Idiopathic inflammatory myopathy: inter-rater variability in muscle biopsy reading

Background: Despite its importance for diagnosing idiopathic inflammatory myopathy (IIM) subgroups, muscle biopsy classification criteria have not been standardized nor validated. Objective: We sought to determine inter-rater variability in diagnosing individual muscle biopsy abnormalities. Methods: We developed a scoring tool to analyze consensus in muscle biopsy reading of an ad hoc workgroup of international experts. Twenty-four samples from suspected IIM patients were randomly selected, providing sections that were stained with standard histological and immuno-histochemical methods. Sections were made available on an online platform and experts were queried about myopathological features within four pathological domains: muscle fibers, inflammation, connective tissue and vasculature. A short clinical presentation of cases was included, and experts were asked to give a tentative diagnosis of polymyositis, dermatomyositis, inclusion-body myositis, anti-synthetase syndrome-related my...