RETROSPECTIVE COHORT STUDY Dr.Bharat Kalidindi MPH 1 st Sem Padmashree School of Public Health. Bangalore
Transcript
1. RETROSPECTIVE COHORT STUDY Dr.Bharat Kalidindi MPH 1st Sem
Padmashree School of Public Health. Bangalore
2. WHAT IS COHORT A cohort is any group of people who are
linked in some way or the other. A cohort is a group of subjects
who have shared a particular event together during a particular
time span OR OR A cohort is a group of people who share a common
characteristic or experience within a defined period
3. EXAMPLE People born in India between 1918 and 1939.
Survivors of an bus accident. Truck drivers who smoked between age
30 and 40. People who are exposed to a drug or vaccine or
pollutant.
4. WHAT IS A COHORT STUDY A cohort study is a form of
longitudinal study (a type of observational study) used in
medicine, social science, actuarial science, business analytics,
and ecology. In a cohort study, a group of individuals exposed to a
putative risk factor and a group who are unexposed to the risk
factor are followed over time (often years) to determine the
occurrence of disease. The incidence of disease in the exposed
group is compared with the incidence of disease in the unexposed
group.
5. Defined population Begin with Exposed Not Exposed Develop
Disease Do not Develop Disease Do not Develop Disease Develop
Disease Identify Follow
6. Cohort studies may be prospective or retrospective.
Prospective cohort study (concurrent cohort study or longitudinal
study): Subjects have been followed up for a period and the
outcomes of interest are recorded. Retrospective cohort study
(Non-concurrent cohort or historical cohort study): Study done
after the occurrence of both the exposure and outcome.
7. AN AMBIDIRECTIONAL COHORT STUDY A cohort study may also be
ambidirectional , meaning that there are both retrospective and
prospective phases of the study. Ambidirectional studies are much
less common than purely prospective or retrospective studies, but
they are conceptually consistent with and share elements of the
advantages and disadvantages of both types of studies
8. The Air Force Health Study (AFHS) - also known as the Ranch
Hand Study - was initiated by the U. S. Air Force in 1979 to assess
the possible health effects of military personnel's exposure to
Agent Orange and other chemical defoliants sprayed during the
Vietnam War. The study was conducted comparing: 1,098 pilots
exposed to dioxin in Vietnam (Operation Ranch Hand) 1,549 men who
flew cargo missions in Southeast Asia during the same time
9. ANALYSIS OF COHORT STUDIES Analysis of a cohort study uses
either the risk or the rate ratio of disease in the exposed cohort
compared with the rate or risk in the unexposed cohort. Rate Ratio
= Incidence rate in exposed group (r1) Incidence rate in unexposed
group (r0)
10. FIRST, IDENTIFY EXPOSED a+b NOT EXPOSED c+d TOTAL
11. THEN, FOLLOW TO SEE WHETHER EXPOSED a b a+b NOT EXPOSED c d
c+d TOTAL Disease does not develop Disease develops
12. CALCULATE AND COMPARE EXPOSED a b a+b a/a+b NOT EXPOSED c d
c+d c/c+d Disease develops Disease does not develop TOTAL Incidence
of disease a/a + b = Incidence in exposed c/c + d = Incidence in
not exposed
13. WHEN IS A COHORT STUDY WARRANTED? When the (alleged)
exposure is known When exposure is rare and incidence of disease
among exposed is high (even if the exposure is rare, determined
investigators will identify exposed individuals) When the time
between exposure and disease is relatively short When adequate
funding is available When the investigator has a long life
expectancy
14. STRENGTHS AND WEAKNESSES OF COHORT STUDIES Strengths:
Multiple outcomes can be measured for any one exposure. Can look at
multiple exposures. Exposure is measured before the onset of
disease (in prospective cohort studies). Good for measuring rare
exposures, for example among different occupations. Demonstrate
direction of causality. Can measure incidence and prevalence.
15. Weaknesses: Costly and time consuming. Prone to bias due to
loss to follow-up. Prone to confounding. Participants may move
between one exposure category. Knowledge of exposure status may
bias classification of the outcome. Being in the study may alter
participant's behaviour. Poor choice for the study of a rare
disease. Classification of individuals (exposure or outcome status)
can be affected by changes in diagnostic procedures.
16. RETROSPECTIVE STUDY In a retrospective study, the outcome
of interest has already occurred at the time the study is
initiated. A retrospective study design allows the investigator to
formulate ideas about possible associations and investigate
potential relationships, although causal statements usually should
not be made.
17. An investigator conducting a retrospective study typically
utilizes administrative databases, medical records, or interviews
with patients who are already known to have a disease or condition.
In general, the reasons to conduct a retrospective study are to:
Study a rare outcome for which a prospective study is not feasible.
Quickly estimate the effect of an exposure on an outcome. Obtain
preliminary measures of association. A retrospective cohort study
allows the investigator to describe a population over time or
obtain preliminary measures of association to develop future
studies and interventions.
18. Defined population Exposed Not Exposed Develop Disease Do
not Develop Disease Do not Develop Disease Develop Disease 1989
2015
19. The investigators jump back in time to identify a cohort of
individuals at a point in time before they have developed the
outcomes of interest, and they try to establish their exposure
status at that point in time. They then determine whether the
subject subsequently developed the outcomes of interest. The
Distinguishing feature of a retrospective cohort study is that the
investigators conceive the study and begin identifying and
enrolling subjects after outcomes have already occurred.
20. Suppose investigators wanted to test the hypothesis that
working with the chemicals involved in tire manufacturing increases
the risk of death. Since this is a fairly rare exposure, it would
be advantageous to use a special exposure cohort text annotation
indicator such as employees of a large tire manufacturing factory.
The employees who actually worked with chemicals used in the
manufacturing process would be the exposed group, while clerical
workers and management might constitute the "unexposed" group.
However, rather than following these subjects for decades, it would
be more efficient to use employee health and employment records
over the past two or three decades as a source of data. In essence,
the investigators are jumping back in time to identify the study
cohort at a point in time before the outcome of interest (death)
occurred. They can classify them as "exposed" or "unexposed" based
on their employment records, and they can use a number of sources
to determine subsequent outcome status, such as death
21. Retrospective cohort studies like this are very efficient
for studying rare or unusual exposures, but there are many
potential problems here. Sometimes exposure status is not clear
when it is necessary to go back in time and use whatever data is
available, especially because the data being used was not designed
to answer a health question. Even if it was clear who was exposed
to tire manufacturing chemicals based on employee records, it would
also be important to take into account (or adjust for) other
differences that could have influenced mortality, i.e., confounding
factor. For example, it might be important to know whether the
subjects smoked, or drank, or what kind of diet they ate. However,
it is unlikely that a retrospective cohort study would have
accurate information on these many other risk factors.
22. Investigator: Uses existing data collected in the past to
identify the population and the exposure status (exposed/not
exposed groups). Determines at present the (development) status of
disease. Investigator spends a relatively short time to: Assemble
study population (and the exposed/not exposed groups) from past
data. Determine disease status at the present time (no future
follow-up).
23. DISADVANTAGES OF RETROSPECTIVE COHORT STUDIES As with
prospective cohort studies, they are not good for very rare
diseases. If one uses records that were not designed for the study,
the available data may be of poor quality. There is frequently an
absence of data on potential confounding factors if the data was
recorded in the past. It may be difficult to identify an
appropriate exposed cohort and an appropriate comparison group.
Differential losses to follow up can also bias retrospective cohort
studies.
24. SELECTION BIAS RETROSPECTIVE COHORT STUDIES Bias can occur
in retrospective cohort studies if subjects in one of the exposure
groups are more or less likely to be selected if they had the
outcome of interest. Ex: Retrospective cohort study with the aim of
measuring the association between exposure to an industrial solvent
and risk of death. study was conducted using employee health
records for the past 20 years from a large company. Some of the
employees were exposed to the solvent, but others were not. During
this span of time, some employee health records were lost, but they
were more likely to retain the records of employees who had been
exposed to the solvent and then subsequently died.
25. real risk ratio would have been found to be 2.0, if all
records had been retained. as a result of selective retention of
records for exposed workers who died prematurely, the apparent risk
ratio was 2.42, i.e. an overestimate of the association.
26. OUTBREAK OF GIARDIA Occurred in Milton The request for
assistance was made some time after the start of the outbreak, and
the outbreak was winding down by the time DPH began their study.
The outbreak was clearly concentrated among members of the
Wollaston Golf Club in Milton, MA , which had two swimming pools,
one for adults and a wading pool for infants and small children.
The investigators thought that contamination of the kiddy pool by a
child shedding Giardia into their stool was the most likely source.
Investigators knew the denominators for the exposure groups, so
they could calculate the cumulative incidence, risk difference, and
the risk ratio. People who had spent time in the kiddy pool had 9.0
more cases per 100 persons than those who spent time in the kiddy
pool. The risk ratio was 3.27.