What is cohort research? Examples

The priority areas for the application of cohort epidemiological studies are rarely encountered prerequisites for the occurrence of diseases, various consequences of the pathology identified in 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 any signs. In observational cohort studies in epidemiology, it always consists of healthy individuals. According to the conditions of the analysis, it is assumed that the entire group or its individual part is exposed or was exposed to the studied risk factors. Therefore, within the association of subjects, certain pathologies should subsequently arise.

Any cohort study (sociological, medical and other) involves a search for the causes of certain phenomena, carried out from the alleged premises to the consequence.

Classification

There are two cohort studies. Separation occurs depending on the type of data that is being studied.

If a group of subjects was formed at the present time, and its observation will be in the future, then we speak of a prospective (parallel) cohort study. In sociology, this option is used quite often.

A cohort can be created based on information about the influence of risk factors, and also analyze it up to the current moment. In this case, they speak of a retrospective cohort study. Consider the characteristics of each of them.

Parallel cohort study in medicine

Such an analysis is based on the identification of new cases of morbidity in a specially selected group of healthy subjects over a period of time.

At the beginning of the cohort study or after the observation phase, the group of individuals is divided into two subgroups: the main and the controlled ones. There may be several of these pairs.

The main subgroup contains subjects exposed to or exposed to the studied risk factor. In this regard, it is called exposed. The control subgroup is formed from subjects in which the influence of the studied factor was not identified.

At the end of a certain period, differences in the frequency of occurrence of diseases in both subgroups are evaluated, conclusions are drawn about the presence or absence of a causal relationship between factors and the disease.

History of development

In the first parallel cohort studies, a causal role of a risk factor for one pathology was revealed. For example, in 1949, an analysis was conducted in New York to establish a connection between rubella in pregnant women and subsequent congenital diseases, death or malformation of the fetus.

Soon they began to conduct cohort studies aimed at detecting several risk factors for several pathologies (within the framework of one analysis). A classic example is the famous Framingham study. It was started in 1949. The purpose 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 phase. Moreover, they were created several times.

Main stages

A parallel cohort study is carried out in several stages:

• the population from which the group will be formed is determined;
• the fact of the influence of each investigated risk factor on an individual subject of the group is revealed, the primary accounting documentation is filled out;
• the observation period is determined;
• a dynamic assessment of the health status of the people who make up the cohort;
• comparison groups are formed (main and control);
• the study of the information received is performed.

Retrospective study

The cohort, identified by archival data, is called historical, and the study, respectively, is historical or retrospective. The key principle of cause-to-effect analysis remains unchanged.

The difference between retrospective and parallel studies is the time of 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. Over a given period, subgroups are tracked according to medical documents, diseased subjects are identified. Further actions are similar to those that are performed as part of a parallel study.

The specifics of retrospective analysis

Information obtained in the course of 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 recording of diseased persons, as well as signs and methods for identifying exposure factors change.

In this retrospective study is simple organization. If historical data on the influence of risk factors and detected cases of disease are reliable, priority is given to historical analysis. For example, a retrospective method is used in the study of occupational diseases, pathologies with severe clinical symptoms, causes of death, etc.

Advantages of cohort analysis

The key advantage of such studies is the opportunity (often the only one) to obtain reliable information about the etiology 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, assess the etiological proportion of situations associated with the alleged cause of the pathology.

These studies can detect rare provocative factors. In this case, several causes of one or more diseases can be detected simultaneously.

The reliability of the information is quite high. This is due to the fact that in a cohort analysis, it is more likely to avoid mistakes when creating the main control subgroups, since they are formed after the detection of consequences (death, disease, etc.).

disadvantages

The main disadvantage of the cohort study is the need to create a group of healthy subjects of large numbers. 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, signs of the population from which individuals will be selected to participate in the study are established. A cohort is formed exclusively from healthy subjects. Moreover, experts proceed from the fact that it will be not just a group of people, but an association in which diseases are expected to occur. This assumption is usually based on the results of descriptive epidemiological observations, within which differences in the incidence of certain population groups were identified.

Definition of signs

If there are assumptions that pathologies will occur in the group, it is assumed that certain factors influence it. The signs of a cohort are determined by specialists in accordance with the working hypothesis about the influence of causes on the likelihood of developing diseases in subjects with these criteria. They can be age, physiological state, gender, time, profession, bad habits, some kind of event, territory of residence, etc.

Suppose that a working hypothesis is the relationship between decreased physical activity and high blood pressure in men 30-40 years old. From this it follows that the cohort should not be created from all citizens and not even from all adult men, but only from those who have reached 30-40 years of age.

If we study factors that obviously do not affect each subject from the population (for example, physical inactivity, smoking, hypertension), one population is determined, and after it one cohort is formed.

If the causal role of a factor that obviously affects all people is investigated, 2 groups will participate in the study. The main one is selected from the exposed persons, the control - from the unexposed, which in all other respects is similar to the first.

Continuous and selective analysis

In a continuous study, the cohort should be formed from all healthy subjects in the selected population. As a rule, general groups are created that are very close to ideal.

To verify the assumption of the relationship of rubella in pregnant and congenital anomalies recorded in newborns, a continuous prospective cohort analysis was performed. The experimental subgroup included almost all pregnancies complicated by pathology. The control subgroup consisted of the remaining pregnant women (more than 5 thousand people).

Selective studies suggest the selection of a representative cohort; they are carried out not from the entire population, but from the general group.

Risk Factor Detection

Before the start of the analysis, the fact of the impact of the alleged causes on individual members of the cohort is only expected. Accordingly, after the separation of the group, it is necessary to determine whether each risk factor acted on a separate subject or not. All of them are included in the composition of signs determined at the preparatory stage of the study.

The method for identifying causes in different individuals depends on the nature of the factors themselves. In practice, interviews are used (direct or conversations with relatives), the study of archival data, clinical studies (pressure measurement, ECG). For medicine, research is important. With its help, it is possible to prevent the development of certain diseases, to minimize them.

As a result, at the initial stage of the study, a primary accounting document will be generated for each subject. It, among other signs, indicates "factorial" criteria. 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 records, if there is a real opportunity to get it.

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