Purpose: To establish non-radioactive assays for detection of antibodies (Abs) to the acetylcholine receptor (AChR) and to muscle specific kinase (MuSK). To show that the assays can be used in Tianjin for testing patients with MG.
Method: AChR and MuSK Abs in 51 Chinese MG patients' sera were tested in Oxford, UK, using the conventional radioimmunoprecipitation assay (RIPA), a recently described cell-based assay (CBA), and a new non-radioactive fluorescence immunoprecipitation assay (FIPA) which can measure both AChR and MuSK antibodies in one step. 102 MG sera were subsequently tested by CBA and FIPA in Tianjin, China. Based on the different serological subgroups, the clinical features of these 153 MG patients were analyzed.
Results: We first confirmed the sensitivity and specificity of the assays in Oxford. There was good agreement between the FIPA and the RIPA for AChR Abs (r² = 0.6; p < 0.0001) with 80% positivity in the RIPA and 76% in the FIPA. Two patients were positive for MuSK Abs (4% of total) by both RIPA and FIPA assays. CBA was more sensitive for MuSK Abs, identifying an additional 3 patients. In Tianjin, using the FIPA and CBA, 84/102 (82%) AChR Ab positive patients and four MuSK Ab positive patients were identified. In the whole group of 153 MG patients, there were no differences in clinical features between different antibody subgroups. However, there were 30 patients with thymomas (20%), of whom one had MuSK antibodies. Moreover, two patients with purely ocular MG also had MuSK antibodies.
Conclusions: Both the FIPA and CBA were established in China and should prove useful for diagnostic testing. Including the CBA for MuSK antibodies increased the number of MuSK-MG patients to 6% of the total, and 33% of the patients without AChR antibodies. The clinical features were mainly relatively mild but thymic tumors were common.
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