The Semmelweis Framework: structuring evidence foundations and expert opinion in clinical practice guidelines

BACKGROUND: For many guideline-developing organisations, expert opinion remains essential when evidence is scarce. However, its unstandardised use remains a significant gap in guideline methodology. This article presents the development of the Semmelweis Framework, a methodological tool for structuring expert opinion in clinical practice guidelines when evidence is limited, indirect, or absent. METHODS: The Semmelweis Framework was developed through a stepwise methodological process. First, a meta-epidemiological study mapped expert opinion use across 2,296 recommendations from 136 organisations. Second, a methodological review of 98 guideline manuals synthesised the functional definition of expert opinion that informed the Framework. Building on this definition, the Framework was refined through independent multi-reviewer application to 30 published recommendations, with calibration meetings, qualitative comparison of classifications, and parallel development of a Preconditions Checklist. RESULTS: The Semmelweis Framework offers an approach for structuring the role of expert opinion in clinical guidelines by classifying recommendations in four classes according to their evidentiary foundation. These classes form a continuum: from recommendations based on direct comparative evidence to those relying entirely on expert opinion when no relevant group-level evidence exists. Class I is grounded in direct comparative evidence. Class II addresses indirect evidence and requires explicit labelling of indirectness while using expert opinion to support applicability. Class III relies on non-comparative evidence and is heavily influenced by expert opinion, whereas Class IV is based entirely on expert opinion, operationalised as structured expert judgement, when no group-level evidence exists. Across all classes, fixed verbs and notations are proposed to standardise communication. A core component of the Framework is the specifically developed Preconditions Checklist, designed to ensure integrity and transparency when recommendations rely substantially on expert opinion. Under the Semmelweis Framework, "expert opinion" should not be used as a generic recommendation label; instead, its roles are redistributed across recommendation classes, each with explicit foundations and labelling. CONCLUSIONS: The Semmelweis Framework reframes the role of expert opinion and offers an approach for structuring its use in clinical guidelines. By introducing safeguards and new recommendation classes according to their evidentiary foundations, the Framework addresses a long-standing methodological gap.