Abstract
BackgroundLine blot immunoassays (LIA) for myositis-specific (MSA) and myositis-associated (MAA) autoantibodies have become commercially available. In the largest study of this kind, we evaluated the clinical performance of a widely used LIA for MSAs and MAAs.MethodsAdults tested for MSA/MAA by LIA at a tertiary myositis centre (January 2016–July 2018) were identified. According to expert-defined diagnoses, true and false positive rates were calculated for strongly and weakly positive autoantibody results within three cohorts: idiopathic inflammatory myopathy (IIM), connective tissue disease (CTD) without myositis, and non-CTD/IIM. Factors associated with true positivity were determined.ResultsWe analysed 342 cases. 67 (19.6%) had IIM, in whom 71 autoantibodies were detected (50 strong positives [70.4%], 21 weak positives [29.6%]). Of the strong positives, 48/50 (96.0%; 19 MSAs, 29 MAAs) were deemed true positives. Of the weak positives, 15/21 (71.4%; 3 MSAs, 12 MAAs) were deemed true positives.In CTD without myositis cases (n = 120), 31/61 (51.0%; 5 MSAs, 26 MAAs) autoantibodies were strongly positive, with 24/31 (77.4%; 0 MSAs, 24 MAAs) true positives. 30/61 (49.2%; 13 MSAs, 17 MAAs) were weakly positive, with 16/30 (53.3%; 0 MSAs, 16 MAAs) true positives. In non-CTD/IIM cases (n = 155), all 24 MSAs and 22 MAAs were false positives; these results included 17 (37.0%; 7 MSAs, 10 MAAs) strong positives.Individual autoantibody specificities were > 98.2 and > 97.5% for weakly and strongly positive results, respectively. True positivity was associated with high pre-test for IIM (odds ratio 50.8, 95% CI 13.7–189.2, p < 0.001) and strong positive (versus weak positive) results (4.4, 2.3–8.3, p < 0.001).ConclusionsWe demonstrated the high specificity of a myositis LIA in a clinical setting. However, a significant burden of false positive results was evident in those with a low pre-test likelihood of IIM and for weakly positive autoantibodies.
Highlights
Line blot immunoassays (LIA) for myositis-specific (MSA) and myositis-associated (MAA) autoantibodies have become commercially available
2/3 of idiopathic inflammatory myopathies (IIM) patients have detectable serum autoantibodies (Abs), termed myositis-specific autoantibodies (MSA), which are unique to IIM and usually mutually exclusive to one another, or myositisassociated autoantibodies (MAA) which can occur in other connective tissue diseases (CTD) [6,7,8]
Cases We retrospectively identified patients tested with the EUROLINE Inflammatory Myopathies 16 Ag (IgG) commercial LIA (Euroimmun, Lubeck, Germany) from January 1st, 2016, to July 30th, 2018, at Salford Royal NHS Foundation Trust (SRFT), United Kingdom
Summary
Line blot immunoassays (LIA) for myositis-specific (MSA) and myositis-associated (MAA) autoantibodies have become commercially available. In the largest study of this kind, we evaluated the clinical performance of a widely used LIA for MSAs and MAAs. The idiopathic inflammatory myopathies (IIM) are heterogeneous multisystem autoimmune diseases often presenting with skeletal muscle weakness, rash, arthritis, and/or interstitial lung disease [1]. Line blot immunoassays (LIA) for MSA/MAA have become commercially available, increasing the availability of testing in clinical practice [19]. Literature suggests LIA is an appropriate substitute to conventional immunoprecipitation for MSA/MAA testing, but only small samples have been studied with variable accuracy demonstrated [13, 19, 20]
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