Myositis specific antibodies - who orders them and what are the results? A retrospective audit of two large referral centers

Abstract

Background: Idiopathic infammatory myopathies are a group of disorders associated with myositis-specifc antibodies (MSA) and myositis-associated antibodies (MAA) (1). MSA and MAA are helpful in classifying clinical syndromes with both treatment and prognostic implications (2). Ordering of MSA and MAA has increased in recent years. We report the hospital services that ordered MSA and MAA and follow-up investigations performed in two large referral centers in Melbourne, Australia. Objective(s): To explore and investigate the testing patterns of MSA and subsequent follow up. Method(s): We identifed all patients who underwent MSA and MAA testing between January 1 2019 and December 31 2020. Relevant demographic and clinical data were obtained by chart review. Result(s): Over the 2-year study period, 294 MSA tests were ordered, alongside 275 ANA and 266 ENA. Of these, 60 (20.4%) were positive and 9 (3.1%) tested positive to more than one MSA. Interstitial lung disease (ILD) was the most common extra-muscular manifestation, 42 (14.2%) had muscle disease manifestations. Respiratory (41%), rheumatology (24%) and neurology (15%) were the top three specialties ordering the most MSA, with the highest proportion of positive MSA originating from rheumatology (38.5%). The rheumatology service was involved in 131 (41.0%) of the MSA ordered as the primary treating unit or as a consultation service. Anti-Scl100 was the most common MSA (n=10, 15.6%). Among the 60 patients with positive MSA, 39 (65%) was given a defni-tive diagnosis, 13 (21.7%) underwent MRI of muscles, 10 (16.7%) open muscle biopsy, and 5 (8.3%) a skin biopsy. Thirty-two (53.3%) of MSA positive patients underwent high-resolution CT chest; 26 (81.3) % had fndings suggestive of a fbrotic process and 12 (20.0%) had a formal diagnosis of ILD. A cancer screen was performed in 23 (38.3%) of the MSA positive patients and no malignancy was identifed within the cohort. Conclusion(s): Among MSA studies performed in two large centers, 20.4% of results were positive. Respiratory ordered the most numerous tests predominantly to evaluate ILD, a higher proportion of tests recommended by the rheumatology service was positive.