who developed myositis had low B cell frequencies (Figure 1, online supplementary tables 2 and 3). A single patient without myositis, but who developed enteritis, also had low B cell levels. Taken together, patients with thymoma who developed myositis or enteritis had lower B cell frequencies (0.19%, 0.12%–0.73%; median, IQR) than patients with thymoma who did not (12.37%, 5.14%–16.5%), those with non-thymic malignancies (8.3%, 2.4%–11.7%) or healthy controls (16.3%, 11.9%–17.65%). These observations suggest that testing for mAChR autoantibodies and/or B cell levels may identify patients with thymoma most at risk for developing myositis with avelumab. Since mAChR autoantibodies cause myasthenia but not myositis or elevated CK levels, and because mAChR autoantibody levels did not increase with myositis, we conclude that they are most likely a marker of pre-existing autoimmunity rather than the direct cause of muscle damage. B cell lymphopaenia, which occurs in half of patients copyright. on November 30, 2023 at Google Indexer. Protected by http://ard. bmj. com/