Cohort Study

COHORT STUDY

Dr. Sijal Fadhil Farhood AL-joborae

F.I.C.M.S (Baghdad)
M.Sc. Community (Nahrain)
M.B.Ch.B (Babylon University)

WHAT IS a COHORT STUDY?

introduction
The term cohort comes from the Latin word cohors , meaning a group of soldiers in ancient Rome .
Today ,we use the ward cohort to characterize any designated group of persons who are followed or traced over a period of time.
In epidemiology, the term cohort is defined as a group of people who share common characteristics or experience within a defined time period (e.g. age, occupation, exposure to drug and vaccine)



COHORT STUDIES
It is an observational analytic design ,it is also called (follow up,longtudenal,incidence and forward looking study) in which a group or groups of individual are defined on the basis of presence or absence of a suspected risk factor for a disease.
At the time exposure status is defined , all potential subjects must be free from the disease under investigation ,and illegible participants are then followed over a period of time to assess the occurrence of that outcome.

The distinguishing features of cohort studies are:

A-a cohort are identified prior to the appearance of the disease under investigation.

B-The study groups are observed over a period of time to determine the frequency of disease among them.

C-the study proceeds forward from cause to effect.

Cohort studies are indicated :
1-when there is a good evidence of an association between exposure and disease(as derived from clinical observations and supported by descriptive and case control studies).
2-when exposure is rare, but the incidence of disease is high among exposed e.g special exposure groups like those in industries ,exposure to x- rays.
3-when attrition of study population can be minimized e.g. follow up is easy, cohort is stable,
co-operative, and easily accessible .
4-where funds are available.
Indication of cohort study

Framework of cohort study:

In contrast to case control studies which proceed from ((effect to cause)),the basic approach in cohort studies is to work from ((cause to effect)), that is in case control study,exposure and disease have already occurred when the study is initiated.
In cohort study,the exposure has occurred,but the disease has not.

Design of cohort study

The exposed group is called index group

The unexposed group is called referent or comparison group

A-The cohorts must be free from disease under study, if the disease under study is coronary heart disease, the cohort members are first examined, and those who already have
evidence of the disease under investigation are excluded.
B-In so far as the knowledge of the disease permits, both the groups(i.e the study and control cohorts) should be equally susceptible to the disease under study.




C-both the groups should be comparable in respect to all the possible variables which may influence the frequency of the disease
D-The diagnostic and eligibility criteria must be defined beforehand ,this will depend upon the availability of reliable methods for recognizing the disease when it develops.

Types of population studied

Defined by
Follow-up
Appropriate measure of disease frequency
Open or dynamic
Changeable characteristic
Member come and go; losses may occur
Incidence rate
Fixed
Irrevocable event
Does not gain members; losses may occur
Incidence rate

Closed

Irrevocable event
Does not gain members; no losses occur


Cumulative incidence
Types of population studied

TYPES OF COHORT STUDY

Prospective cohort study

Retrospective cohort study

Advantages:

1- incidence can be calculated
2-several possible outcomes related to exposure can be studied simultaneously
3-direct estimate of relative risk.
4-dose-response ratio can be calculated
5-since the comparison groups are formed before disease develops, certain forms of bias can be minimized like miss-classification of individuals into exposed and unexposed groups


3-Certain administrative problems such as loss of experienced staff, loss of funding and extensive record keeping are inevitable.
Disadvantages:
1-Unsuitable for investigating uncommon disease or diseases with low incidence in the population.
2-It takes a long time to complete the study and obtain results(loss of follow
up)

4-Loss of substantial proportion of the original cohort, they may migrate lose interest in the study, or simply refuse to provide any required information
5-There may be changes in the standard methods or diagnostic criteria of the disease over prolonged follow up
6-Expensive
7-Ethical problems with varying importance
8-Practical considerations dictate that we must concentrate on a limited number or factors possibly related to disease outcome

Elements of cohort study

1-selection of study subjects.
2-obtaining data on exposure.
3-selection a comparison groups.
4-follow-up.
5-analysis.

1-selection of the exposed population

It depends on hypothesis under study,the exposure frequency and feasibility considerations such as availability of records and ease of follow up.
1-Special cohorts:
2-General cohorts:


Special cohorts: are assembled to study the health effects of rare exposures such as uncommon occupational chemicals, unusual diets or life styles, medical procedures….etc
General cohorts: are typically assembled for common exposures such as use of oral contraceptives, dietary factors such as vitamin use and habits such as cigarette smoking and alcohol consumption

2-obtaining data on exposure

Information about exposure may be obtained from:
1-cohort members: through personal interviews and mailed questionnaire
2-review of record: certain kinds of information (dose of radiation, kinds of surgery, or details of medical treatments) can be obtained only from medical records.

3-Medical examination or special tests: (eg.blood pressure, serum cholesterol,ECG)

4-Enviromental surveys: This is the best source of obtaining information on the exposure level of suspected factor in the environment where the cohort lived or worked

3-selection of comparisons groups

They should be similar to the study group in all the factors
related to the disease EXCEPT the factor under study.
1-Internal Comparison :


strength:

2-External comparison :

the comparison with general population in the same geographic area as the exposed with people ,it is commonly used in occupational studies strength:accessible,stable data.Limitation:lack of comparability with exposed group,results may suffer from healthy worker effect,data on key variable may be missing.
3-comparison with general population:

4-Follow-up:by

e.g. Periodic medical examination, reviewing hospital and medical record, routine surveillance of death records.
5-ANALYSIS:
The data are analyzed in terms of:
A-Incidence rates of outcome among exposed and non exposed.
B-estimation of risk.

A-incidence rates

Relative risk(RR)=incidence among exposed
---------------------------------------------------
incidence among non exposed

• a / a+b RR =------------- c/ c+d

Where:
incidence in exposed= a/a+b

Incidence in non exposed= c/c+d

Interpretation of RR
If RR=1 risk in exposed equal to risk in non exposed(no association).
If RR>1 risk in exposed greater than risk in non exposed(positive association ,possibly causal).
If RR<1 risk in exposed less than risk in non exposed(negative association, possibly protective)


2-Attributable risk(ar):
Is the difference in the incidence rates of disease between an exposed group and non exposed group ,some authors use the term (risk difference) to attributable risk.
AR=(incidence in exposed group)-(incidence in non exposed group)

3-attributable risk percent(ar%)

AR%=(Incidence in exposed)-(incidence in non exposed)
----------------------------------------------------------------------------x100% Incidence in exposed group

example

•

• Bacteruria

•

• Yes

• No
• total
• OC use
• Yes
• 27
• 455
• 482


• No
• 77
• 1831
• 1908
• Total
•
• 104
• 2286
• 2390
• Cohort study for bacteruria in oral contraceptive pill users:
• RR = Ie / Io
• = [a / (a + b)]/ [c / (c + d)]
• = [27/482] / [77 / 1908]
• = 1.4 times bacteruria among OC users compared to non-users
• Or to interpret with: those using OC are 1.4 times more to develop bacteruria than those do not use OC
• AR% = [(Ie - Io) / Ie] * 100 = [{(27/482) – (77 / 1908)} / (27/482)] * 100
• = 28.57% could be prevented if not used OC, or there is excess risk of 28.57% to develop bacteruria on using OC.

References

-Epidemiology for public health practice
-Lecture notes-Epidemiology , Evidence-based Medicine and Public health 6th edition
-Jekel’s Epidemiology ,Biostatistics Preventive Medicine and Public health