements that support the hypothesis of a false negative
– The disease (condition) is common (GAS, Q fever)
– The test is not sufficiently sensitive(Gram staining/PCR)
A test was negative only for the pathogens
that were looked for
• If the culture on a specific medium was not done, the test
cannot be interpreted as negative for the specific
pathogen
• If you did not ask for Campylobacter culture, the
“negative” stool culture is not really “negative” for
Campylobacter
Host-pathogen relationship
• Presence of an organism may have different interpretation
according to the context
• Immune system
– Immunocompetent patient
• Opportunistic pathogens may be innocent
– Immunocompromised patient
• Opportunistic pathogens may be the cause of the infection
• Age (pertussis)
• Physiological status (e.g., Urinary infection in pregnancy)
Possible objectives of joint laboratory
epidemiology investigations
• Test a hypothesis (Qualitative outcome)
– Test a hypothesis
• About an etiologic agent
(e.g., Is West Nile virus the cause of the outbreak?)
• About the relatedness of isolates
(e.g., Are the cases caused by an identical pathogen?)
• Measure a quantity (Quantitative outcome)
– Estimate a quantity
• Prevalence
• Incidence
Using laboratory evidence to confirm the
relatedness of isolates
• Generate hypotheses using epidemiological evidence
– Studies allowing the use of statistical tests (Large sample size)
– Studies not allowing the use of statistical tests (Small sample size)
• Test hypotheses using laboratory evidence
– Use typing technique adapted to:
• Hypothesis
• Pathogen
Nosocomial IGAS infections,
Skåne, Southern Sweden, 2012
• 43 cases of invasive Group A Streptococcus (iGAS)
between 9 January and 29 April 2012
• 27 cases between 3 January and 24 April 2011
• Thirteen of the 43 cases in 2012 were treated in an
Intensive Care Unit. One case, a 84 year old already
hospitalised prior to iGAS diagnosis, died
• Hospital rejects the hypothesis of nosocomial infections
IGAS cases typing results,
Skåne, Sweden, 2012
Typing
(EMM/PFGE)
emmst1 / P7
emmst3 / P1
emmst28 / P6
emmst81 / P2
emmst89 / P5
emmst4 / P3
emmst1 / P4
N
%
20
15
2
2
2
1
1
47%
35%
5%
5%
5%
2%
2%
Cases of IGAS by date of onset,
Skåne, Sweden, 2012
Toxin (superantigen) pattern,
IGAS, Skåne, Sweden, 2012
PHASE
1
emmst3
/ P1
PHASE
2
emmst1
/ P7
other
subtype
s
# (n=15) %
#(n=20) %
# (n=8)
Total
%
13%
# (n=43) %
SPE_A
15 100%
20 100%
1
SPE_B
15 100%
20 100%
8 100%
43 100%
SPE_C
15 100%
20 100%
3
38
SPE_F
SPE_G
SPE_H
SPE_I
SPE_J
15 100%
14 93%
5 33%
0
0%
7 47%
20 100%
19 95%
19 95%
0
0%
3 15%
8 100%
7 88%
5 63%
0
0%
3 38%
43 100%
40 93%
29 67%
0
0%
13 30%
SSA
15 100%
20 100%
1
36
38%
13%
36
84%
88%
84%
Possible objectives of joint laboratory
epidemiology investigations
• Test a hypothesis (Qualitative outcome)
– Test a hypothesis
• About an etiologic agent
(e.g., Is West Nile virus the cause of the outbreak?)
• About the relatedness of isolates
(e.g., Are the cases caused by an identical pathogen?)
• Measure a quantity (Quantitative outcome)
– Estimate a quantity
• Prevalence
• Incidence
Interpreting prevalence and incidence
• A study estimating the frequency of a disease on the basis
of a laboratory test (e.g., serological survey) must be
interpreted according to:
– Predictive value positive
– Predictive value negative
• These will depend upon:
– The test used (sensitivity and specificity)
– The frequency of the disease
Be careful about what the manufacturer may
say about the predictive values
• The manufacturer may report values of
– Sensitivity
– Specificity
• These probably come from panel testing
• Be careful with values of predictive values positive and
negative reported by manufacturers
– These values depends upon specific prevalence settings
– They may come from a combination of a positive and negative
panels that generate an artificial prevalence of 50%
Take home message:
Interpret epidemiological and laboratory
evidence as a team
• Positive tests are likely to rule in the diagnosis if the test
is specific and the disease is common
• Negative tests are likely to rule out the diagnosis if the
test is sensitive and the disease is uncommon
• Emergent pathogens are discovered in the laboratory and
assessed according to additional studies
• Laboratory investigations of relatedness must be based on
hypotheses developed on the basis of the epidemiology
• Interpret incidence and prevalence indicators according to
predictive values positive and negative