A cohort study (can/cannot) be both retrospective and prospective in nature.
USMLE® Step 1 style questions USMLE
A 10-year study is being developed to investigate the consequences of electronic cigarette use. The researchers plan to enroll patients from 100 primary care physicians in 10 major cities in the United States. Individuals aged 18-65 will be included in the study. Patients with concurrent standard cigarette use or marijuana use, currently being treated for cancer, or cognitive inability to answer survey questions independently, will be excluded. The patients will receive an initial survey about their electronic cigarette use and then a physical exam. Their medical records will subsequently be evaluated annually over the study period and they will receive a full physical exam every two years. At the end of the study period, the patients will receive a follow-up survey and final physical exam. Which of the following best describes the design of this study?
Content Reviewers:Rishi Desai, MD, MPH
A group of people who share a common characteristic is called a cohort.
For example, people born in the year 1981 make up a birth cohort, and people who work in construction make up an occupational cohort.
Now, cohort studies or longitudinal studies are a type of study design that follows a cohort of people over time to figure out if there’s an association between an exposure and an outcome.
Typically, cohort studies look at individuals in a cohort who have a certain exposure, as well as individuals in a cohort who have not had that exposure, to compare their rates of a certain outcome in the future.
We can then compare the groups by dividing the probability of lung cancer for people who smoked - 0.00082 - by the probability of lung cancer for people who didn’t smoke - 0.00003 - and determine that people that smoked had 27 times the risk of developing lung cancer during that ten period.
Now, there are two main types of cohort studies.
The first type is called prospective cohort or concurrent cohort, because individuals are followed forward in time.
Then the two groups are followed for ten years, until 2028, and the number of people who develop lung cancer is compared between the two groups.
In retrospective cohort studies, participants are recruited in the past and then followed until the present day.
This can be done either with time-travel or by simply by looking at medical records from the past.
For example, you might look at medical records to find a cohort of young adults in 2008 and then divide them into a group of people that smoke cigarettes, the exposed group, and a group of people that don’t smoke cigarettes, the non-exposed group.
Then we can follow the medical records of these individuals over the next ten years, until 2018, and compare the number of individuals in each group who develop lung cancer.
So even though retrospective cohort studies use data from the past, they still have the same basic structure as prospective cohort studies.
In other words, they still start with a cohort of exposed and non-exposed individuals and follow them over time to assess their outcomes.
Now, this is different from case-control studies, which start with individuals who do or don’t have the outcome, and then use data to investigate past exposures.
One advantage of cohort studies is that they’re able to clearly show the timing or temporality of the relationship between the exposure and the outcome.