Defining expert opinion in clinical guidelines: insights from 98 scientific societies - a methodological study.

Background: The use of Expert Opinion (EO) in clinical guidelines is highly variable and lacks standardization, leading to ongoing controversy. A clear and universally accepted definition of EO is also lacking. To date, no research has systematically assessed how guideline-developing societies conceptualize and apply EO. This study aims to map methodological manuals, evaluate their rationale for EO use, examine its foundations, and synthesize a comprehensive definition.

Methods: Systematic searches for clinical guidelines were conducted in PubMed to identify guideline-developing societies, supplemented by additional searches. Systematic searches were then conducted to identify methodological manuals from these societies. Screening was performed independently by two reviewers, and data extraction was conducted using piloted forms. Findings were summarized through narrative evidence synthesis using descriptive statistics.

Results: A total of 473 national and international societies were identified, and methodological manuals from 98 societies were mapped and analysed. These manuals included 61 handbooks, 29 journal articles, and 8 websites. EO is mentioned in 65 (66%) manuals, with substantial variation in its utilization and terminology. EO is primarily used in two contexts: (1) filling evidence gaps (72%), and (2) interpreting existing evidence (8%). In the remaining 20%, EO use is unclear. Five main foundations could be identified as a potential basis for EO (clinical experience, indirect evidence, low-quality evidence, mechanism-based reasoning, and expert evidence/witnesses). Based on these findings, a novel comprehensive definition of EO was synthesized.

Conclusions: EO is widely used to address evidence gaps and interpret ambiguous evidence, underscoring its importance in guideline development. However, the variability in its application and conceptualization across societies highlights the need for standardization. We propose a comprehensive EO definition as a first step towards standardization to improve consistency, transparency, and clinical decision-making.