Validation of the diagnostic accuracy of myositis-related antibodies in a large patient-cohort

Lackner, A; Tiefenthaler, V; Mirzayeva, J; Graninger, W; Stradner, MH

Background/Purpose: Myositis-specific antibodies (MSA) and myositis-associated antibodies (MAA) are used for the diagnosis of idiopathic inflammatory myopathies (IIM). A careful evaluation of these antibodies is needed, because of their relevance for establishing the diagnosis and stratification into specific disease subsets. The aim of this study was to assess the diagnostic accuracy of a line immunoassay for IIM in a large real-life patient cohort and to determine the clinical significance of the Myositis-autoantibodies.

Methods: In this retrospective analysis, we retrieved the clinical diagnoses of all patients submitted to our diagnostic laboratory for MSA and MAA testing between October 2014 and October 2017. A line-immunoassay (Euroline Autoimmune Inflammatory Myopathies Immunoblot, Lübeck, Germany) was used to detect autoantibodies directed against Jo-1, Mi-2α, Mi-2β,TIF1γ, SRP, MDA-5, NXP-2, SAE, PL-7, PL-12, EJ, OJ, PM-Scl100, PM-Scl75, Ku as indicated by the manufacturer. We calculated specificity, sensitivity, negative (NPV) -and positive predictive values (PPV) as well as the positive and negative likelihood ratios (LR) for each autoantibody.

Results: In total, 3167 samples were analyzed. After exclusion of samples with repeated measurements, records were reviewed and patients without sufficient clinical data were excluded. In total 1111 patient were included in the final analysis. A total of 64 IIM patients were identified. 242 patients had at least one positive antibody testing result, of which 39 patients had an IIM diagnosis. 25 patients with a diagnosis of IIM tested negative for all autoantibodies. The test accuracy of the line-immuno-assay is shown in table 1.

Conclusion: Using the line-immuno-assay for diagnostic work-up for IIM in a real-life setting revealed that this method is a suitable alternative to more time-consuming procedures. However, clinicians should be aware that PPVs for most autoantibodies are low, due to a low pre-test probability.

Table 1: Test-accuracy of Myositis-antibodies