Impact of Concordance Between Antinuclear Antibody Indirect Immunofluorescence Patterns and Myositis Antibodies in Idiopathic Inflammatory Myopathies: Study Protocol

Idiopathic inflammatory myopathies (IIM) are a heterogeneous group of systemic autoimmune rheumatic disorders in which chronic inflammation of skeletal muscle leads to muscle weakness. Many other organs, including the skin, heart, lungs, and joints, may be affected. Patients with IIM may be positive for myositis antibodies (MAs), including myositis-specific and/or associated antibodies. Although helpful for establishing the diagnosis of IIM in the appropriate clinical setting, the presence of MAs does not always predict the occurrence of connective tissue diseases. Additionally, commonly used techniques such as line blot are known to have high rates of false positivity, especially for rare MAs. The accuracy of MAs tests such as line blot may be improved by cross-checking their results with antinuclear antibody (ANA) patterns on HEp-2 indirect immunofluorescence (IIF). This study aims to examine the concordance between ANA IIF patterns and myositis-specific antibodies in a Portuguese cohort of patients with IIM. We will assess whether concordance between methods is associated with increased fulfillment of IIM classification criteria, greater disease severity, or distinct patterns of organ involvement among affected patients.