Question:

Over the course of 2 days, 25 elderly individuals came to the emergency department of an urban medical center with acute respiratory tract infection (ARTI)—mainly bronchiolitis or pneumonitis. Public health authorities investigated the outbreak and used standardized questionnaires to collect relevant clinical and sociodemographic information from the 25 individuals with ARTI and 50 elderly individuals without ARTI matched by age, sex, and county of residence. Preliminary results showed that 92% of the elderly individuals with ARTI had attended a social gathering the previous week, and 20% of the elderly individuals without ARTI had attended the same social gathering. Which of the following statements is correct about this study design?

Because individuals with and without ARTI were matched by age, sex, and county of residence, this study is a randomized controlled trial

Because it collected data on attendance at the social gathering and presence of ARTI using a standardized questionnaire, this study is a cross-sectional study

Because it compares information about attendance at the social gathering among individuals with and without ARTI, this study is a case-control study

Because it compares the incidence of disease among elderly individuals who attended the social gathering and those who did not, this study is a cohort study

Because it describes individuals with and without ARTI in terms of clinical and sociodemographic characteristics, this study is a case series

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Explanation:

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In this study, elderly patients with acute respiratory tract infection (ARTI) are cases (individuals with disease), and elderly patients with no ARTI are controls (individuals with no disease). A questionnaire was used to collect relevant clinical and sociodemographic information to compare exposure to ≥1 risk factors of interest (ie, attendance at social gatherings) between cases and controls.

Because individuals were initially selected based on their status for a particular disease (ie, case: ARTI; control: no ARTI) and then categorized based on past exposure to a risk factor (ie, attendance at a social gathering), the study has a case-control design.

(Choice A) In a randomized controlled trial (RCT), participants are randomly assigned to ≥1 interventions to evaluate the effects of those interventions on outcomes of interest. Participants in a RCT may or may not be matched by certain characteristics (eg, age, sex, residence) to control for confounders. However, this study does not involve comparing treatments.

(Choice B) In a cross-sectional study, a snapshot of a population is taken to estimate the prevalence of risk factors and disease simultaneously. A questionnaire is often used to collect data in cross-sectional studies, but they are also common in other study designs (eg, case-control study).

(Choice D) In a cohort study, 2 groups of individuals are initially identified as "exposed" or "nonexposed" according to their exposure status to a specific risk factor and then followed over time to assess development of the outcome (ie, incidence of disease). In this study, individuals were initially identified according to disease status (not exposure status); therefore, it is a case-control study, meaning that it is not possible to estimate incidence of disease.

(Choice E) In a case series, a small group of patients with a similar diagnosis or treatment is described at a point in time or followed over a certain period. This observational study has no comparison group, so it cannot establish associations between risk factors and disease. However, in this study there is a comparison group and an implicit purpose to evaluate association between exposure (ie, social gathering) and disease (ie, ARTI).

Educational objective:
Case-control studies can consider only 1 outcome (ie, disease) per study but can evaluate exposure to ≥1 risk factors.