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  • This was a retrospective cross-sectional multi-center study. Clinical data and sera of adult patients with a high index of suspicion for seronegative generalized MG were collected from 18 hospitals between January 2014 and May 2016. Data were entered into a standard case report form designed to record the clinical characteristics of patients with seronegative generalized MG. MG was diagnosed based on the presence of exertional muscle weakness and an abnormal decremental response to low-frequency repetitive nerve stimulation (RNST), or positive pharmacological tests (amelioration of symptoms after intravenous or intramuscular administration of anti-cholinesterase). AChR antibodies should be negative on RIPA. Collected data were reviewed and assessed for inclusion by two authors (KH Park, YH Hong) who had access to all the clinical and laboratory data, including disease course and therapeutic response. Disease severity was evaluated by the Myasthenia Gravis Foundation of America (MGFA) clinical classification, and the MG composite scale (MGCS) [15,16]. All patients provided written informed consent. Sera were stored at -80°C at the central laboratory of Boramae Medical Center, Seoul, before batch transport to the UK for testing. This study was approved by the local ethics committee of Seoul National University, Seoul Metropolitan Government Boramae Medical Center (IRB 16-2014-29). All serum samples were tested for the three autoantibodies to MuSK, LRP4, and clustered AChR at the Autoimmune Neurology Diagnostic Laboratory, Nuffield Department of Clinical Neurosciences, John Radcliffe Hospital, Oxford, UK. All antibody testing was performed blinded to the clinical information. Antibodies to MuSK were tested by both RIPA (RSR Ltd, Cardiff, UK) and CBA, and antibodies to clustered AChR and LRP4 were tested by CBA. Measurement of antibody binding in CBA was performed by indirect immunofluorescence, as previously described [3,6,17]. Results were measured by two observers on a nonlinear visual scale from 0 to 4 with the mean result given. A score of less than 1 was considered to be negative and scores from 1 to 4 were considered to be positive with 1 considered to be weak positive and 4 strong positive. All positive tests were repeated. Differences between groups were assessed by Fisher’s exact test or Kruskal-Wallis test. Pearson’s correlation analysis was performed to determine the relationship between CBA scores and radioimmunoassay (RIA) values for anti-MuSK antibodies. A two-tailed P value < 0.05 was considered to be statistically significant. All statistical analyses were performed with SPSS for Windows version 21.0 (IBM Corp., Armonk, NY).
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