OBJECTIVES: Case report data have traditionally been considered low-quality evidence due to their anecdotal nature and potential for bias. However, case reports provide detailed clinical data that may otherwise be lacking – from patients with rare diseases, or those experiencing infrequent events, for example. These data may be especially informative when consolidated, although formal methodologic guidance is lacking, and existing systematic literature reviews (SLRs) typically use descriptive syntheses. This scoping review characterized contemporary methodologies used for quantitatively synthesizing case report data, as well as noted key strengths and limitations.

METHODS: A search strategy was implemented in MEDLINE and Embase (October 2022) using indexed terms and keywords related to case reports/series, SLRs, and meta-analyses. Abstracts were screened and studies/SLRs describing or employing methodologies for quantitative synthesis of case report data were included. Studies were characterized, and the potential benefits and methodologic challenges of quantitative case report syntheses were tabulated.

RESULTS: Of 2,235 abstracts identified, 13 (0.6%) were included; 10 SLRs and 3 methodological articles. The majority (60%) of included SLRs (n, 5-477) investigated rare conditions; however, complications of COVID-19 were also commonly studied (30%). Within SLRs, synthesis methods included Kaplan-Meier curves, regression analyses, and pooled proportions. Across included articles, 6 (46%) highlighted the potential role of quantitative case report synthesis to generate hypotheses and 4 (31%) to identify rare associations. Key methodologic challenges included publication bias (46%), clinical heterogeneity of cases (39%) and small sample sizes for individual outcomes (31%).

CONCLUSIONS: This scoping review highlighted common approaches, potential value, and limitations of quantitative evidence synthesis of case report data. Such methods may be important in aggregating data on outcomes among those with rare diseases, summarizing infrequent events, or exploring risk factors for these. The issues identified with synthesizing case report data suggest that guidance is needed to provide greater clarity for rigorously implementing this methodology.