In epidemiology, the term “cohort” is used to define a set of people followed for a certain period of time. W. H. Frost, a 20th century epidemiologist, was the first to adopt the term in a 1935 publication, when he assessed age-specific and tuberculosis-specific mortality rates. The epidemiological definition of the word currently means “a group of people with certain characteristics, followed up in order to determine incidence or mortality by any specific disease, all causes of death or some other outcome.” [1]
Cohort studies design is observational, unlike clinical studies, there is no intervention. [2] Because exposure is identified before outcome, cohort studies have a temporal framework to assess causality and thus have the potential to provide strong scientific evidence. [1] A fundamental characteristic of the study is that at the starting point subjects are identified and exposure to a risk factor is assessed. Subsequently, the frequency of the outcome, usually the incidence of disease or death over a period of time, is measured and related to exposure status. [3]
Advantages of cohort study include the possibility of assessing causality; examine multiple results from a given exposure; investigate rare exposures and determine disease rates in exposed and unexposed individuals over time. Disadvantages are the need for large samples, the susceptibility to selection bias and the long follow-up time. Cohort studies may be prospective, retrospective [1] or ambidirectional. [4]
Prospective cohort studies
Prospective cohort studies are characterized by the selection of the cohort and the measurement of risk factors or exposures before the outcome occurs, thus establishing temporality an important factor in determining causality. This design provides a different advantage over case-control studies in which exposure and disease are assessed at the same time. [5]
Main disadvantage to prospective cohort study is the cost. It requires a large number of individuals to be followed up for long periods of time, [5] that can be difficult due to loss to follow-up or withdrawal by the individuals studied. [1] Biases may occur, especially if there is significant loss during follow-up. [5]
It is important to minimize loss of follow-up, a situation in which the researcher loses contact with the individual, resulting in missing data. When loss to follow-up of many individuals occurs, the internal validity of the study is reduced. A general rule requires that the loss rate does not exceed 20% of the sample. Any systematic differences related to the outcome or exposure of risk factors for those who drop and those who remain in the study should be examined, if possible, by comparing individuals who remain in the study and those who were loss to follow-up or dropped out. Therefore, it is important to select individuals who can be followed for the entire duration of the cohort study. Strategies to avoid loss to follow-up are to exclude individuals who are likely to be lost, such as those who plan to move, to obtain information to enable future tracking and to maintain periodic contact. [1]
Prospective design is inefficient and inappropriate for the study of rare diseases, but it becomes efficient when there is an increase in the frequency of the disease in the population. [5]
Retrospective cohort studies
Cohort studies may also have retrospective design. Retrospective cohorts are also called historical cohorts. [1,6] A retrospective cohort study considers events that have already occurred. Health records of a certain group of patients would already have been collected and stored in a database, so it is possible to identify a group of patients – the cohort – and reconstruct their experience as if it had been prospectively followed up. Although patient information was probably collected prospectively, the cohort would not have initially identified the goal of following individuals and investigating the association between risk factor and outcome. In a retrospective study, it is likely that not all relevant risk factors have been recorded. This may affect the validity of a reported association between risk factor and outcome when adjusted for confounding. In addition, it is possible that the measurement of risk factors and outcomes would not have been as accurate as in a prospective cohort study. [2]
Many of the advantages and disadvantages of retrospective cohort studies are similar to those of prospective studies. As previously described, retrospective cohort studies are typically constructed from previously collected records, in contrast to prospective design, which involves identification of a prospectively followed group, with the objective of investigating the association between one or more risk factors and outcome. However, an advantage to both study designs is that exposure to risk factors is recorded before the outcome occurs. This is important because it allows the sequence of risk and outcome factors to be evaluated. [6]
Use of previously collected and stored records in a database indicates that the retrospective cohort study is relatively inexpensive, quick and easy to perform. However, a consequence of retrospective cohort is that possibly not all relevant risk factors have been identified and recorded. Another disadvantage is that many health professionals will have become involved in patient care, making the measurement of risk factors and outcomes less consistent than that achieved with a prospective study design. [6]
Ambidirectional cohort studies
A cohort study may also be ambidirectional or ambispective, which means that there are prospective and retrospective phases in the study. Ambidirectional studies are less common than either prospective or retrospective studies, but are conceptually consistent and share the advantages and disadvantages of both types of studies. [4]
Ambidirectional cohorts present retrospective and prospective components. Analysis of an outcome shortly after exposure can be approached retrospectively. In a same study, other outcomes that may not appear until a certain time after exposure may be followed prospectively to see if they have a higher incidence. [4]
As with prospective and retrospective designs, none of the followed subjects has the outcome of interest at the beginning of the follow-up period, the compared groups differ in the exposure status, and incidence of the outcome is measured and compared to determine whether there was an association between exposure and outcome. [4]
Conclusions
- Cohort studies are appropriate studies to evaluate associations between multiple exposures and multiple outcomes.
- Main advantages of prospective and retrospective designs are, respectively, higher accuracy and higher efficiency.
References
- Song JW, Chung KC. Observational Studies: Cohort and Case-Control Studies. Plast Reconstr Surg. 2010;126(6):2234–2242 DOI: 10.1097/PRS.0b013e3181f44abc.
- Sedgwick P. Prospective cohort studies: advantages and disadvantages. BMJ 2013;347:f6726-f6727 DOI: 10.1136/bmj.f6726
- Euser AM, Zoccali C, Jager KJ, Dekker F. Cohort Studies: Prospective versus Retrospective. Nephron Clin Pract. 2009;113:c214–c217 DOI: 10.1159/000235241
- Boston University School of Public Health. Prospective and Retrospective Cohort Studies. Disponível em: <https://http://sphweb.bumc.bu.edu/otlt/MPH-Modules/QuantCore/PH717_CohortStudies/PH717_CohortStudies3.html/> Acesso em: 06 de agosto de 2018
- Firestein GS, Budd R, Gabriel SE, McInnes IB, O’Dell JR. Kelley and Firestein’s Textbook of Rheumatology. 10 ed. Philadelphia: Elsevier, 2017. E-book. ISBN: 978-0-323-31696-5. Disponível em: <https://books.google.com.br/books?isbn=032341494X>. Acesso em: 06 de agosto de 2018
- Sedgwick P. Retrospective cohort studies: advantages and disadvantages. BMJ 2014;348:g1072-g1072 DOI: 10.1136/bmj.g1072
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