Can Post-Training Turn LLMs into Good Medical Coders? An Empirical Study of Generative ICD Coding

Ziqing Wang, Weihao Li, Shijie Chen, Yuan Luo, Kaize Ding


Abstract
Automated International Classification of Diseases (ICD) coding is a core medical-coding task for billing, epidemiology, and clinical decision support. Generative large language models (LLMs) are often reported as weak medical coders, but this finding mainly comes from inference-time settings such as prompting, retrieval, reranking, or tool use; the role of task-specific post-training has received limited systematic study. We present a controlled empirical study of post-training for generative ICD coding, comparing discriminative baselines with LLM coders across prompting, supervised fine-tuning (SFT), and reinforcement learning (RL) under a common protocol and metric set. To our knowledge, this is the first comprehensive controlled study of post-training for generative ICD prediction. We further introduce Progressive Hint Injection (PHI), a diagnostic missed-code curriculum within Group Relative Policy Optimization (GRPO) training. Our results show that prompting-only evaluation substantially underestimates the potential of LLMs for ICD coding. SFT provides the main capability jump, GRPO further improves code-set prediction beyond SFT, and PHI provides targeted gains on macro-level performance. These findings suggest that the main bottleneck is not the generative formulation alone, but how the model is adapted and optimized for full-taxonomy recall. We release our code, data splits, and checkpoints at https://anonymous.4open.science/r/LLM4ICD.
Anthology ID:
2026.findings-emnlp.459
Volume:
Findings of the Association for Computational Linguistics: EMNLP 2026
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
Findings
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
9643–9658
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URL:
https://aclanthology.org/2026.findings-emnlp.459/
DOI:
Bibkey:
Cite (ACL):
Ziqing Wang, Weihao Li, Shijie Chen, Yuan Luo, and Kaize Ding. 2026. Can Post-Training Turn LLMs into Good Medical Coders? An Empirical Study of Generative ICD Coding. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 9643–9658, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Can Post-Training Turn LLMs into Good Medical Coders? An Empirical Study of Generative ICD Coding (Wang et al., Findings 2026)
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PDF:
https://aclanthology.org/2026.findings-emnlp.459.pdf