Why Clinical Question Answering Requires Both Lexical and Semantic Search

A comprehensive study of 500 clinical queries reveals that BM25 (keyword-based) and dense (semantic) retrieval methods solve fundamentally different problems—dense retrieval misses 22% of questions that keyword search answers. This finding demonstrates that hybrid retrieval approaches, combined using Reciprocal Rank Fusion, are essential for robust clinical question answering systems. For healthcare applications, implementing hybrid search with deterministic citation verification can significantly improve diagnostic accuracy and clinical documentation retrieval.