Case series – Insights on value and use

Manuscripts in medical journals are published in a spectrum of formats ranging from original research to individual case reports. Among these, a “case series” is a lesser understood and possibly underutilized form of research methodology. It can be interpreted in various ways and we aim to clarify both definition and utility.

A case series cannot be considered merely a combination of case reports. It is an observational and descriptive research methodology. At its bones, a case series is a collection of cases with a common factor or outcome. The cases may or may not have common exposure factors and can be selected prospectively or retrospectively. Dekkers et al. infer that the absence of subjects without the specified “outcome” means that absolute risk cannot be calculated.^[1-4]

To offer perspective, cohort studies will follow a group of exposed/unexposed subjects to study the possibility of an outcome. This permits the calculation of absolute risk (as well as relative risk if controls are included).^[1,5]

Over 3 cases are required for a case series to be accepted as research.^[6] It can include from 3 to 100 cases, however, case series is not mentioned under the strengthening the reporting of observational studies in epidemiology (STROBE) guidelines.^[7,8]

There is no formally defined process for case selection, and often includes any relevant case encountered by the researcher, not necessarily consecutive or over a defined period of time. This offers the opportunity for some amount of bias (in particular, selection bias).^[2,9,10]

Although strong data-backed conclusions cannot be made from a case series, they contribute in other valuable ways. For example, often, this is the first way of presenting a new hypothesis or identifying new patterns. Subsequently, attention can be brought to the topic and further detailed research may be performed.^[3] Case series can aid in the design of future prospective trials and funding applications. It can lead to the recognition of new pathologies or uncommon presentations of common pathologies.

It is inexpensive and can be done in low-resource settings with ease. In view of straightforward methodology, a case series can also be published rapidly for new pathologies requiring urgent study. For example, during Covid-19, information about the clinical presentation, virology, and management of complications were essential to be disseminated quickly.^[11,12] The source of life-threatening hemoptysis in post-Covid-19 patients was from pulmonary pseudoaneurysms, unlike bronchial arteries found in a post-tubercular squeal. Embolization could be planned accordingly.^[13]