Introduction

A case series is an uncontrolled study that samples participants—with both a specific outcome and a specific intervention (exposure), or one that samples participants with a specific outcome of interest, regardless of their exposure status.1 These are commonly a retrospective review of a string of interesting cases with a unifying feature—which may be exposure, intervention, treatment or outcome. It is unclear whether the definition implies that the cases should be consecutive. These are frequent within the medical literature but are also present within social sciences and the humanities.1 As with case reports, their value has been debated.2 ,3 In the age of evidence-based medicine and the randomised controlled trial as the criterion standard to show the efficacy of a particular treatment, what is the role of a case series study?

In the summer of 1999, the use of a case series in the recognition of a new disease was exemplified by the epidemic of West Nile encephalitis in New York.4 Historically, case series were important in identifying the impact of maternal drinking and pregnancy outcome, and the role of vitamin C in preventing scurvy.5 ,6 More recently, a study by Albrecht et al7 of case series published in the Lancet found that a high proportion went on to have follow-up trials and that they were useful in establishing an early evidence base for treatments of rare diseases in which trials would not be feasible. For some specialties, establishing control groups may be difficult, for example, accident and emergency medicine. In the social sciences, many social psychology studies have been case series, for example, Yale Psychologist Milgram's seminal work on obedience to authority figures.8

In a 2005 report, Dalziel et al9 outlined that case series are used in 30% of health technology assessments—assessments used in the provision and suitability of care. However, poor reporting in the case series included in their study severely constrained their analysis and investigation of the hypothesis that findings in case series may be affected by methodological characteristics.9 Currently, no standardised reporting criteria developed within a robust methodological framework exist for case series. In the ongoing drive to improve the evidence base for clinical practice, a number of tools have been developed to improve the quality of reporting research. For example, publication of CONSORT (Consolidated Reporting Standards of Randomised controlled Trials) has seen the quality of articles in some fields improve significantly.10 ,11 The CONSORT statement has also been used to highlight and raise awareness of poor compliance in some fields.12–16 More recently, our group has shown the deficiency of observational studies in plastic surgery using the guideline STROBE (Strengthening the Reporting of Observational Studies in Epidemiology).17

A wide variety of reporting guidelines are now available across different research study types, except for case series. Problems in the reporting of surgical case series, in particular, have been highlighted to us from our recent experience in conducting a systematic review of autologous fat grafting for breast reconstruction.18 In this study, 25 of the 31 included studies were case series, yet 20% did not mention the age of the participants and 48% did not mention whether the participants had been treated with radiotherapy, an important prognostic factor. Surgery has the additional complexity of involving learning curves. The technique selected is not the sole factor affecting outcome. Patients need to be carefully selected appropriately worked-up the technique has to be meticulously worked out and implemented in an appropriate setting, with an appropriate postoperative regimen.

Readers need complete, transparent information and failure to provide this will short circuit critical appraisal, assessment of external validity and impact decisions on whether, for instance, a surgeon should change their practice. We aim to close this gap and help produce a reporting guideline for case series that is methodologically robust, easy to use and accepted internationally across a broad range of specialties and disciplines. According to Moher et al's19 guidance on guideline development, the early steps in this process require an analysis of previous literature to identify previous guidance (if any), and to analyse relevant evidence on the quality of reporting of published research articles within the domain of interest. We report our protocol for such a systematic review.