Automating expert-level medical reasoning evaluation of large language models

As large language models (LLMs) become increasingly integrated into clinical decision-making, ensuring trustworthy reasoning is paramount. However, current evaluation strategies of LLMs' medical reasoning capability either suffer from unsatisfactory assessment or poor scalability, and a rigorous benchmark remains absent. To address this, we present MedThink-Bench, a benchmark designed for rigorous and scalable assessment of LLMs' medical reasoning. MedThink-Bench comprises 500 high-complexity questions spanning ten medical domains, accompanied by expert-authored, step-by-step rationales that elucidate intermediate reasoning processes. Further, we introduce LLM-w-Rationale, an evaluation framework that combines fine-grained rationale assessment with an LLM-as-a-Judge paradigm, enabling expert-level fidelity in evaluating reasoning quality while preserving scalability. Results show that LLM-w-Rationale correlates strongly with expert evaluation (Pearson coefficient up to 0.87) while requiring only 1.4% of the evaluation time. Overall, MedThink-Bench establishes a rigorous and scalable standard for evaluating medical reasoning in LLMs, advancing their safe and responsible deployment in clinical practice.