Cohort Study PDF

Cohort study By Dr. Eman Fareehah Dr. Suhaila Ataweel Climb the Evidence Pyramid for better research One more step forward Preliminary results from the cross-sectional and case-control studies suggest an association between the exposure and the outcome. What is missing? St...

Cohort study By Dr. Eman Fareehah Dr. Suhaila Ataweel Climb the Evidence Pyramid for better research One more step forward Preliminary results from the cross-sectional and case-control studies suggest an association between the exposure and the outcome. What is missing? Strengthening the evidence for a causal link between the exposure and the outcome. We use the results from our previous studies to propose for a COHORT STUDY Definition: It is a type of non-experimental or observational epidemiologic study design used to inform patients, clinicians and policy makers on a wide variety of topics. The term COHORT is derived from the Latin word “cohort” “ a group of soldiers” refers to a group of people who have been included in a study on the definition decided by the researcher. Cohort studies: Cohort study evaluate a possible association between exposure and outcome by following two groups of individuals ( exposed and unexposed) over a period of time ( often years ) to see whether they develop the disease or outcome of interest. The incidence of disease among the exposed and unexposed groups are determined and compared. Subjects should not have outcome variable ( disease free) on entry and should have potential to develop the outcome..  Though there are more challenges in establishing causal inferences with observational studies compared ( cohort study) to randomized controlled trials( RCT) These studies are the only ethical way to gain insights on the effects of exposures known to have harmful effects that would be of interest to clinicians Example: Observe the effect of exposure to tobacco smoke among smoker population Most often clinical trials are not adequate to estimate treatment side effect or adverse events. Example: Observe renal failure as side effect among patients who already using corticosteroids for long term period General Design Framework: A cohort study typically consists of subjects at risk for a disease whose exposure status is assessed at baseline, and who are followed up over time to detect incident disease. Elements of a prospective cohort study design Selection of sample from population/ or self allocated groups. Measuring the exposure variable in the sample Ensuring that the outcome is not present ( non of the participants have the disease) Follow up the different exposure groups for a period of time. Measure the occurrence of the outcome variable in each group. Cohort study structure Exposure evaluation: Typically, a cohort study is initiated by a cross-section study to determine the baseline exposure status and to exclude subjects who are not at risk for the disease. The exposure status can have two levels: 1. The exposed group; those who observed to have the exposure at baseline. 2. The unexposed group; those who are observed to not have the exposure at the baseline. Outcome evaluation: A central goal of epidemiology is to identify causes ( or exposures ) that can explain outcomes such as disease occurrence, which are referred to as the outcome or disease. Cohort study can examine the effect of an exposure on more than one outcome. Other types of cohort studies: Cohort studies may be prospective or retrospective, but both types define the cohort on the basis of exposure, not outcome Retrospective cohort study ( Historical Cohort) Exposure and outcome have already occurred at the start of the study. Pre-existing data, such as medical notes, can be used to assess any causal links. Much less costly than prospective cohorts. Investigator has minimal control over study design ( data may not be correctly collected or there may be no collected data on some confounding variables. Retrospective-prospective study A cohort study may combine both retrospective and prospective data. Advantages & strength of Cohort study Knowing that predictor variable was present before outcome variable occurred ( some evidence of causality) Valuable in studying rare exposures. Directly measure incidence of a disease outcome. Can study multiple outcomes of a single exposure Relative risk (RR) is measure of association. Disadvantages & limitations: Expensive and inefficient for studying rare outcomes A large number subjects is usually needed Often need long follow-up period or a very large population Loss to follow-up can affect validity of findings Experimental ( RCT) and observational (cohort) studies : The main difference In cohort study; no intervention, treatment, or exposure is given to participants. We just observe for the exposure and the occurrence of outcome. In RCT; participants are randomly allocated to either receive the treatment/intervention or not. While in cohort study; there is no randomization, we do not allocate participants to groups, there is either pre-exist exposure, or the participants self-select themselves ( for example, some women might choose to be on hormonal contraception while others choose the mechanical contraception). Analytic framework of cohort study Two-by-two tables are generally used to organize the data from a cohort study as shown below to estimate the effect of exposure on the outcome. Outcome No outcome total Exposed A B A+B Unexposed C D C+D Total A+C B+D A+B+C+D  Two measures of incidence are risks and rates. Risks and rates can be further manipulated to provide additional information on the effects of the exposure of interest, such as risk or rate ratios, risk or rate differences, and attributable risk fractions. The risk or incidence of the outcome Risk is defined as the number of new cases divided by the total population-at-risk at the beginning of the follow-up period. An individual's risk of developing the outcome of interest is 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑛𝑒𝑤 𝑐𝑎𝑠𝑒𝑠 𝐴 The ris for expose = = 𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 𝑎𝑡 𝑟𝑖𝑠𝑘 𝐴+𝐵 𝐶 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑛𝑒𝑤 𝑐𝑎𝑠𝑒𝑠 𝑓𝑜𝑟 𝑢𝑛𝑒𝑥𝑝𝑜𝑠𝑒𝑑 The risk for unexposed = = 𝐶+𝐷 𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑢𝑛𝑒𝑥𝑝𝑜𝑠𝑒𝑑 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 The rate of outcome: A rate is the number of new cases of an outcome divided by the total person-time-at-risk for the population. Person-time is calculated by the sum total of time all individuals remain in the study without developing the outcome of interest (the total amount of time that the study members are at risk of developing the outcome of interest). Person-time can be measured in days, months, or years, depending on the unit of time that is relevant to the study.  A rate measures the rapidity of health outcome occurrence in the population. 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑛𝑒𝑤 𝑐𝑎𝑠𝑒𝑠 𝑅𝑎𝑡𝑒 = 𝑇𝑜𝑡𝑎𝑙 𝑝𝑒𝑟𝑠𝑜𝑛−𝑡𝑖𝑚𝑒 𝑎𝑡 𝑟𝑖𝑠𝑘 Relative risk (RR) or risk ratio The relative risk (RR) or Risk Ratio is used in cohort studies to estimate the strength of the association between exposure and outcome. 𝐴 𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑎𝑚𝑜𝑛𝑔 𝑒𝑥𝑝𝑜𝑠𝑒𝑑 (𝐴+𝐵) Relative risk(RR) = = 𝐶 𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑎𝑚𝑜𝑛𝑔 𝑛𝑜𝑛 𝑒𝑥𝑝𝑜𝑠𝑒𝑑 (𝐶+𝐷) The following table may be applied to both risk and rate ratios. RR=1 The null value where the exposure is not associated with the outcome is reflected by value of 1The exposure ( neither preventive nor harmful) RR>1 The exposure increasing the risk for the outcome ( risk factor) RR