Transcript Prospective cohort

Prospective Study
Cohort Study
Assis.Prof.Dr Diaa Marzouk
Community Medicine
What is a cohort?
A cohort is a group of persons
who share a common
experience within a defined
time period.
Healthy cohorts are followed
up forward in time for the
development of a disease
Examples of cohorts:
Birth cohort:
All persons born within a
given period of time.
Marriage cohort:
All persons married within a
given period of time
Occupational cohort:
Selection of cohorts:
Volunteers:
Because it is acssesable
From armed forces:
Because its medical records
are available
Have particular exposue:
At work
Prospective Study
Cohort Study
It starts with a group of
people / cohort:
All considered to be free of a
given disease.
But vary in exposure to a
supposed risk factor.
Information is gathered
about their exposure to the
suspected risk factor
The individuals are divided
into exposed and not exposed
To the factor of interest
The cohort is followed over
time in order to determine
differences in the rate at
which disease develops in
relation to exposure to the
factor
Retrospective Studies
Case Control St.
Diseased and nondiseased
groups (cases and controls)
are selected and compared
for presence or absence of
antecedent factor (risk
factor)
Advantages of
Prospective Study
1. The cohort is classified in
relation to exposure to the
factor before the disease
develops
This classification cannot
be influenced by the
knowledge that disease
exists.
2. It permits calculation of
Incidence rates among
exposed and not exposed.
3. Theabsolute difference
between both is the
attributable risk.
3. The relative risk.
4. Permits observation of many
outcomes,
e.g smoking and lung cancer,
showed that smoking is
associated with other otcomes
as emphysema, CHD, peptic
ulcer, cancer larynx, oral cavity,
oesopphagus and urinary
bladder.
Disadvantages of
Prospective studies
1. Long, expensive and large
scale undertaking.
2. The problem of attrition:
loss of patients due to lack
of interest, migration or
death from other causes.
3. Changes in diagnostic
criteria and methods over
time
4. Administrative problems as
loss of staff, loss of funding,
and the high costs of record
keeping
Analysis of Results
Anlytic studies are designed
to determine whether an
association exsists between
a factor or exposure and a
disease and to determine the
strength of the association.
Relative Risk
It is an important measure of
association that relates the
incidence rates of the
disease under study among
those with and without the
factor or exposure.
Relative Risk
It is defined as the ratio of
the incidence rate for
persons exposed to the
incidence rate for those not
exposed.
Relative Risk (RR)=
Incidence among exposed
Incidence among unexposed
Diseased
Not Diseased
Exposed
a
b
Not Exposed
c
d
RR=
a ÷ c
a+b
c+d
=
ad
bc
Historical Prospective
Studies
It combines the advantages
of both retrospective and
prospective study desgins.
It involves following healthy
exposed and unexposed
cohorts for the development
of the disease.
These cohorts are
constructed retrospectively
through existing records that
permit correct classification
of the exposure status of
individuals
Study subjectd are traced to
the present time or
sometimes to the future as
well.
The analysis of this study is
as the prospective study.
Examples of Cohort Studies
I- OBJECTIVE: To identify risk factors for breast
cancer among female survivors of childhood
cancer.
Exposure:
Survivors of childhood cancer are at risk for
secondary breast cancer.
DESIGN: Retrospective cohort study.
SETTING: The Childhood Cancer Survivor Study
(CCSS), a multicenter study of persons who
survived more than 5 years after childhood cancer
diagnosed from 1970 to 1986.
PARTICIPANTS: Among 6068 women in the
CCSS, 95 women had 111 confirmed cases of
breast cancer.
MEASUREMENTS: Standardized incidence
ratios for breast cancer were calculated by using
age-specific incidence rates in the general
population.
Breast cancer incidence was evaluated with
respect to primary cancer diagnosis and therapy,
age at and time since primary diagnosis, menstrual
and reproductive history, and family history of
cancer.
RESULTS:
Breast cancer risk was increased in
survivors who were treated with chest
radiation therapy (standardized incidence
ratio, 24.7 [95% CI, 19.3 to 31.0]) and
survivors of bone and soft-tissue sarcoma
who were not treated with chest radiation
therapy (standardized incidence ratios, 6.7
and 7.6, respectively).
CONCLUSION:
Survivors of childhood sarcomas and those
who received chest radiation therapy are at
risk for secondary breast cancer. When
assessing a survivor's risk, clinicians
should consider primary diagnosis,
previous radiation therapy, family cancer
history, and history of thyroid disease.
II-Growth in prepubertal
children with cystic fibrosis
In cystic fibrosis, growth and lung function
have been identified as prognostic markers
of both severity of pulmonary disease and
survival.
Objective:
longitudinal design to determine whether
normalisation of growth could be found in
the genetic subgroup of prepubertal
children with CF with the homozygous Delta
F508 mutation, which is one of the known
severe mutations.
METHODS:
Data of all children born after 1980 with the
homozygous Delta F508 mutation,
diagnosed in early childhood at the
specialised centre of the Children's Hospital
of Berne were systematically assessed up
to the age of 11 years and retrospectively
analysed. Follow-up data of height, weight
and BMI were compared to the Swiss
reference population using z-scores.
RESULTS:
In the study, cohort growth (height, weight
and BMI) was significantly below that of the
normal Swiss population. A significant
decline of lung function with age was also
found, however, no association between
lung function and growth could be seen.
Compared to an earlier cohort, an improved
growth over the last decade could be shown
but no improvement on lung function could
be detected
CONCLUSION:
In contrast to sequential crosssectional studies of children with CF,
the present longitudinal study of
children with homozygous for the
Delta F508 mutation failed to confirm
normalisation of growth over time.