Cohort Study PDF

COHORT STUDY Dr. Sijal Fadhil Farhood AL- joborae F.I.C.M.S (Baghdad) M.Sc. Community (Nahrain) M.B.Ch.B (Babylon University)  Objectives: The Distinguishing Features Of Cohort Studies Indication Of Cohort Study Design of cohort study Types Of Cohort Study Advantages And Disadvantages...

COHORT STUDY Dr. Sijal Fadhil Farhood AL- joborae F.I.C.M.S (Baghdad) M.Sc. Community (Nahrain) M.B.Ch.B (Babylon University)  Objectives: The Distinguishing Features Of Cohort Studies Indication Of Cohort Study Design of cohort study Types Of Cohort Study Advantages And Disadvantages Analysis Of Results 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  COHORT SUDIES 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  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.  INDICATION OF COHORT STUDY 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 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 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 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) 3-Certain administrative problems such as loss of experienced staff, loss of funding and extensive record keeping are inevitable. 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 : 3-comparison with general population: 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. 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  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