Background
Inflammatory myositis (IIM) is rare and can be associated with other conditions such as interstitial lung disease (ILD) and malignancy. Autoantibodies specific for IIM, or myositis specific antibodies (MSAs), are useful biomarkers that assist with clinical diagnosis. The utility of MSA has increased as new autoantibodies are being discovered. Currently there is no data on the positive predictive value of MSA in general patient population where the overall prevalence of inflammatory myositis is low.
Aim:
To assess the rate of true positive MSA and the clinical consequences of false positive results.
Method
This was a retrospective review of positive MSA assays reported by South East Sydney Area Laboratory Service (SEALS) between 2017 and 2019. Relevant laboratory results such as antinuclear antibody pattern, ESR, CRP and CK were also retrieved. A review of the corresponding clinical notes, biopsy results, and radiological findings was conducted.
Results
In total, 150 positive MSA were reported in 118 patients over the study period with Ro52 (n=42), Mi2 (n=21) and Pm-scl100 (n=12) being the most common autoantibodies. Twenty-seven patients (22.8%) had clinical diagnosis of inflammatory myositis with dermatomyositis the most common subtype. Twenty-four patients (20%) had interstitial lung disease, of which 10 were associated with underlying rheumatologic condition.
Conclusion
Positive MSA can be detected in a number of conditions besides inflammatory myositis. There is growing use of MSA as part of ILD assessment. Positive results should be interpreted with care to avoid over-diagnosis and unnecessary investigation.