Title: When Case Gets Rare: A Retrieval Benchmark for Off-Guideline Medical Question Answering
Primary area: datasets and benchmarks
Keywords: medical LLMs, retrieval augmentation, benchmarking

Abstract: Across medical specialties, clinical practice is anchored in evidence-based guidelines that codify best studied diagnostic and treatment pathways. These pathways work well for the majority of patients but routinely fall short for the long tail of real-world care not covered by guidelines. Most medical large language models (LLMs), however, are trained to encode common, guideline-focused medical knowledge in their parameters. Current evaluations test models primarily on recalling and reasoning with this memorized content, often in multiple-choice settings. Given the fundamental importance of evidence-based reasoning in medicine, it is neither feasible nor reliable to depend on such memorization in practice. To address this gap, we introduce OGCaReBench, a long-form retrieval-focused benchmark aimed at evaluating LLMs at answering clinical questions that require going beyond typical guidelines. Extracted from published medical case reports and validated by medical professionals, OGCaReBench contains long-form clinical questions requiring free-text answers, providing a systematic framework for assessing open-ended medical reasoning in rare, case-based scenarios. Our experiments reveal that even the best-performing baseline (GPT-o3-mini) correctly answers only 51% of our benchmark with open-source models only reaching 36%. Augmenting the models with retrieved medical articles improves this performance to up to 75% (using GPT-5) highlighting the importance of evidence-grounding for real-world medical reasoning tasks. OGCaReBench thus establishes a foundation for benchmarking and advancing both general-purpose and medical language models to produce reliable answers in challenging clinical contexts.

TL;DR: We present OGCaReBench, a rare-case based physician-validated medical QA benchmark. RAG significantly improves performance upon retrieval quality, context processing, and reasoning.