Neither cancer nor myositis are common in patients testing positive for anti-TIF1γ by line blot in real-world laboratory settings

Immune-mediated myositis (IMM) is a rare, often severe multi-system autoimmune disease, characterized by presence of a number of classical autoantibodies. Concurrent with increased disease awareness has been an increase in testing for myositis-specific antibodies (MSA). However, outside of research laboratories where gold-standard immunoprecipitation assays can be performed, most diagnostic laboratories rely on the EuroImmun (Lubeck, Germany) line immunoblot assay (LIA). Although the EuroImmun LIA has been validated in reference to other methods of testing within IMM cohorts, outside of stringently confirmed IMM cohorts, the specificity of positive results has been recognized to be poor. Therefore, testing in low pre-test probability circumstances may result in a high proportion of positive myositis LIA being non-specific or “false positive.” Assessment and follow-up of patients with positive myositis autoantibodies with low clinical suspicion of IMM places increasing burden on rheumatology services.