Priority areas of application of cohort epidemiological studies are rarely occurring prerequisites for the onset of diseases, various consequences of the cause of the pathology identified in the course of one analysis. Such studies are the shortest way to identify the etiology of pathologies and quantitative risk analysis. Consider the features of cohort studies, examples and types.
General information
The concept of "cohort" is used in medicine to refer to a group of subjects united by some characteristics. In observational cohort studies in epidemiology, it always consists of he althy individuals. Under the terms of the analysis, it is assumed that the entire group or a separate part of it is exposed or has been exposed to the studied risk factors. Therefore, certain pathologies must subsequently arise within the association of subjects.
Any cohort study (sociological, medical, etc.)involves the search for the causes of certain phenomena, is carried out from the alleged prerequisite to the consequence.
Classification
There are two methods of cohort studies. The division occurs depending on the type of data being studied.
If a group of subjects has been formed at the present time, and its observation will be in the future, then one speaks of a prospective (parallel) cohort study. In sociology, this option is used quite often.
A cohort can be created based on knowledge of the impact of risk factors, as well as analyze it up to the current moment. In this case, one speaks of a retrospective cohort study. Consider the characteristics of each of them.
Parallel Cohort Study in Medicine
This analysis is based on the detection of new cases in a specially selected group of he althy subjects over a certain period.
At the beginning of a cohort study or after the observation phase, a group of people is divided into two subgroups: the main and controlled. There may be several of these pairs.
In the main subgroup are subjects exposed or exposed to the risk factor under investigation. In this regard, it is called exposed. The control subgroup is formed from subjects in whom the influence of the studied factor was not revealed.
At the end of a certain period, differences in the incidence of diseases in both subgroups are evaluated, conclusions are drawn about the presence orno causal relationship between factors and disease.
Development history
The first parallel cohort studies identified the causal role of any risk factor for a single pathology. For example, in 1949, an analysis was conducted in New York to establish an association between rubella in pregnant women and subsequent congenital diseases, death or malformation of the fetus.
Soon began to conduct cohort studies aimed at finding multiple risk factors for multiple pathologies (within the same analysis). The famous Framingham Study is a classic example. It was started in 1949. The aim of this cohort study was to identify risk factors for cardiovascular disease. The scheme of this analysis assumed the formation of the main and control subgroups not immediately, but after the observation stage. However, they were created several times.
Main Stages
A parallel cohort study is conducted in several stages:
- the population from which the group will be formed is determined;
- the fact of the influence of each studied risk factor on a separate subject of the group is revealed, the primary accounting documentation is filled in;
- observation period is determined;
- dynamic assessment of the he alth status of people in the cohort;
- comparison groups are formed (main and control);
- The received information is being studied.
Retrospective study
A cohort selected from archive data is calledhistorical, and the study, respectively, historical or retrospective. The key principle of analysis "from cause to effect" remains unchanged.
The difference between retrospective and parallel studies is the timing of the creation of the main and control subgroups.
Due to the fact that cases of morbidity have already been recorded, it is possible to divide the cohort immediately after its formation. During a given period, subgroups are traced through medical documents, sick subjects are identified. Further actions are similar to those performed in the framework of a parallel study.
Specific lookback analysis
Information obtained from historical research is not considered as reliable as the findings of a prospective study. This is due to the fact that over time, the criteria for the quality of detection, diagnosis and registration of ill individuals, as well as the signs and methods for identifying impact factors, change.
At the same time, a retrospective study is distinguished by its simplicity of organization. If historical data on the influence of risk factors and identified cases of morbidity are reliable, priority is given to historical analysis. For example, the retrospective method is used in the study of occupational diseases, pathologies with severe clinical symptoms, causes of death, and so on.
Advantages of cohort analysis
The key advantage of such research is the opportunity (often the only one) to obtain reliable information aboutetiology of pathologies. This is especially important in cases where it is impossible to conduct an experiment.
Cohort studies are the only way to establish indicators of the relative, attributive and absolute risks of a disease, to assess the etiological proportion of situations associated with the alleged cause of the pathology.
These studies allow the detection of rare triggers. In this case, several causes of one or more diseases can be simultaneously detected.
The reliability of the information received is quite high. This is because cohort analysis is more likely to avoid errors in the creation of the main control subgroups, since they are formed after the discovery of consequences (death, disease, etc.).
Flaws
The main disadvantage of a cohort study is the need to create a large group of he althy subjects. This is especially necessary in cases of relatively rare pathologies. The less often the disease is detected, the higher the physical impossibility to form the desired cohort. Important disadvantages are the duration and high cost of research.
Population definition
At the beginning of the study, the characteristics of the population from which individuals will be selected for participation in the study are established. The cohort is formed exclusively from he althy subjects. At the same time, experts proceed from the fact that it will be not just a group of individuals, but an association in whichdiseases are expected. This assumption is usually based on the results of descriptive epidemiological observations, which revealed differences in the incidence of certain population groups.
Identification of features
If there is an assumption that pathologies will occur in a group, it is assumed that certain factors influence it. The characteristics of the cohort are determined by experts in accordance with the working hypothesis of the impact of causes on the likelihood of developing diseases in subjects who have these criteria. They can be age, physiological state, gender, time, profession, bad habits, some event, area of residence, and so on.
Assume that the working hypothesis is the existence of a link between reduced physical activity and high blood pressure in men aged 30-40. It follows from this that a cohort should be created not from all citizens and not even from all adult men, but only from those who have reached 30-40 years old.
If factors are studied that obviously do not affect each subject from the population (for example, physical inactivity, smoking, hypertension), one population is determined, and then one cohort is formed from it.
If the causal role of any factor that obviously influenced all people is investigated, 2 groups will participate in the study. The main one is selected from the exposed faces,control - from unexposed, which in all other respects is similar to the first.
Complete and sample analysis
In a complete study, the cohort should be formed from all he althy subjects in the selected population. As a rule, general groups are created that are very close to ideal.
A continuous prospective cohort analysis was conducted to test the hypothesis of an association between rubella in pregnant women and congenital anomalies in newborns. The experimental subgroup included almost all pregnancies complicated by pathology. The control subgroup consisted of the rest of the pregnant women (more than 5 thousand people).
Sampling studies involve the selection of a representative cohort, they are carried out not from the entire population, but from the general group.
Detection of the fact of the influence of a risk factor
Before analysis begins, the impact of putative causes on individual members of the cohort is only expected. Accordingly, after the selection of the group, it is necessary to determine whether each risk factor acted on an individual subject or not. All of them are included in the characteristics determined at the preparatory stage of the study.
The way to identify causes in different individuals depends on the nature of the factors themselves. In practice, surveys are used (direct or conversations with relatives), the study of archival data, clinical studies (pressure measurement, ECG). For medicine, research is essential. With its help, you can prevent the development of certain diseases, minimize them.
As a result, at the initial stage of the study, a primary accounting document will be generated for each subject. In it, among other features, "factorial" criteria are indicated. The influence of each factor is taken into account not only by the principle of presence / absence, but also by the duration / strength of the impact. Of course, this information is recorded in the accounting documentation, if there is a real opportunity to obtain it.
GMT
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