Case series are studies that involve the retrospective review of medical records data or the prospective collection of case data as a means of characterizing certain patient groups (eg, patients with a particular condition or disease) and their outcomes. Such studies provide researchers a relatively quick, inexpensive, and uncomplicated way to obtain clinically useful information not previously reported or appreciated.

The JAVMA has a long history of publishing case series, and although the journal's treatment of retrospective case series reports has changed over time,¹ case series remain among the most common types of original studies considered for publication. Indeed, of the 363 manuscripts submitted to the JAVMA in 2016 as original studies, 104 (29%) were submitted as case series or “retrospective studies” that would qualify as case series.

Despite the popularity of case series, little guidance exists on how to report such studies. This lack of guidance could stem from confusion within the medical—and not just veterinary—community regarding what exactly constitutes a case series and how case series differ from other types of observational studies. However, distinguishing case series from other observational studies is important not only for pedagogical purposes but also for assessment of evidentiary merit.²

To be clear, case series are purely descriptive studies for which the objective is to characterize or summarize the typical clinical course of a particular condition or disease through to some outcome.³ Their value lies in the descriptive data (eg, frequencies or ranges) generated, which can be useful when the study population reflects a larger patient population. The data can be obtained either retrospectively, through review of medical records, or prospectively, as cases are encountered. Because case series lack a control group, no conclusions can be drawn regarding the effect of any applied intervention. However, the findings can be useful for generating hypotheses to be explored in subsequent observational or experimental studies. Consequently, case series rank low in the hierarchy of evidence,⁴ but higher than case (or clinical) reports. Note that case reports can include multiple cases as well. However, case series involve consecutive and complete inclusion of all cases seen over a specific period, whereas case reports do not.²

In reviewing manuscripts submitted to the JAVMA in 2016, we found that at least half of those submitted as case series could be better classified as observational studies such as cross-sectional, case-control, and cohort studies, and a previous study⁵ found that this misclassification was common in other journals as well. In contrast with case series, observational studies involve explicit hypothesis testing through formal statistical comparison of 2 or more groups with respect to exposure variables (eg, treatments, environmental conditions, or putative risk or protective factors), outcome variables, or both. Consequently, we strongly recommend that researchers consider the intent of their study (ie, description or explicit contrast) when contemplating the study's design and, if questions remain, consult an epidemiologist. In some instances, a case series–observational study hybrid may be an appropriate classification, depending on the intended purpose of the study. Note that post hoc addition of comparisons to case series studies as an afterthought or a means of strengthening the evidentiary value is discouraged, and a poorly designed observational study is unlikely to yield stronger evidence than a well-designed and well-reported case series.

Given the limited attention paid to the reporting of case series in the past and the ongoing confusion regarding this study design, it is understandable that case series submitted to the JAVMA have been generally lacking in transparency, prompting queries from reviewers or the editors for clarification. Clear and thorough descriptions of inclusion criteria, case definitions, data collection approaches, interventions, evaluated outcomes, and other components are important to permit reviewers and others to determine the validity of any conclusions drawn and to allow readers to appreciate whether the findings may apply to their own patients.

To assist authors, readers, and reviewers when reporting or critically appraising case series, the JAVMA editors have developed a guidance document for reporting case series, and this document is now posted with the JAVMA instructions for authors (www.avma.org/News/Journals/Pages/javma-ifa.aspx). Going forward, the JAVMA editors will evaluate the quality of submitted case series reports in the context of this guidance document and will encourage reviewers to do so as well.

We appreciate that a lack of consensus exists among the scientific community regarding the usefulness of reporting criteria for scientific studies. The aim of the new JAVMA guidance document is not to create a disincentive or impediment for researchers but, rather, to provide assistance to ensure that studies are transparently reported to facilitate appropriate evaluation and interpretation. We believe the suggestions provided therein support our commitment to publishing the highest quality veterinary research.