Test-Time Adaptation of Medical Vision-Language Models with Mixture of Modality Experts

Test-time adaptation (TTA) has emerged as a promising solution to improve the robustness of deep learning models under domain shifts, particularly in real-world scenarios where source data or true labels are unavailable. In this paper, we propose a novel TTA framework tailored for medical vision-language models (VLMs) that leverages a Mixture-of-Experts (MoE) mechanism. Building upon a frozen, pre-trained BiomedCLIP backbone, our method integrates parallel MoE adapters of different medical imaging modalities in each vision MLP block, enabling expert-specific adaptation without disrupting the core model representation. At inference time, only the MoE router is optimized through an entropy-regularized objective, which is further augmented by pseudo-label guidance and adaptive scaling strategies. Additionally, we propose an entropy-aware MoE scaling policy that dynamically adjusts expert influence based on prediction uncertainty, improving model adaptability. Extensive experiments on multiple medical imaging benchmarks demonstrate that our approach achieves substantial performance improvements over existing TTA baselines, while maintaining high efficiency and parameter sparsity. Our results highlight the potential of MoE-enhanced TTA to achieve robust and generalizable medical VLMs in unseen domains without access to source data. Code is available at https://openi.pcl.ac.cn/OpenMedIA/MoME-TTA.git